Trends Sci. 2025; 22(11): 10991

Carbon Capture at the Point Source: CO₂ Gas Bubbling vs. Bicarbonate Supplementation in Microalgal Cultivation

Yudatomo Tri Nugroho¹, Ari Hardianto², Abu Bakar Muhammad Ibnu Syihab¹,

Saifa Aprilia Sidquni¹, Ivani Nurjannah³, Lucy Adinisa³ and Toto Subroto^2,*

¹PT. YTL Jawa Timur, East Java 67291, Indonesia

²Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran,

West Java 45363, Indonesia

³Department of Biotechnology, Graduate School, Universitas Padjadjaran, West Java 40132, Indonesia

(^*Corresponding author’s e-mail: [email protected])

Received: 11 June 2025, Revised: 1 July 2025, Accepted: 8 July 2025, Published: 5 August 2025

Abstract

Mitigating greenhouse gas (GHG) emissions from industrial point sources is a key challenge for effective climate action. Among emerging carbon capture, utilization, and storage (CCUS) technologies, large-scale microalgal cultivation offers a sustainable pathway to converting carbon emissions into valuable biomass. This review evaluates 2 practical carbon supply strategies for microalgal systems - CO₂-based and bicarbonate-based approaches - highlighting their respective technical performance and operational requirements. By comparing these systems in the context of practical integration with industrial emitters, we aim to clarify key trade-offs and operational considerations. This assessment provides a framework for optimizing microalgae-based carbon capture technologies and informs future design and deployment of emission-integrated microalgal cultivation systems. In comparison to the CO₂-based. system, the bicarbonate-based carbon supply system is a more effective choice for microalgae cultivation system in carbon capture project, according to the study’s findings. The Net energy Ratio (NER) of this bicarbonate-based cultivation system is 7.29, significantly greater than that of the CO₂-based bubbling system, which only has a NER value of 0.85. This extremely high value can contribute to the project’s overall financial viability.

Keywords: Carbon capture, CO₂, CCUS, GHG emission, Microalgae, Cultivation, Bicarbonate, Bubbling

Introduction

Anthropogenic greenhouse-gas (GHG) emissions, particularly carbon dioxide (CO₂) from industrial point sources, are a major driver of global warming [1]. These emissions, predominantly resulting from fossil fuel combustion, account for approximately 40% of global anthropogenic CO₂ output [2]. Therefore, reducing emissions directly at the point of release is a key priority in effective climate action strategies. Although the natural greenhouse effect, sustained by atmospheric CO₂ and CH₄, maintains Earth’s habitable temperature [3], continued reliance on fossil fuels is pushing

atmospheric concentrations beyond safe thresholds. As a result, large-scale carbon capture at industrial point sources has gained increasing attention as a promising mitigation approach to accelerate the transition toward decarbonization in sectors such as power generation, aiming to limit further GHG emissions.

Among emerging carbon capture, utilization, and storage (CCUS) strategies, microalgal photobioreactor systems have attracted growing interest as a sustainable and scalable biological solution. Microalgae convert light, CO₂, and nutrients into biomass with relatively high efficiency compared to terrestrial plants, and exhibit rapid growth rates, morphological flexibility, and adaptability to varying environmental conditions [4]. Certain species, such as Chlorella vulgaris, not only tolerate operational fluctuations but also accumulate commercially valuable compounds, including proteins, lipids, and carbohydrates [5]. Due to these advantages, microalgal biomass holds considerable potential across diverse sectors, from food and biofuel production to pharmaceuticals [6], positioning these systems as promising platforms for integrated carbon capture efforts.

In microalgal carbon capture systems, inorganic carbon is primarily supplied in the form of dissolved carbon dioxide (CO₂) or bicarbonate ions (HCO₃⁻) [7]. CO₂ can be sourced from ambient air, purified gas, or more practically, from industrial flue gas, and delivered through technologies such as microbubble generators, membrane spargers, or direct gas bubbling [8]. Alternatively, bicarbonate-based systems capture CO₂ into alkaline solutions, converting it chemically into HCO₃⁻, which is then supplied to the algal culture [9]. This strategy, exemplified by the Bicarbonate-based Integrated Carbon Capture and Algae Production System (BICCAPS), offers improved pH stability and reduced carbon loss, although it requires additional processing steps and chemical inputs. In both systems, the carbon supply method plays a critical role in determining energy consumption, carbon fixation efficiency, and the overall feasibility of microalgae-based carbon capture systems.

A comparison between CO₂-based and bicarbonate-based cultivation systems is essential due to their distinct operational characteristics, energy demands, and suitability for integration with industrial processes. Previous studies have reported varying outcomes in terms of biomass productivity, carbon utilization efficiency, and techno-economic performance [9-11], highlighting the need for context-specific evaluation. Such comparison are crucial for identifying optimal pathways to implement microalgal CCU technologies under different industrial and environmental conditions.

This review presents a comparative evaluation of 2 carbon capture strategies - flue gas-based and bicarbonate-based systems - for photoautotrophic microalgal cultivation. Relevant research articles were systematically reviewed, and key technical and economic data were extracted, interpreted, and compared. Additionally, the author conducted an experiment on microalgae cultivation at the BioCCU Unit at Paiton Power Plant Units 5 & 6 in Indonesia. The experiment compared the technical performance of the bicarbonate-based system with CO₂-based system. Several parameter values ​​generated in this experiment were used to conduct technical simulations. To ensure consistency and comparability across studies, standardized parameters were employed, focusing on carbon fixation efficiency and energy consumption for technical evaluation. Economic assessment considered energy costs, chemical costs, and the overall cost of CO₂ emission reduction per ton. Particular emphasis was placed on applications relevant to the energy sector, a major contributor to global GHG emissions.

Microalgae cultivation systems for carbon fixation

Microalgae are unicellular, photosynthetic organisms that containi organelles such as chloroplasts, where photosynthesis occurs [12]. Through this process, they capture carbon dioxide (CO₂) and convert it into organic biomass, with carbon comprising approximately 50% of the dry weight [13]. Notably, microalgae can utilize CO₂ from both the atmosphere and aquatic environments, maintaining high fixation efficiencies even under low CO₂ concentrations [14]. In the photosynthetic process, light-dependent reactions use solar energy to split water molecules, producing protons, electrons, and oxygen. The Calvin cycle then fixes CO₂ into glucose, which is subsequently converted into biomolecules such as lipids, proteins, and polysaccharides through various metabolic pathways [15,16]. Altogether, the rapid growth, high carbon fixation capacity, and the ability to thrive in diverse water sources, whether potable or non-potable, position microalgae as a highly promising platform for sustainable carbon capture and utilization [17].

Dissolved Inorganic Carbon (DIC) and its availability

In microalgal cultivation systems, carbon supply primarily relies on dissolved inorganic carbon (DIC), which consists of 4 main species: Dissolved carbon dioxide (CO₂), carbonic acid (H₂CO₃), bicarbonate ions (HCO₃⁻), and carbonate ions (CO₃²⁻). The relative distribution of these species is strongly influenced by the pH of the medium: Dissolved CO₂ is predominates under acidic conditions, bicarbonate becomes predominant in near-neutral to moderately alkaline environments, and carbonate ions become more prevalent at high pH levels (Figure 1) [7]. In typical cultivation settings, where the pH is maintained around 7 - 9, bicarbonate is the most abundant and bioavailable form, supporting efficient carbon fixation. Moreover, the ability of microalgae to assimilate different forms of DIC varies by species. For example, Nannochloropsis oculata primarily transports HCO₃⁻, whereas Chlamydomonas reinhardtii can utilize both CO₂ and HCO₃⁻ [18]. In cultivation systems utilizing continuous flue gas injection, the dynamic supply of CO₂ can also influence the pH, often reesulting in acidification if not properly regulated. Therefore, a comprehensive understanding of both DIC speciation and species-specific carbon uptake mechanisms is essential for optimizing carbon fixation efficiency in microalgal cultures.

