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Review

Microalgal Valorization of CO2: A Sustainable Pathway to Biofuels and High-Value Chemicals

by
Shutong Wu
,
Kaiyin Ye
,
Xiaochuan Zheng
and
Lei Zhao
*
State Key Laboratory of Urban-Rural Water Resources & Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China
*
Author to whom correspondence should be addressed.
Fermentation 2025, 11(7), 371; https://doi.org/10.3390/fermentation11070371
Submission received: 19 May 2025 / Revised: 17 June 2025 / Accepted: 20 June 2025 / Published: 27 June 2025
(This article belongs to the Special Issue Algae—The Medium of Bioenergy Conversion: 2nd Edition)
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Abstract

The escalating climate crisis and the imperative to transition from a fossil fuel-dependent economy demand transformative solutions for sustainable energy and carbon management. Biological CO2 capture and utilization (CCU) using microalgae represents a particularly compelling approach, capitalizing on microalgae’s high photosynthetic efficiency and remarkable product versatility. This review critically examines the principles and recent breakthroughs in microalgal CO2 bioconversion, spanning strain selection, advanced photobioreactor (PBR) design, and key factors influencing carbon sequestration efficiency. We explore diverse valorization strategies, including next-generation biofuel production, integrated wastewater bioremediation, and the synthesis of value-added chemicals, underscoring their collective potential for mitigating CO2 emissions and achieving comprehensive resource valorization. Persistent challenges, such as economically viable biomass harvesting, cost-effective scale-up, and enhancing strain robustness, are rigorously examined. Furthermore, we delineate promising future prospects centered on cutting-edge genetic engineering, integrated biorefinery concepts, and synergistic coupling with waste treatment to maximize sustainability. By effectively bridging carbon neutrality with renewable resource production, microalgae-based technologies hold considerable potential to spearhead the circular bioeconomy, accelerate the renewable energy transition, and contribute significantly to achieving global climate objectives.

1. Introduction

The rapid advancement of industrial civilization has driven exponential growth in global energy demand. Currently, approximately 84% of the world’s energy supply still relies on fossil fuels such as coal, oil, and natural gas. According to the statistics in the Global Carbon Budget 2024, the CO2 emissions generated from fossil fuel combustion reached 4.16 billion tons in 2022. Moreover, the atmospheric CO2 concentration climbed to 422.5 ppm, showing an increase of around 52% compared with the industrial level [1]. Against this backdrop, carbon capture, utilization, and storage (CCUS) technology has emerged as a core technological pathway for global carbon neutrality efforts. However, traditional chemical carbon capture and utilization (CCU) methods often require high temperature and pressure conditions and present issues of heavy metal catalyst contamination [2,3], while photo/electrochemical techniques remain constrained by energy conversion efficiency bottlenecks [4,5]. By contrast, biological carbon fixation technologies, represented by microalgae, are attracting increasing attention in the CCU field, owing to their photoautotrophic characteristics and metabolic plasticity [6,7].
Microalgae are a diverse group of prokaryotic or eukaryotic, single-celled or simple multicellular photosynthetic microorganisms. Thriving in nearly every habitat on Earth, from freshwater and marine ecosystems to hypersaline lakes and terrestrial soils, their defining characteristic is the ability to convert light energy and CO2 into organic biomass. These microorganisms are typically microscopic, with cell sizes ranging from 2 to 200 μm. This small size results in a high surface-area-to-volume ratio, which facilitates efficient nutrient uptake and gas exchange with the surrounding environment [8,9,10].
Microalgal carbon fixation systems assimilate CO2 via photosynthesis to generate biomass, with carbon fixation rates reported to be 10–50 times [11] higher than those of terrestrial plants and they are less restricted by land utilization rate [12]. For example, the high CO2 concentrations found in industrial flue gases (10−20%) can be directly utilized as a carbon source for microalgae cultivation. Intracellular metabolic pathways enable the conversion of CO2 into lipids (as biodiesel precursors), polysaccharides (as bioplastic feedstocks), and high-protein feed [13], thereby achieving “negative-carbon production”. Moreover, certain microalgae are capable of coupling wastewater nitrogen and phosphorus removal with CO2 fixation [14,15]. Despite these advantages, the large-scale application of microalgal carbon fixation still faces several bottlenecks, such as suboptimal PBR energy efficiency [16], unstable performance of genetically engineered strains [17], and high biomass harvesting costs [18,19], necessitating interdisciplinary technological breakthroughs.
This review provides a comprehensive overview of the latest advances in microalgae-based CO2 bioconversion and valorization. We systematically explore the synergistic integration of cutting-edge strain engineering, next-generation photobioreactor design, and smart digital process control. Beyond technological innovations, this review critically evaluates recent breakthroughs in biorefinery processes and the diversification of value-added products. A thorough analysis of the prevailing techno-economic bottlenecks is presented, alongside a forward-looking perspective on pathways toward sustainable, large-scale implementation. By bridging fundamental biological mechanisms with applied engineering perspectives, this review delineates the primary technological hurdles and proposes future research trajectories, offering both theoretical grounding and technical guidance for the advancement of a low-carbon circular economy.