In microalgal cultivation systems supplied with flue gas, carbon assimilation occurs in 2 stages: Physical dissolution of CO₂ into the culture medium, followed by biological uptake of dissolved inorganic carbon (DIC) by the cells. CO₂ dissolution is governed by the 2-film theory, where diffusion across thin gas and liquid boundary layers limits mass transfer [19]. The transfer flux is described by:

where T_CO2 represents the rate of CO₂ transfer from the gas phase (C_g) to the liquid phase (C_L), driven by the concentration difference (C_g − C_L), with kt denoting the overall transfer coefficient and S the effective interfacial area. The equilibrium solubility of CO₂ in the liquid is determined by Henry’s law:

where C_eq is the maximum dissolved CO₂ concentration achievable under a given partial pressure P_CO2 and H is Henry’s constant. Due to inherently low solubility of CO₂, several strategies such as bubble size reduction, increased gas-phase CO₂ concentration, nanobubble technology, and extended gas retention time are employed to enhance mass transfer (Table 1) [7,20]. Once dissolved, CO₂ undergoes hydration and ionization into DIC species (CO₂(aq), H₂CO₃, HCO₃⁻, CO₃²⁻), with the distribution strongly pH-dependent [7,21]. However, at near-neutral pH, the conversion of CO₂ into bicarbonate is kinetically slow, increasing the risk of CO₂ loss to the atmosphere. To mitigate this, chemical enhancement using alkaline absorbents (e.g., NaOH) is applied to rapidly convert dissolved CO₂ into stable HCO₃⁻ through the following reaction:

This reaction improves both carbon retention and bioavailability. Consequently, optimizing both physical mass transfer efficiency and chemical stabilization which is crucial for effective carbon capture in microalgal flue gas systems.

Figure 1 (A) CO₂ dissolution from the gas phase into microalgae suspension [7]. (B) The percent distribution of DIC in microalgae suspension in various pH conditions [21].

Table 1 Summary of strategies to improve CO₂ dissolutions into microalgae suspension.

DIC such as CO₂ and HCO₃⁻, is assimilated into microalgal cells through 3 principal pathways: (1) direct diffusion of CO₂ across the plasma membrane; (2) enzymatic conversion of extracellular HCO₃⁻ into CO₂ by carbonic anhydrase (CA) near the cell surface, facilitating subsequent CO₂ diffusion into the cell; and (3) active transport of HCO₃⁻ into the cytosol via bicarbonate-specific transporters such as BCT1, particularly under carbon-limited conditions [7,22,23]. Recent findings in Chlamydomonas have clarified that an extracellular form of carbonic anhydrase (CAH1) plays a key role in the conversion of HCO₃⁻ and CO₂ at the cell surface, thereby enhancing initial DIC uptake in aquatic environments [24]. On the other hand, the active transport of HCO₃⁻ typically serves as the initial step of carbon concentrating mechanisms (CCMs) evolved in diverse microalgal species. CCMs develop physiological strategies to elevate the CO₂ concentration around Rubisco in the chloroplast, enhancing carboxylation efficiency and reducing photorespiration losses [23,25]. In the well-established CCM pathway, HCO₃⁻ actively imported into the cytosol is transported into the chloroplast and accumulated within the pyrenoid, a Rubisco-associated microcompartment. Carbonic anhydrase inside the pyrenoid catalyzes the conversion of HCO₃⁻ to CO₂, generating a localized high-CO₂ environment that promotes carbon fixation. A proteinaceous shell surrounding the pyrenoid acts as a diffusion barrier, minimizing CO₂ leakage and further optimizing fixation efficiency [7].

Operating a photobioreactor at large scale and maintaining stable biomass productivity requires regulation of multiple environmental parameters. Successful carbon fixation in microalgae depends on the coordinated optimization, such as: (1) energy capture via light absorption, (2) enzymatic carbon metabolism regulated by temperature and pH, and (3) the continuous supply of substrates such as CO₂ and essential nutrients. Light provides the primary energy source for photosynthesis. Microalgal growth increases with light intensity up to a saturation threshold, beyond which photoinhibition occurs [18]. Optimal light intensities typically range between 26 and 400 μmol photons m⁻²s⁻¹, depending on species and reactor configuration. Temperature and pH regulate the activity of carbon fixation enzymes. Most microalgae achieve optimal growth between 20 - 35 °C [26], while deviations impair carbon assimilation. Similarly, maintaining culture pH between 6 and 8 is critical to sustain DIC availability and enzymatic stability [27,28].

Nutrient supply - especially of carbon, nitrogen, and phosphorus—supports biosynthesis and biomass accumulation. Although microalgae primarily fix inorganic carbon through photosynthesis, optimizing overall productivity, particularly at industrial scales, often benefits from a flexible approach where supplemental organic carbon sources can be utilized. Microalgae’s ability to grow autotrophically, heterotrophically, or mixotrophically enables strategic modulation of carbon inputs to enhance biomass yields depending on process goals [29]. Finally, CO₂ concentration directly impacts substrate availability for the Calvin cycle. Insufficient CO₂ limits growth, whereas excess CO₂ can acidify the medium, disrupting cellular metabolism [30]. Careful management of CO₂ levels is therefore essential for stable cultivation. In practice, environmental parameters often fluctuate, particularly in large-scale or outdoor systems. Moreover, the regulation of these operational parameters is closely linked to reactor energy demand, input costs, and ultimately affects the overall carbon footprint and economic feasibility of microalgae-based carbon capture technologies. Therefore, robust microalgal strains capable of tolerating environmental variations, combined with efficient reactor operation strategies, are essential to achieve sustainable and scalable cultivation systems.

Reactor design in microalgae-based carbon capture system

In microalgae-based carbon capture systems, the design of the cultivation platform significantly affects operational efficiency, scalability, and cost-effectibeness. Cultivation systems are generally classified into open ponds and photobioreactors (PBRs) (Figure 2), each offering distinct advantages and challenges [31,32]. Open pond systems are the oldest and most widely used method for large-scale microalgae cultivation. They valued for their low construction and operational costs and use of simple infrastructure [33]. However, open systems require large land areas and are highly susceptible to environmental fluctuations and contamination, which can affect cultivation stability. Bicarbonate-based supplementation particularly well suited for open ponds systems, as it provides more stable pH conditions, minimizes CO₂ volatilization losses, and avoids the direct flue gas release, thus improving both safety and carbon retention efficiency [31]. In contrast, photobioreactors (PBRs) are enclosed cultivation systems that provide precise control of environmental parameters such as light, temperature, nutrient levels, and pH. This controlled environment improves gas-liqued mass transfer efficiency and minimizes carbon loss, makin PBRs compatible with both direct CO₂ supplementation and bicarbonate-based systems. PBRs also reduce the risk of contamination and can operate under wider range of climatic conditions. However, PBRs require higher capital and operating costs and may pose scalability challenges for large-volume production [31,32].

Figure 2 Several types of Open Pond ((1.a); (1.b); (1.c)) and PBR ((2.a); (2.b); (2.c); (2.d)) cultivation system [32].

CO₂-based cultivation system

In CO₂-based cultivation systems, carbon directly supplied as CO₂ gas to the microalgae culture (Figure 3). Sources of CO₂ include atmospheric air, purified CO₂, or industrial flue gas. While atmospheric air is inexpensive and readily available, its low CO₂ concentration results in poor biomass productivity and carbon fixation efficiency, making it impractical for large-scale applications. In energy sector application, flue gas from fossil fuel combustion serves as the primary CO₂ source, offering higher concentrations and better alignment with emission mitigation goals. However, supplying CO₂ gas presents operational challenges. Due to its limited solubility in water, CO₂ has low mass transfer rates, and a significant portion of the supplied CO₂ escapes before being fixed by the microalgae. This reduces carbon utilization efficiency and increases operational costs [34]. Furthermore, excessive CO₂ bubbling can lower the pH of the culture medium, creating suboptimal conditions for microalgal growth. To optimize CO₂ uptake while minimizing losses and pH instability, reactor design must carefully balance gas-liquid contact efficiency with operational simplicity. Despite these limitations, CO₂-based systems remain attractive for large-scale deployment where inexpensive and concentrated CO₂ sources, such as industrial flue gas, are readily available.

Figure 3 The schematic of the CO₂-based microalgae cultivation system in carbon capture technology [35].

Bicarbonate-based cultivation system

To address the poor mass transfer efficiency of gaseous CO₂ into aqueous culture media, bicarbonate ions (HCO₃⁻) have been proposed as an alternative carbon source, offering greater solubility and stability in solution [36]. One notable implementation of this approach is the Bicarbonate-based Integrated Carbon Capture and Algae Production System (BICCAPS), where CO₂ from flue gas is absorbed by an alkaline solution to produce a bicarbonate-rich medium for microalgae cultivation (Figure 4) [37]. BICCAPS offers operational advantages by eliminating the need for continuous CO₂ bubbling, allowing for simpler and more cost-effective photobioreactor designs. However, the economic feasibility of this system strongly depends on the efficient recycling of the cultivation medium and the alkaline absorbent. Sodium hydroxide (NaOH) is often proposed as an effective CO₂ absorbent due to its high reactivity and widespread availability. Theoretically, capturing 1 ton of CO₂ requires approximately 0.9 tons of NaOH, which is more efficient than the 1.39 tons needed when using conventional monoethanolamine (MEA) solvents [37]. Moreover, NaOH is relatively inexpensive and already used at industrial scales, making it an attractive option for large-scale integration. While bicarbonate-based systems show promising potential for improving carbon fixation efficiency and operational manageability, future development must address the challenges of chemical cost management and system sustainability to enable broader deployment.

Figure 4 General process flow diagram of BICCAPS [38].

Several types of microalgae species for BioCCU

Various microalgae species have been explored for their potential in carbon capture and biomass production, each with different physiological characteristics influencing their compatibility with CO₂ or bicarbonate-based cultivation systems. Chlorella vulgaris is one of the most widely studied strains, demonstrating robust growth and carbon fixation capabilities under both CO₂ and bicarbonate conditions [10]. Spirulina platensis, known for its tolerance to high pH environments, is particularly well-suited to bicarbonate-based systems. However, CO₂ bubbling can lower the pH of the growth medium, potentially inhibiting or damaging Spirulina cells if pH levels drop too far [39]. Dunaliella salina is known for its halotolerance and flexibility in carbon source utilization, making it compatible with both strategies. The capacity of Dunaliella salina to thrive in extremely high CO₂ concentrations (up to 30%) is another benefit. This is because it can reduce Radical Oxygen Species (ROS) by producing antioxidants (carotenoids) [40]. Euglena gracilis is another promising candidate for carbon capture agent with their benefit to be able for growth under very acidic conditions, avoiding contamination from other organisms [41].

While some species such as Chlorella vulgaris and Dunaliella salina are suitable for both CO₂-based and bicarbonate-based systems, the choice of species is not universally interchangeable between the 2. Differences in environmental conditions - such as pH stability, carbon availability, and CO₂ absorption dynamics - make certain species more favorable for one system over the other. For example, Spirulina performs best in bicarbonate-rich media due to its alkaliphilic nature, whereas species like Euglena gracilis have shown greater productivity in CO₂-enriched environments [39,41]. Therefore, selecting an appropriate species should align with the specific characteristics of the cultivation system. Ultimately, the selection of microalgal strains should also reflect system priorities, including the type of carbon source, cultivation mode, biomass productivity goals, and intended product valorization - especially in the context of industrial-scale bio-based carbon capture and utilization (Table 2).

Table 2 Types of microalgae that are commonly utilized in carbon capture technologies.

Technical review

Key technical parameters such as biomass productivity, carbon fixation rate, carbon fixation efficiency, and net energy ratio must be thoroughly evaluated and optimized to ensure the economic viability of microalgae-based carbon capture technologies [43-45]. The design of the cultivation system and its supporting infrastructure is inherently linked to these critical factors.

This study focuses on comparing the effects of CO₂-based and bicarbonate-based cultivation methods on these parameters. Operational simulations were conducted based on a defined set of assumptions (Table 2), derived from previous studies with cultivation conditions similar to those used in this work. These simulations specifically focus on the carbon supply system, while other supporting subsystems assumed to operate identically in both scenarios (Figure 5).

The simulations involved calculating the total gas flow requirements in each cultivation scenario to determine the power requirements of each equipment. Once power requirement of each equipment was identified, the estimated energy consumption of the carbon supply system was calculated using the following formula:

where E is energy consumption in kWh/day, P is power requirement in kW and t is operating hour of the equipment (hours/day). The power requirements for the equipment are estimated using the following formula:

where P is the power requirement (kW) for carbon supply equipment, Q is blower/pump flowrate (Nm³/h for blower and m³/h for pump), ΔP is the operating pressure (kPa), ρ is the density of fluid (kg/m³), g is gravitational constant (m/s²), H is head of the pump (meter) and η is blower/pump efficiency (no unit). The efficiency of all equipment in this simulation is assumed to be 70%.

Based on the simulation results, the CO₂-based cultivation system requires power 3,571 W of ring blower to deliver flue gas from the power plant to the PBR (Figure 5(A)). In contrast, the bicarbonate-based system requires a blower, a NaOH circulation pump for the CO₂ absorption system, ring blower for mixing, and a bicarbonate supply pump for delivering bicarbonate ions to the PBR, with respective power capacity of 56, 7.04, 3,571 and 0.04 W (Figure 5(B)).

Using the assumed inlet gas data and the ideal gas law, the mass flow rate of supplied CO₂ was calculated. Meanwhile, the carbon fixation rate for each cultivation method was estimated using the algal biomass characteristics and the following equation [9]:

where M_CO2 is the molar mass of CO₂ (g/mole), M_C is the molar mass of carbon (g/mole), F_C is the carbon content in the algal biomass (%), BP is the biomass productivity (g/L/day), and FR_CO2 is the fixation rate of CO₂ (g/L/day).

The simulation outputs include values for carbon fixation rate, carbon fixation efficiency, estimated energy consumption, and net energy ratio for both cultivation scenarios. These results are discussed in the following sections.