2. Photoautotrophic Microalgae-Driven CO2 Fixation

The evolutionary trajectory of industrial biomass feedstocks (as shown in Figure 1) clearly illustrates the technological advancement pathway of microalgae-driven CO2 bioconversion. First-generation biomass relied upon food crops such as sugarcane and corn. While it initially validated the feasibility of photosynthetic carbon fixation, it triggered the ethical controversy of “food versus fuel”. [20]. Second-generation technologies shifted to lignocellulosic biomass and other terrestrial plant residues [21]. While this mitigated the pressure of food competition, it was limited by the recalcitrance of the lignin–cellulose complex, necessitating high-energy pretreatment [22]. Furthermore, carbon conversion efficiency remained below 15%. The third-generation revolutionary breakthrough stemmed from the large-scale application of microalgae and other photosynthetic microorganisms [13]. Their light energy conversion efficiency reached 10–20%, and their annual carbon fixation rate could reach 50–200 tones/hectare [23,24]. The rapidly growing rate far exceeded that of higher plants and microalgae could be cultivated ex situ in saline–alkaline water and industrial wastewater, thereby entirely bypassing arable land constraints [25]. Crucially, high-value components in microalgae biomass can be selectively converted into products like biodiesel (fatty acid methyl ester yield > 90%) [26], biodegradable plastics [27], and natural antioxidants (such as astaxanthin) [28], establishing a carbon-negative emission value chain. Currently, fourth-generation technology is centered on the synergistic integration of genetically engineered microalgae and intelligent PBRs. Utilizing synthetic biology tools such as CRISPR-Cas9, engineered algal strains can enhance photosynthetic carbon fixation efficiency [29].