Table 3 List of assumptions used in operational simulations.

Figure 5 Process flow diagram and the boundary (dotted red lines) of the simulations of CO₂-based (A) and Bicarbonate-based (B) microalgae cultivation system for carbon capture technology.

Biomass productivity & carbon fixation rate

Microalgae biomass productivity (BP) refers to the increase in algal biomass concentration over time and is calculated using the following equation [55]:

where BP is biomass productivity (mg/L/day), X_t is the biomass concentration (mg/L) at the end of the cultivation period, X₀ is the initial biomass concentration (mg/L), and Δt is the duration of cultivation (days).

BP varies across the different growth phases of microalgae, typically reaching its peak during the exponential phase and declining thereafter [52]. Therefore, it is advisable to calculate BP using data from the start of cultivation through the end of the exponential growth phase [48].

BP influenced by both reactor design and operational parameters, such as CO₂ concentration (in gas bubbling), pH, temperature, nutrient availability, illumination, and culture mixing. Reactor design is especially critical, as it affects light penetration (light path length), mixing efficiency (which affects the distribution of nutrient, light, and CO₂), and the ability to control environmental conditions such as temperature and pH—factors essential for maintaining optimal growth conditions [56,57].

The carbon fixation rate (FR_CO2) can be derived from BP using the carbon content fraction (F_C) of algal biomass, as shown in Eq. (6) [9]. A positive linear relationship exists between BP and FR_CO2: as BP increases, so does the carbon fixation rate. For example, if the carbon content in algal biomass is 50.39% by weight (dry basis), then a BP of 1 g of dry biomass per day corresponds to the carbon fixation rate of approximately 1.83 g of CO₂ per day.

In the context of carbon capture technology, this positive correlation offers dual benefits. First, a higher BP leads to increased biomass yield, which—if the biomass is of high quality- can enhance the economic viability of the technology. Second, a higher FR_CO2 results in greater CO₂ capture, thereby contributing to a larger reduction in greenhouse gas (GHG) emissions. These environmental benefits can be monetized if the emissions reductions are registered within a carbon credit program.

Many studies show CO₂-based generally yield higher BP and maximum algal cell concentrations than bicarbonate-based cultivation systems. Lam and Lee [10] found that cultivating Chlorella vulgaris with 5% CO₂ bubbling yielded a BP value of 0.072 g/L/day, but bicarbonate-based culture yielded only 0.014 g/L/day. Other example, Do et al. [9] reported a maximum BP of 0.639 g/L/day and a peak cell concentration of approximately 3.4 g/L in a CO₂-based system using Scenedesmus acuminatus TH04, compared to just 0.186 g/L/day under bicarbonate-based conditions. Several research, however, reveal the opposite results, such as the study by Zhu et al. [11], which found that the BP of bicarbonate-based cultivation of Trebouxiophyte microalgae was 0.8 g/L/day, which is greater than CO₂-based cultivation with 2% CO₂ concentration at 0.61 g/L/day. Similarly, the results of our experiment demonstrate that the BP value of the bicarbonate-based system was 0.053 g/L/day, slightly higher than that of the CO₂-based system at 0.049 g/L/day. This disparity is primarily attributed to differences in system optimization. In our experimental setup, the bicarbonate-based system was specifically optimized to enhance productivity, while the CO₂-based system was maintained under more generic conditions without targeted optimization. This highlights the importance of system-specific parameter tuning, as performance outcomes may significantly vary depending on operational conditions and cultivation strategies.

Higher cell concentrations in CO₂-based systems also benefit downstream processes by reducing harvesting volumes, thus improving operational efficiency. Although it can produce algal cultures with high cell concentrations, the CO₂-based cultivation method takes more energy for carbon supply system. This is due to the very poor carbon fixation efficiency value, which will be detailed further in the next section. However, the current study focuses specifically on the technical and economic comparison of carbon supply systems and does not delve into other subsystems or downstream processes.

Based on simulation results using previously established assumptions, the CO₂-based system consumed 3.57 kWh/day to operate the ring blower for carbon supply system, resulting in a dry biomass yield of 0.49 kg/day. In comparison, the bicarbonate-based system required 0.67 kWh/day for carbon supply system—via both a ring blower and a bicarbonate pump - producing 0.53 kg/day of dry biomass. When normalized to biomass output, the energy requirements were 7.29 kWh/kg dry biomass (DBM) for the CO₂-based system and 1.27 kWh/kg DBM for the bicarbonate-based system. The lower energy demand per unit biomass in the bicarbonate-based system indicates superior energy efficiency, which is a critical factor in assessing the feasibility of carbon capture applications. The comparison of BP, FR_CO2 and maximum algae cell concentration of CO₂-based and bicarbonate-based cultivation system is shown in Table 4.

Table 4 Comparison of BP value between CO₂-based and bicarbonate-based system.

*Based on the assumption that the carbon content of microalgae biomass is 50.39 %w/w [54], the calculation was performed using Eq. (6).

Carbon fixation efficiency

Carbon fixation efficiency, often referred to as CO₂ fixation efficiency, indicates how effectively microalgae convert inorganic carbon (typically CO₂ or bicarbonate) into organic compounds through photosynthesis. It calculated by comparing the amount of carbon fixed in biomass to the total carbon supplied, as shown in the following equation [9]:

where E_CO2 is the CO₂ fixation efficiency (%), F_CO2 is the amount of CO₂ fixed during the entire cultivation period (g), and m_CO2 is the amount of CO₂ supplied to the system over the same period (g).

Carbon fixation efficiency influenced by both internal and external factors. Internal factors are species-specific and relate to the inherent efficiency of the photosynthetic machinery and carbon assimilation pathways of a given microalgal strain. External factors include cultivation conditions such as CO₂ concentration, light intensity and photoperiod, temperature, pH, and nutrient availability [18,26,30].

Improving fixation efficiency from an internal standpoint involves selecting microalgal strains with high carbon assimilation capacity, rapid growth rates, and resilience to environmental fluctuations. Techniques such as random mutagenesis, adaptive laboratory evolution (ALE), and genetic engineering are widely used to enhance specific metabolic traits and stress tolerance, in addition to selecting naturally superior strains [18].

Externally, optimization of cultivation parameters is key to improve fixation efficiency. Suboptimal conditions reduce carbon assimilation and lower biomass productivity, which in turn diminishes biomass yield per unit volume or area. This increases operational costs due to the need for more energy input - e.g., prolonged lighting, extended cultivation periods, or higher CO₂ dosing rates to compensate for inefficiencies. Consequently, low carbon fixation efficiency negatively affects both technical and economic performance, ultimately reducing return on investment (ROI) and limiting commercial feasibility.

In CO₂-based systems, large volumes and concentrations of CO₂ gas typically required, especially in photobioreactors (PBRs) and open pond systems. However, due to limited solubility of CO₂ in water and the low carbon utilization rate of microalgae, only a small fraction of the supplied CO₂ actually fixed and most of it lost to the atmosphere. This evidenced by the typically low fixation efficiencies reported in CO₂-based systems, often around 10% or even below (Table 5) [9,11]. Additionally, increasing the CO₂ concentration (in bubbled gas) can significantly lower the culture pH of medium and potentially inhibiting algal growth. To mitigate this, a pH control system involving alkaline reagents must be implemented, adding to both capital and operational costs.

Despite these limitations, some studies demonstrate that fixation efficiency in CO₂-based systems can be significantly improved through reactor arrangements. For example, arranging PBRs in a cascade or series configuration has been shown to enhance CO₂ utilization. Do et al. [9] achieved a fixation efficiency of 64.8% by connecting ten PBRs in series using Scenedesmus acuminatus. This was further improved to 93.9% by introducing triethylenetetramine (TETA), a CO₂ absorption enhancer, into the cultivation medium. However, this setup increases the complexity of reactor construction and operation. The need for a highly intricate CO₂ distribution system - with multiple interconnected blowers and pipelines - significantly elevates both capital and maintenance costs (Figure 6).

Figure 6 Process flow diagram of a CO₂-based cultivation system using 3 photobioreactors (PBRs) in series. Increasing the number of PBRs in series necessitates a more complex construction design and additional equipment. PBR S1: photobioreactor Series 1, and so on.

Figure 7 Process flow diagram of a CO₂ absorber column to produce bicarbonate ions from power plant’s flue gas.

Alternatively, bicarbonate-based systems offer notable advantages in improving carbon fixation efficiency. Bicarbonate has higher solubility in water compared to gaseous CO₂, resulting in improved carbon availability for algal uptake and more stable pH conditions - favorable for algal growth. This benefit illustrated in Do et al. [9], who reported a carbon fixation efficiency of 100% when cultivating microalgae S. acuminatus with 4.2 g/L sodium bicarbonate (Table 5).

Moreover, bicarbonate-based cultivation can be integrated with a CO₂ absorption system that uses flue gas and an alkaline solution to generate bicarbonate ions. When properly designed, this approach can be more compact, cost-effective, and operationally efficient (Figure 7). Unlike the complex piping and gas distribution systems required for CO₂ bubbling into multiple PBRs, bicarbonate systems involve a more centralized CO₂ absorption unit followed by a simpler bicarbonate solution distribution line for delivering bicarbonate-enriched media to the reactors (PBR).

Table 5 Summary of various study on carbon fixation efficiency: bicarbonate-based vs CO₂-based.

Net energy ratio

The net energy ratio (NER) is a system calculated by dividing the total energy generated (i.e., the energy content of the product - in this case, algal biomass) by the total energy consumed during the production process [45]. This study compares the NER values of CO₂-based and bicarbonate-based systems using the operational simulation data described previously. To focus on the core aspect of our study, only the energy consumption of the carbon supply system is considered in the simulation, as shown in Figure 5. Energy use from other subsystems excluded under the assumption that these are identical in both scenarios. The NER in this study calculated using the following equation [45]:

As shown in Figure 5, the CO₂-based system’s energy consumption includes only the energy required to operate a ring blower that transports flue gas from the power plant to the PBR. In contrast, the bicarbonate-based system accounts for the energy used by the ring blower, the alkali solution circulation pump in the CO₂ absorption system, and the bicarbonate distribution pump, which delivers the bicarbonate solution from the absorber to the PBR. Energy demands for other subsystems - such as harvesting, drying, and nutrient supply - are not included in this analysis.

The NER calculation in this study specifically focuses on the energy consumption of the carbon supply system, excluding energy inputs from downstream processes such as biomass harvesting and drying. These stages are widely recognized as significant contributors to the total energy demand in microalgae production systems. As such, the resulting NER values primarily reflect partial energy efficiency specific to the carbon supply stage. To obtain a more comprehensive assessment of overall system viability, a full life cycle energy balance approach would be necessary - one that includes all production stages, particularly downstream operations. Incorporating these elements in future analyses may yield NER estimates that are more representative of real-world industrial applications.

According to our simulations results, the total energy usage per day in the CO₂-based system is 35.71 kWh, while in the bicarbonate-based system is 4.23 kWh. These values calculated using Eq. (3). While the energy content in algae biomass of each scenario assumed to be the same of 5,303 kcal/Kg or 6.16 kWh/Kg DBM [54]. The total energy generated was calculated by multiplying the energy content of the biomass with BP value. Based on this calculation, the NER value is 0.85 for the CO₂-based system and 7.72 for the bicarbonate-based system, indicating that the bicarbonate-based system is superior to the CO₂-based system in terms of energy efficiency of the carbon supply system.

For a microalgae cultivation project to be considered energetically and economically viable - particularly for energy applications such as biofuel or biomass fuel production - the NER must be greater than one. A higher NER indicates greater energy efficiency and feasibility [45]. However, if the biomass product intended for non-energy applications (e.g., food, nutraceuticals, or chemicals), a low NER may still be acceptable, as the value lies in the product’s functionality rather than its energy content.

The NER strongly influenced by the design and complexity of the carbon capture system. Systems with multiple energy-intensive components typically result in lower NER values. Conversely, systems that produce biomass with high calorific value (e.g., lipid-rich strains) can improve the NER. Additionally, operational efficiency - achieved through reduced energy consumption - can also enhance NER values [45].

A comparative simulation by Jorquera et al. [45] illustrates this concept well. They evaluated the NER of 3 microalgae cultivation systems - raceway ponds, flat-plate PBRs, and tubular PBRs - each designed to produce 100,000 kg of dry biomass (DBM) per year. Their results showed high NER values for raceway ponds (8.34) and flat-plate PBRs (4.51), while the tubular PBR system exhibited a much lower NER of 0.20, indicating it was not viable for net energy production. This demonstrates that the appropriate carbon capture system design has a significant impact on the NER value.

Slade and Bauen [46] reviewed several Life Cycle Assessment (LCA) studies ofn microalgae cultivation projects which covers all of production aspects such as cultivation, harvesting and drying systems. Their findings indicate that 6 out of 8 raceway ponds cultivation project reviewed in their study have shown energy input less than energy generated in biomass content which indicates that these systems might attain a positive energy balance. However, in the same study, most PBR projects have shown energy generated less than energy consumed in all cases of their study which indicates negative energy balance. The top-performing PBR in their study is the flat-plate system, which outperformed tubular PBRs due to its extensive illumination surface area and reduced oxygen accumulation.

From the perspective of NER, and based on the 2 references above, the order of potential project feasibility based on reactor type is: Raceway pond, PBR-Flat plate and, the least feasible, is tubular PBR. However, the use of raceway pond is generally infeasible for carbon capture application in coal-fired power plants due to spare limitations, layout constraints, the complexity gas distribution piping, and safety concerns - particularly the risk of hazardous gases such as SO₂, escaping into the environment and posing risk to workers. Due to this infeasibility of using raceway ponds, the PBR flat plate (flat planar) emerges as the most viable option.

To improve the overall feasibility of the project, alternative strategies to the conventional CO₂-based system are being explored, with particular emphasis on the development of bicarbonate-based system. One of the most well-known technological developments based on the bicarbonate-based system is BICCAPS (The bicarbonate-based integrated carbon capture and algae production system) as explained previously. This process enables a more sustainable and cost-effective means of collecting and supplying the nutrients for cultivating microalgae [38].

Cordoba-Perez and de Lasa [60] evaluated the performance of Chlorella vulgaris grown with NaHCO₃ as the inorganic carbon source. The study reported a maximum carbon conversion efficiency of 29.6%, demonstrating that nearly one-third of the supplied bicarbonate was assimilated into microalgal biomass, reinforcing the potential of bicarbonate-based systems for biofuel and carbon capture applications. These improvements in carbon fixation efficiency will directly enhance the NER value and consequently, improve the overall feasibility of the project. The following table (Table 6) presents a comparison of the NER results obtained from our simulation.