2.1. Microalgal Species

Species selection for microalgal carbon fixation technology follows three criteria: photosynthetic efficiency, environmental adaptability, and metabolic plasticity. Current mainstream industrial strains encompass four major phyla, Cyanobacteria, Chlorophyta, Haptophyta, and Rhodophyta, exhibiting significant differentiation in phylogenetic characteristics and metabolic advantages (Table 1).
Focusing firstly on the phylum Cyanobacteria, their prokaryotic nature and established gene editing systems make them an ideal chassis for synthetic biology. CRISPR-Cas9 has been successfully used to enhance succinic acid production in Synechococcus elongatus PCC 7942 and free fatty acid production in Synechococcus elongatus UTEX 2973 [30]. The Cpf1 system has also been used for cyanobacteria genome editing. It offers advantages such as lower toxicity, easier colony formation, and the ability to remove editing plasmids without multiple passages. It is suitable for various operations, including gene knockout, single-nucleotide alterations, and direct gene replacement [31].
Representative species within the phylum Chlorophyta include Chlorella vulgaris and Chlamydomonas reinhardtii. Using a Cas9-sgRNA construct targeting the omega-3 fatty acid desaturase (fad3) gene in strain C.FSP-E showed 46% (w/w) higher lipid accumulation [32]. Targeted knockout of the zeaxanthin epoxidase (ZEP) gene in Chlamydomonas strain CC-4349 via the CRISPR/Cas9 RNP system enabled simultaneous lutein and zeaxanthin accumulation, specifically boosting zeaxanthin content by 56-fold and yield by 47-fold [33].
The phyla Haptophyta and Rhodophyta excel in adapting to extreme environments. The CRISPR/Cas9 technology has been successfully used for the genetic modification; through the establishment of a genetic transformation system, characteristics like cold tolerance and pest and disease resistance have been realized, paving the way for large-scale cultivation [34]. The diversity of microalgae from these different phyla provides rich genetic resources and technical route options for CO2-based bioconversion.
Table 1. Current research of microalgae used for CO2 bio-fixation.
Table 1. Current research of microalgae used for CO2 bio-fixation.
PhylaSpeciesCarbon SourceType of ReactorCO2 Fixation RateProducts/EfficiencyReferences
yanophyta
(Cyanobacteria)		Synechococcus elongatus PCC 7942CO2-enriched airPBR/8.9 g L−1 succinic acid[35]
Synechococcus elongatus UTEX 2973Flue gas (3–6% CO2)PBR/75.2 mg L−1 d−1 PHB[36]
Synechococcus elongatus UAM-C/S03Pure CO2PBR674 mg L−1 d−158.1 mg L−1 d−1 PHB[37]
Synechococcus 2973-phaCAB5% CO2PBR/6.9 g m−2 d−1 PHB[38]
Thermosynechococcus sp. CL-1Inorganic carbonFlat panel PBR21.98 mg L−1 h−198.1 mg g−1 phycocyanin[39]
Thermosynechococcus elongatus E5425–15% CO2PBR/Biomass[40]
Thermosynechococcus sp. CL-1Inorganic carbonPBR11.41 mg L−1 h−10.043 mg L−1 h−1 carotenoid[41]
Spirulina platensisFlue gas (99% CO2)Incubator/38.3 g m−2 d−1 biomass[42]
Spirulina. sp.Flue gas (99% CO2)Raceway ponds51.3 g m−2 d−1Biomass[43]
Phormidium alkaliphilumCapture ambient CO2Tubular PBR/5.8 g m−2 d−1 dry weight[44]
ChlorophytaChlorella vulgaris5% CO2Flask/989.4 mg L−1 lipid[45]
Chlorella vulgaris FACHB-3115% CO2PBR/2.1 g L−1 biomass[46]
Chlorella vulgaris FACHB 2410% CO2B-PAMFC605.3 mg L−1 d−1105.9 mg L−1 d−1 lipid[47]
Chlorella sorokiniana TH012% CO2Flat-panel PBR/3.0 mg L−1 d−1 lutein[48]
Chlorella fusca LEB 11115% CO2PBR89.2 mg L−1 d−126.8 mg L−1 carbohydrate[49]
Chlorella mutant PY-ZU115% CO2Tubular PBR/5.5 g L−1 biomass[50]
Immobilized Scenedesmus obliquus5% CO2Incubator/628 mg g−1 protein[51]
Scenedesmus obliquus CPCC051–25% CO2PBR0.44 kg m−3 d−10.36 kg m−3 d−1 biomass[52]
Haematococcus pluvialis15% CO2PBR/Astaxanthin[53]
Chlamydomonas reinhardtii5% CO2PBR/Lutein and lipid[54]
ChrysophytaEmiliania huxleyi CCMP 371Pure CO2PBR/Biomass[55]
Nannochloroposis oculata CCMP525Capture ambient CO2PBR/Lipid[56]
RhodophytaColaconema formosanumInorganic carbonIncubator/6 mg g−1 phycobiliprotein[57]