Table 6 The comparison of NER value between CO₂-based and bicarbonate-based.

Technical, economical and environmental comparison

The selection of carbon supplementation strategies - whether through direct CO₂ or bicarbonate-based delivery - plays a pivotal role in determining the overall feasibility and sustainability of microalgae-based carbon capture systems. Each approach presents distinct trade-offs in terms of technical design, energy requirements, operational complexity, and environmental performance [66]. Technically, bicarbonate-based systems provide greater carbon fixation efficiency and better pH stability, as CO₂ converted into bicarbonate via chemical absorption. This allow for more controlled carbon dosing and reduced carbon loss. This system minimizes gas stripping and energy consumption related to continuous aeration, making it more suitable for open pond systems [67]. In contrast, CO₂-based systems are simpler in infrastructure and lower in initial cost, utilizing direct flue gas injection into the photobioreactor. However, these systems often suffer from inefficient CO₂ utilization and pH fluctuations, which can hinder algal productivity. Closed PBR systems are compatible with both approaches but require careful gas management in CO₂-based setups to maintain cultivation stability [66,67].

Economically, these approaches associated with different cost structures. CO₂-based systems typically involve lower upfront capital investment and chemical cost but incur higher long-term operational expenditures (OPEX) due to the energy demands of gas compression, storage, and distribution [67,68]. In contrast, bicarbonate supplementation requires additional investment in absorber units and alkaline reagents such as NaOH. Nevertheless, this system can significantly reduce energy consumption - by as much as 80% - 90% compared to conventional CO₂ systems - due to the minimal power requirements for liquid dosing rather than gas injection. This leads to significantly lower OPEX and addresses a major bottleneck in the economic scalability of algae-based production [69,70]. From an environmental perspective, bicarbonate supplementation associated with lower upstream carbon footprints, particularly when integrated with industrial waste streams or recycling systems. CO₂-based systems, while chemically simpler, tend to exhibit higher carbon intensity in upstream processes due to energy use in gas handling. Moreover, they suffer from high fugitive emissions - over 90% of unabsorbed CO₂ and other gases can be lost during cultivation - whereas bicarbonate systems localize emissions to the absorption stage, allowing for easier monitoring and mitigation. In terms of resource consumption, CO₂ systems may require intermittent alkalinity addition to correct acidification, while bicarbonate systems involve routine chemical input that can be minimized through recycling strategies [66,70]. These comparative insights are summarized in Table 7.

Table 7 Comparison of technical, economical and environmental aspects.

Future perspectives

The transition toward more sustainable and energy-efficient carbon capture strategies necessitates further development and scaling up of bicarbonate-based microalgae cultivation systems. As demonstrated in this review, bicarbonate-based systems outperform conventional CO₂-based systems in terms of carbon fixation efficiency, net energy ratio (NER), and operational stability. Future research should focus on enhancing the integration of CO₂ absorption technologies with microalgal cultivation units, particularly through the optimization of absorber design, reagent regeneration, and bicarbonate dosing control. Genetic and metabolic engineering of microalgal strains to further improve bicarbonate uptake kinetics and stress tolerance could also enhance system performance. Moreover, techno-economic and life cycle assessments (LCA) must be expanded to include the entire production chains - from flue gas conditioning to downstream biomass utilization - to validate long-term feasibility of these systems. Innovative systems such as BICCAPS offer promising solution, and future pilot-scale or industrial demonstrations will be critical to establish commercial viability. Ultimately, advancing bicarbonate-based BioCCU technologies will support the dual goals of mitigating industrial CO₂ emissions and producing valuable bio-based products, contributing to global efforts toward a circular carbon economy and net-zero targets.

Conclusions

Based on technical comparison in this study, the bicarbonate-based system superior to the CO₂-based system in terms of simplicity of construction and operation. This system also has a very high carbon fixation efficiency (can reach 100%), which has an influence on lowering particular operational costs in the carbon supply system, supporting the economic viability of the technology. On the other hand, the CO₂-based system, while having a poor carbon fixation efficiency (below 10%), produces algal biomass with high cell densities, making harvesting process easier and more efficient. Although the carbon fixation efficiency of CO₂-based system can be improved by PBR arrangement, it necessitates a highly extensive PBR structure, making the building and operation processes extremely complex and expensive, rendering it ineffective.

In terms of economic efficiency, the bicarbonate-based system outperforms the CO₂-based system, with a NER value of 7.79 vs 0.85 for the CO₂-based system. It was further backed by a low specific energy consumption of 7.29 kWh/Kg DBM generated for the bicarbonate-based system, as opposed to 0.79 kWh/Kg DBM for the CO₂-based system. Although bicarbonate-based carbon supply systems use less energy than CO₂-based systems, it is crucial to highlight that in this system, the alkali material recycling process must be used to reduce chemical purchase costs.

Acknowledgements

This work was supported by Universitas Padjadjaran, Indonesia, PT. Jawa Power, Indonesia, Japan International Corporation Agency (JICA), Japan and Japan Science and Technology Agency (JST), Japan, Japan in the framework of Science and Technology Research Partnership for Sustainable Development (SATREPS).

Declaration of Generative AI in Scientific Writing AI was used in writing this paper to help streamline the language style because the authors are not native English speakers.

CRediT Author Statement

Yudatomo Tri Nugroho: Conceptualization, Writing – original draft, Editing

Ari Hardianto: Conceptualization, Supervision, Writing – original draft, Validation.

Abu Bakar Muhammad Ibnu Syihab: Conceptualization, Writing – original draft, Visualization, Editing.

Saifa Aprilia Sidquni: Writing – original draft, Visualization,

Ivani Nurjannah: Writing – original draft, Editing.

Lucy Adinisa: Writing – original draft, Editing.

Toto Subroto: Conceptualization, Supervision, Writing – original draft, Validation.

References

1. A Valavanidis. Global warming and climate change. Fossil fuels and anthropogenic activities have warmed the. Scientific Reviews 2022. https://www.researchgate.net/publication/358498680

2. V Foster and D Bedrosyan. Understanding CO₂ emissions from the global energy sector. World Bank, Washington DC, 2014.

3. L Bruhwiler and AM Michalak. Overview of the global carbon cycle. national oceanic and atmospheric administration. Annual Greenhouse Gas Index 2018; 7(15), 1-29.

4. R Sayre. Microalgae: The potential for carbon capture. Bioscience 2010; 60(9), 722-727.

5. B Clément-Larosière, F Lopes, A Gonçalves, B Taidi, M Benedetti, M Minier and D Pareau. Carbon dioxide biofixation by Chlorella vulgaris at different CO₂ concentrations and light intensities. Engineering in Life Sciences 2014; 14(5), 509-519.

6. JCM Pires and AL Gonçalves. Microalgae cultures: Environmental tool and bioenergy. Energies 2022; 5(16), 5809.

7. W Wu, L Tan, H Chang, C Zhang, X Tan, Q Liao, N Zhong, X Zhang, Y Zhang and SH Ho. Advancements on process regulation for microalgae-based carbon neutrality and biodiesel production. Renewable and Sustainable Energy Reviews 2023; 171, 112969.

8. Q Zheng, X Xu, GJO Martin and SE Kentish. Critical review of strategies for CO₂ delivery to large-scale microalgae cultures. Chinese Journal of Chemical Engineering 2018; 26(11), 2219-2228.