2.2. Pathways of Photosynthetic Carbon Fixation in Microalgae

The technological advantages of microalgal carbon fixation systems stem from their evolutionarily developed multi-layered carbon assimilation mechanisms (Figure 2). The core pathway is the Calvin cycle, driven by Photosystem II (PSII), where Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) acts as the rate-limiting enzyme, fixing CO2 into 3-phosphoglycerate (3-PGA) powered by ATP and NADPH [58,59]. Macroalgal Rubisco exhibits significantly enhanced carboxylation rates (kcat ≈ 3–5 s−1) and specificity factors, resulting in a reduced CO2/O2 competitive inhibition threshold [60,61]. However, the catalytic efficiency of native Rubisco (≈3.5 × 103 M−1s−1) remains far below the theoretical limit. This has prompted researchers to construct mutants (e.g., the L8S8 variant in Synechococcus PCC7942) through directed evolution techniques, leading to a 2.3-fold increase in carbon fixation flux [62].
Aquatic photosynthetic microorganisms, including microalgae, face significant limitations in CO2 acquisition due to the low diffusion rate of all types of inorganic carbon (Ci) in water. To cope with this challenge, and with fluctuating CO2 concentrations (which can occur naturally or in anthropogenic systems like industrial flue gas [63]), microalgae have evolved a unique CO2 concentrating mechanism (CCM) [64]. This mechanism operates through the synergistic action of plasma membrane HCO3− transporters (e.g., LCIA, NAR1.2) and chloroplast-localized carbonic anhydrase (CAH3), converting extracellular CO2 into HCO3− and concentrating it within the carboxysome, thereby increasing the CO2 concentration at the Rubisco active site [59,65]. Genetic engineering to enhance CCM (e.g., by overexpressing the LciB subunit in Chlamydomonas reinhardtii) can increase microalgal growth rates [66]. Furthermore, some diatoms, such as Phaeodactylum tricornutum, can utilize extracellular carbonic anhydrase (eCA) to convert dissolved HCO3- to CO2, coupled with a proton pumping effect from their cell surface nanostructured silica shells [67,68].
The current research frontier is shifting from optimizing single pathways to global metabolic network regulation. For instance, through CRISPRi-mediated downregulation of PEPC1 combined with overexpression, the biomass, lipid, and lutein productivity of Chlamydomonas reinhardtii was successfully enhanced, with the constructed PGi strain performing best at 35 °C [69], while overexpressing glycerol-3-phosphate acyltransferase (GPAT) in Nannochloropsis oceanica successfully enhanced non-polar lipid content (up to 51%), increased lipid productivity (up to 42%), and boosted PUFA accumulation [70]. Notably, the energy balance between carbon fixation pathways and product synthesis remains a key challenge: When the NADPH requirement of the lipid synthesis pathway exceeds the supply capacity of the photosynthetic system, it triggers oxidative stress [71]. Dynamic flux balance analysis (dFBA) based on genome-scale metabolic models (GSMM) can quantify the impact of different engineering strategies on carbon partitioning, providing theoretical guidance for the rational design of coupled high-carbon fixation and high-product systems [72,73].

2.3. Microalgal Carbon Fixation Systems

Current mainstream microalgal cultivation systems can be broadly classified into three categories: open systems, closed photobioreactors (CPBRs), and integrated wastewater treatment systems, each presenting a unique balance of benefits and drawbacks (Table 2).

2.3.1. Open Cultivation Systems

Open systems, most commonly exemplified by open raceway ponds (ORPs), offer primary advantages of low construction costs and operational simplicity, which makes them suitable for the large-scale production of low-value biomass [85]. However, they face significant technical bottlenecks: (1) insufficient CO2 mass transfer efficiency, leading to injected CO2 escaping to the atmosphere; (2) excessive light path, causing severe light attenuation; (3) a high risk coefficient for microbial contamination [86]; and (4) high water losses due to evaporation and significant land area requirements, posing challenges for sustainability and scalability [85].

2.3.2. Closed Photobioreactors

In contrast, closed systems are designed to overcome many of the limitations of open ponds by isolating the culture from the external environment, which allows for precise parameter control through modular design. For example, helical-tubular PBR is designed with a combination of a large culture volume to surface area ratio and optimized light penetration depth. It incorporates features for effective spatial fresh air and CO2 distribution, leading to enhanced CO2 transfer through the extensive culture–liquid interface. This design also facilitates easy control of temperature and contaminants and allows for automated monitoring of cell concentration. Despite these advantages, the primary bottleneck for CPBRs is their high capital expenditure (CAPEX) and operational expenditure (OPEX), largely driven by high energy consumption for mixing, pumping, and cooling [87]. Furthermore, technical challenges such as biofouling on the reactor surface, which reduces light availability, and the accumulation of photosynthetically produced oxygen to inhibitory levels can severely limit long-term productivity and operational stability [19].

2.3.3. Integrated Wastewater Treatment Systems

A paradigm shift towards a circular bioeconomy has spurred the development of integrated systems that couple microalgal cultivation with wastewater treatment. This approach, often termed phycoremediation, leverages the ability of microalgae to assimilate inorganic nitrogen and phosphorus from municipal, industrial, or agricultural wastewater, thus simultaneously treating the effluent and producing valuable biomass without the need for freshwater or synthetic fertilizers [88]. The dual benefit of pollution mitigation and resource recovery makes this an attractive and sustainable model. Performance data shows that microalgae-based systems can achieve high removal efficiencies for nutrients and COD [89].
However, this integrated approach carries its own set of risks and limitations. Biomass contamination with heavy metals, pathogens, or persistent organic pollutants from the influent severely limits its use in high-value applications like food and feed, often necessitating costly purification [90]. Furthermore, process instability arises from the fluctuating composition and potential toxicity of wastewater, which can inhibit algal growth and compromise treatment efficiency [90,91]. Harvesting complexity remains a major bottleneck, as separating algal cells from large wastewater volumes is energy-intensive and expensive, potentially negating the economic advantages of nutrient recovery [92].
To overcome the bottlenecks of current systems, third-generation “smart” PBRs are achieving technological advancements. Future PBR development will focus on a “biomimetic-digital” integration. On the biomimetic front, 3D-printed PBRs inspired by the fractal structures of leaf veins can simultaneously optimize light distribution and fluid dynamics to increase volumetric productivity [93,94]. On the digital front, virtual PBRs based on digital twin technology can use machine learning to predict the spatio-temporal coupling between light gradients and cell density, enabling the adaptive optimization of culture parameters [95,96]. These innovations aim to overcome many of the aforementioned limitations by enhancing resource efficiency and process stability, thus paving the way for economically viable and sustainable large-scale microalgal cultivation.

3. Factors Influencing Microalgal Carbon Fixation

The efficiency of CO2 fixation by microalgae is not invariant. Instead, it is significantly affected by a complex array of environmental and operational factors. Collectively, these factors determine photosynthetic activity, metabolic rates, and the capacity of microalgal cells to acquire inorganic carbon sources. Therefore, in-depth understanding and effective regulation of these key parameters are crucial for enhancing microalgal carbon fixation performance and accelerating its industrial application.

3.1. Light and Photoperiod

Light serves as the fundamental energy source for microalgal photosynthesis, and its intensity and photoperiod exert a direct and profound impact on microalgal growth and carbon fixation rate. In the light-limiting region, lower light intensity restricts the generation of ATP and NADPH during the photosynthetic light reactions, leading to low carbon fixation efficiency and slow biomass accumulation [97]. As light intensity increases, the photosynthetic rate improves until it reaches the light saturation point. A further increase in light intensity can induce photoinhibition, resulting in a burst of reactive oxygen species (ROS) and damage to photosynthetic membrane structure [98]. Optimal light intensity varies considerably depending on microalgal species and cultivation objectives. For instance, Chlorella sorokiniana shows good growth at 12,000 lx [99], while Isochrysis galbana performs best under 30–50 μmol/(m2·s) [100]. Many microalgae species require higher irradiance to achieve optimal growth rates and valuable biomass accumulation. For example, Desmodesmus sp. achieves high biomass at 600 μmol/(m2·s) [101]. For Chlorella species, biomass and fatty acid profiles show significant differences under varying light intensities. Maximum biomass (2.05 ± 0.1 g L−1) was achieved at 62.5 μmol photons/(m2·s), while the maximum percentage of total saturated fatty acids (SFA) (33.38%) was recorded at 100 μmol photons/(m2·s) [102]. Furthermore, optimal light intensity varies with different applications. For biogas upgrade and biogas effluent nutrient reduction, 450 μmol/(m2·s) is recommended [100], whereas for biogas upgrading applications, 2000 μmol/(m2·s) proves optimal [103]. Therefore, light intensity selection should be optimized based on specific species and target products, balancing growth rate, biomass yield, and energy costs to achieve the desired cultivation outcomes.
Photoperiod, on the other hand, influences carbon flow distribution by regulating the balance between dark reactions and photorespiration [104,105]. Studies have found that Chlorella pyrenoidosa maintained optimal growth with the highest mean biomass of 0.516 ± 0.069 g/L in the combination of a photoperiod of L/D 16 h/8 h and light intensity of 8000 lux, with an increase in CO2 biomass fixation rate of about 2-fold in comparison to the lowest light intensity (2000 lux; long/short cycle 8/12 h). The highest mean biomass was 0.516 ± 0.069 g/liter [106].
In practical high-density microalgae cultivation systems, mutual shading among cells leads to significant light intensity gradients within the reactor. Surface cells may experience photoinhibition, while cells in deeper layers are under light-limiting conditions. Therefore, regulating light conditions requires the integrated consideration of light source selection, managing the self-shading effect through controlling cell density or employing continuous/semi-continuous culture, and achieving dynamic cell circulation in different light intensity regions through mixing or gas agitation. A reasonable light–dark cycle period may be in the millisecond range, as suggested by research [107]. The flashing light effect describes the significant increase in microalgae proliferation rate observed when light alternates between light and dark at high frequency [108]. For example, Botryococcus braunii reached its highest biomass values of 0.22 and 0.39 mg/(mL·d), respectively, and highest relative growth rates of 9.20 ± 2.16 and 9.47 ± 2.67 µg/(mL·d), respectively, when subjected to monochromatic blue and mixed red–green–blue LED illumination [109]. Furthermore, innovations in PBR design (e.g., optimizing light path, surface area to volume ratio, thin layer configuration in flat-panel reactors, or fiber optic illumination systems) can improve light distribution uniformity, enhancing volumetric light energy utilization [86].