9. CVT Do, NTT Nguyen, TD Tran, MHT Pham and TYT Pham. Capability of carbon fixation in bicarbonate-based and carbon dioxide-based systems by Scenedesmus acuminatus TH04. Biochemical Engineering Journal 2021; 166, 107858.

10. MK Lam and KT Lee. Effect of carbon source towards the growth of Chlorella vulgaris for CO₂ bio-mitigation and biodiesel production. International Journal of Greenhouse Gas Control 2013; 14, 169-176.

11. C Zhu, Y Xi, X Zhai, J Wang, F Kong and Z Chi. Pilot outdoor cultivation of an extreme alkalihalophilic Trebouxiophyte in a floating photobioreactor using bicarbonate as carbon source. Journal of Cleaner Production 2021; 283, 124648.

12. KE Achyuthan, JC Harper, RP Manginell and MW Moorman. Volatile metabolites emission by in vivo microalgae - an overlooked opportunity? Metabolites 2017; 7, 39.

13. MF de Souza, E Meers and S Mangini. The potential of microalgae for carbon capture and sequestration. EFB Bioeconomy Journal 2024; 4, 100067.

14. UB Singh and AS Ahluwalia. Microalgae: A promising tool for carbon sequestration. Mitigation and Adaptation Strategies for Global Change 2013; 18(1), 73-95.

15. A Rinanti, E Kardena, DI Astuti and K Dewi. Improvement of carbon dioxide removal through artificial light intensity and temperature by constructed green microalgae consortium in a vertical bubble column photobioreactor. Malaysian Journal of Microbiology 2014; 10, 29-37.

16. MP Johnson. Photosynthesis. Essays in Biochemistry 2016; 60(3), 255-273.

17. BS Yu, S Pyo, J Lee and K Han. Microalgae: A multifaceted catalyst for sustainable solutions in renewable energy, food security, and environmental management. Microbial Cell Factories 2024; 23(1), 308.

18. G Li, W Xiao, T Yang and T Lyu. Optimization and process effect for microalgae carbon dioxide fixation technology applications based on carbon capture: A comprehensive review. C-Journal of Carbon Research 2023; 9(1), 35.

19. PM Doran. Bioprocess engineering principles. Academic Press, New York, 1995.

20. P Pal, A Kioka, S Maurya and RA Doong. Innovative nanobubble technology: Fuelling the future of bioenergy and carbon mitigation. Renewable and Sustainable Energy Reviews 2025; 209, 115118.

21. C Kim, J Kim, S Joo, Y Bu, M Liu, J Cho and G Kim. Efficient CO₂ utilization via a hybrid Na-CO₂ system based on CO₂ dissolution. IScience 2018; 9, 278-285.

22. W Klinthong, YH Yang, CH Huang and CS Tan. A review: Microalgae and their applications in CO₂ capture and renewable energy. Aerosol and Air Quality Research 2015; 15, 712-742.

23. R Prasad, SK Gupta, N Shabnam, CYB Oliveira, AK Nema, FA Ansari and F Bux. Role of microalgae in global CO₂ sequestration: Physiological mechanism, recent development, challenges, and future prospective. Sustainability 2021; 13(23), 13061.

24. D Shimamura, T Ikeuchi, A Matsuda, Y Tsuji, H Fukuzawa, K Mochida and T Yamano. Periplasmic carbonic anhydrase CAH1 contributes to high inorganic carbon affinity in Chlamydomonas reinhardtii. Plant Physiology 2024; 196, 2395-2404.

25. K Mallikarjuna, K Narendra, R Ragalatha and BJ Rao. Elucidation and genetic intervention of CO₂ concentration mechanism in Chlamydomonas reinhardtii for increased plant primary productivity. Journal of Biosciences 2020; 45(1), 115.

26. KH Chowdury, N Nahar and UK Deb. The growth factors involved in microalgae cultivation for biofuel production: A review. Computational Water, Energy, and Environmental Engineering 2020; 9(4), 185-215.

27. R Qiu, S Gao, PA Lopez and KL Ogden. Effects of pH on cell growth, lipid production and CO₂ addition of microalgae Chlorella sorokiniana. Algal Research 2017; 28, 192-199.

28. SV Mohan, MV Rohit, P Chiranjeevi, R Chandra and B Navaneeth. Heterotrophic microalgae cultivation to synergize biodiesel production with waste remediation: Progress and perspectives. Bioresource Technology 2015; 184, 169-178.

29. FG Acién, JM Fernández, JJ Magán and E Molina. Production cost of a real microalgae production plant and strategies to reduce it. Biotechnology Advances 2012; 30(6), 1344-1353.

30. R Pourjamshidian, H Abolghasemi, M Esmaili, HD Amrei, M Parsa and S Rezaei. Carbon dioxide biofixation by Chlorella sp. In a bubble column reactor at different flow rates and CO₂ concentrations. Brazilian Journal of Chemical Engineering 2019; 36(2), 639-645.

31. JS Tan, SY Lee, KW Chew, MK Lam, JW Lim, SH Ho and PL Show. A review on microalgae cultivation and harvesting, and their biomass extraction processing using ionic liquids. Bioengineered 2020; 11(1), 116-129.

32. SA Razzak, K Bahar, KMO Islam, AK Haniffa, MO Faruque, SMZ Hossain and MM Hossain. Microalgae cultivation in photobioreactors: Sustainable solutions for a greener future. Green Chemical Engineering 2024; 5, 418-439.

33. TJ Johnson, S Katuwal, GA Anderson, L Gu, R Zhou and WR Gibbons. Photobioreactor cultivation strategies for microalgae and cyanobacteria. Biotechnology Progress 2018; 34, 811-827.

34. C Zhu, S Chen, Y Ji, U Schwaneberg and Z Chi. Progress toward a bicarbonate-based microalgae production system. Trends in Biotechnology 2022; 40(2), 180-193.

35. YJ Sung, JS Lee, HK Yoon, H Ko and SJ Sim. Outdoor cultivation of microalgae in a coal-fired power plant for conversion of flue gas CO₂ into microalgal direct combustion fuels. Systems Microbiology and Biomanufacturing 2021; 1(1), 90-99.

36. I Umetani, E Janka, M Sposób, CJ Hulatt, S Kleiven and R Bakke. Bicarbonate for microalgae cultivation: A case study in a chlorophyte, Tetradesmus wisconsinensis isolated from a Norwegian lake. Journal of Applied Phycology 2021; 33(3), 1341-1352.

37. C Zhu, R Zhang, L Cheng and Z Chi. A recycling culture of Neochloris oleoabundans in a bicarbonate-based integrated carbon capture and algae production system with harvesting by auto-flocculation. Biotechnology for Biofuels 2018; 11(1), 204.

38. MJD Latagan, D Nagarajan, WM Huang, MDG de Luna, JH Chen, AP Rollon, IS Ng, DJ Lee and JS Chang. Bicarbonate-based microalgal cultivation technologies: Mechanisms, critical strategies, and future perspectives. Chemical Engineering Journal 2024; 502, 157998.

39. B Muthu, A Kamalanathan and S Janarthanan. Effect of pH on Arthrospira platensis production. Alochana Chakra Journal 2020; 9(5), 2297-2235.

40. B Huang, G Qu, Y He, J Zhang, J Fan and T Tang. Study on high-CO₂ tolerant Dunaliella salina and its mechanism via transcriptomic analysis. Frontiers in Bioengineering and Biotechnology 2022; 10, 1086357.

41. SR Chae, EJ Hwang and HS Shin. Single cell protein production of Euglena gracilis and carbon dioxide fixation in an innovative photo-bioreactor. Frontiers in Bioengineering and Biotechnology 2006; 97, 322-329.

42. Spinola MP, Mendes AR, Prates JAM. Chemical composition, bioactivities, and applications of Spirulina (Limnospira platensis) in food, feed, and medicine. Foods 2024; 13(22), 3656.

43. RK Goswami, S Mehariya and P Verma. Advances in microalgae-based carbon sequestration: Current status and future perspectives. Environmental Research 2024; 249, 118397.

44. PA Kusi, D McGee, S Tabraiz and A Ahmed. Bicarbonate concentration influences carbon utilization rates and biochemical profiles of freshwater and marine microalgae. Biotechnology Journal 2024; 19(8), 2400361.

45. O Jorquera, A Kiperstok, EA Sales, M Embiruçu and ML Ghirard. Comparative energy life-cycle analyses of microalgal biomass production in open ponds and photobioreactors. Bioresource Technology 2010; 101(4), 1406-1413.

46. R Slade and A Bauen. Micro-algae cultivation for biofuels: Cost, energy balance, environmental impacts and future prospects. Biomass and Bioenergy 2013; 53, 29-38.

47. G Fekete, S Klátyik, A Sebők, AB Dálnoki, A Takács, M Gulyás, I Czinkota, A Székács, C Gyuricza and L Aleksza. Optimization of a Chlorella vulgaris-based carbon sequestration technique using an alkaline medium of wood biomass ash extract. Water 2024; 16(24), 3696.

48. YK Dasan, MK Lam, S Yusup, JW Lim, PL Show, IS Tan and KT Lee. Cultivation of Chlorella vulgaris using sequential-flow bubble column photobioreactor: A stress-inducing strategy for lipid accumulation and carbon dioxide fixation. Journal of CO₂ Utilization 2020; 41, 101226.

49. J Fu, Y Huang, Q Liao, X Zhu, A Xia, X Zhu and JS Chang. Boosting photo-biochemical conversion and carbon dioxide bio-fixation of Chlorella vulgaris in an optimized photobioreactor with airfoil-shaped deflectors. Bioresource Technology 2021; 337, 125355.

50. KE Dickinson, K Stemmler, T Bermarija, SM Tibbetts, SP MacQuarrie, S Bhatti, C Kozera, SJB O’Leary and PJ McGinn. Photosynthetic conversion of carbon dioxide from cement production to microalgae biomass. Applied Microbiology and Biotechnology 2023; 107, 7375-7390.

51. U Ojaniemi, A Tamminen, J Syrjänen and D Barth. CFD modeling of CO₂ fixation by microalgae cultivated in a lab scale photobioreactor. Bioresource Technology 2025; 415, 131715.

52. K Kumari, S Samantaray, D Sahoo and BC Tripathy. Modulation of photosynthesis, biomass and lipid production by sulfur and CO₂ supplementation in Chlorella vulgaris. Journal of Applied Phycology 2024; 37(1), 135-150.

53. IA Lawal. Analyzing CO₂ capture using sodium hydroxide in a spray column: Operational influences on absorption efficiency and regeneration. Digitala Vetenskapliga Arkivet 2024; 313, 1-69.

54. A Raheem, S Sivasangar, WW Azlina, YHT Yap, MK Danquah and R Harun. Thermogravimetric study of Chlorella vulgaris for syngas production. Algal Research 2015; 12, 52-59.

55. AM Youssef, M Gomaa, AKSH Mohamed and ARA El-Shanawany. Enhancement of biomass productivity and biochemical composition of alkaliphilic microalgae by mixotrophic cultivation using cheese whey for biofuel production. Environmental Science and Pollution Research 2024; 31(30), 42875-4288.

56. M Chen, Y Chen and Q Zhang. A review of energy consumption in the acquisition of bio-feedstock for microalgae biofuel production. Sustainability 2021; 13(16), 8873.

57. P Xu, J Li, J Qian, B Wang, J Liu, R Xu, P Chen and W Zhou. Recent advances in CO₂ fixation by microalgae and its potential contribution to carbon neutrality. Chemosphere 2023; 319, 137987.

58. V Lukyanov, S Gorbunova and A Avsiyan. Biotechnological and economic assessment of the productivity of Chlorella vulgaris IBSS-19 microalgae under different cultivation regimes. Bioresource Technology Reports 2024; 27, 101907.

59. P Ratomski, M Hawrot-Paw and A Koniuszy. Utilisation of CO₂ from sodium bicarbonate to produce chlorella vulgaris biomass in tubular photobioreactors for biofuel purposes. Sustainability 2021; 13(16), 9118.

60. M Cordoba-Perez and H de Lasa. CO₂-derived carbon capture using microalgae and sodium bicarbonate in a photobioCREC unit: Kinetic modeling. Processes 2021; 9(8), 1296.

61. P Zhang, Q Sun, Y Dong and S Lian. Effects of different bicarbonate on spirulina in CO2 absorption and microalgae conversion hybrid system. Frontiers in Bioengineering and Biotechnology 2023; 10, 1119111.

62. WM Huang, YK Leong, JH Chen, MJD Latagan, CY Chen, DJ Lee and JS Chang. Bicarbonate-based cultivation of Chlorella sorokiniana SU-1: Optimizing culture conditions and utilizing bicarbonate from flue gas chemical absorption. Chemical Engineering Journal 2025; 503, 158651.

63. A Ughetti, F Roncaglia, B Anderlini, V D’Eusanio, AL Russo and L Forti. Integrated carbonate-based CO₂ capture-biofixation through Cyanobacteria. Applied Sciences 2023; 13(19), 10779.

64. HPS Alarde, KJC Bartolabac, DP Acut, JFA Cane and JR Magsayo. Development of an Arduino-based photobioreactor to investigate algae growth rate and CO₂ removal efficiency. IAES International Journal of Robotics and Automation 2022; 11(2), 141.

65. G Penloglou, A Pavlou and C Kiparissides. Recent advancements in photo-bioreactors for microalgae cultivation: A brief overview. Processes 2024; 12(6), 1104.

66. X Yu, W Guo, Z Hu, P Li, Z Zhang, J Cheng, C Song and Q Ye. Flue gas CO₂ supply methods for microalgae utilization: A review. Clean Energy Science and Technology 2023; 1(2), 78.

67. Y Yang, S Tang and JP Chen. Carbon capture and utilization by algae with high concentration CO₂ or bicarbonate as carbon source. Science of the Total Environment 2024; 918, 170325.

68. M Hanifzadeh, MH Sarrafzadeh, Z Nabati, O Tavakoli and H Feyzizarnagh. Technical, economic and energy assessment of an alternative strategy for mass production of biomass and lipid from microalgae. Journal of Environmental Chemical Engineering 2018; 6(1), 866-873.

69. H Leflay, J Pandhal and S Brown. Direct measurements of CO₂ capture are essential to assess the technical and economic potential of algal-CCUS. Journal of CO₂ Utilization 2021; 52, 101657.

70. MD Somers and JC Quinn. Sustainability of carbon delivery to an algal biorefinery: A techno-economic and life-cycle assessment. Journal of CO₂ Utilization 2019; 30, 193-204.