3.2. Temperature

Temperature, as a ubiquitous environmental factor, profoundly influences the rates of all biochemical reactions within microalgal cells, including enzymatic reactions in photosynthesis (e.g., catalytic activity of RuBisCO, carbonic anhydrase activity), respiration, nutrient absorption, and cell membrane fluidity, among others.
Each microalgal species possesses its unique optimal growth temperature (Topt) range. Most microalgae utilized for carbon fixation studies are mesophilic species, whose Topt is typically between 25–35 °C [110]. Within this range, the maximum carboxylation rate (Vc, max) and CO2/O2 specificity factor (Sc/o) of Rubisco achieve an optimal balance. For instance, a high-temperature-adapted strain of Chlorella sorokiniana exhibits extremely high growth rates and CO2 tolerance at 35–40 °C, making it suitable for carbon fixation using power plant flue gas [111]. However, psychrophilic algae (e.g., those adapted to polar environments, Topt < 15 °C) [112] and thermophilic algae (e.g., found in hot springs or thermal power plant effluents, Topt > 45 °C) [113] also exist in nature. These specialized algal species provide possibilities for microalgal carbon fixation in different climatic zones or under specific industrial conditions.
Temperature also indirectly affects CO2 solubility [114] and the microalgal CCMs [115]. Generally, increasing temperature decreases gas solubility in water, potentially exacerbating CO2 supply limitations. Furthermore, drastic temperature fluctuations can cause stress to microalgae [116], affecting their physiological homeostasis. Thus, in large-scale outdoor cultivation systems, diurnal temperature differences and seasonal temperature variations are significant challenges for maintaining culture stability and efficiency. Simultaneously, when utilizing high-temperature flue gas for microalgal carbon fixation, the flue gas temperature must be pre-adjusted to a range suitable for microalgae, for example, through heat exchange.
Based on this, in closed PBRs, precise culture temperature is maintained through built-in or external heating/cooling systems [117]. For open systems, considerations include geographic location selection, pond design, the use of covering materials, or utilizing industrial waste heat for heating, realizing cascaded energy utilization. Concurrently, obtaining thermotolerant or cold-tolerant algal strains through molecular breeding and adaptive evolution selection is also an effective approach to enhance system robustness and carbon capture efficiency.

3.3. Gas–Liquid Mass Transfer Efficiency

The transfer of CO2 from the gas to liquid phase is a physical process, and its efficiency depends on the gas–liquid contact area, mass transfer driving force, and mass transfer resistance. Most microalgae transport extracellular dissolved inorganic carbon (DIC) into the cell via CCMs, increasing the CO2 concentration around Rubisco, thereby enhancing Rubisco’s carboxylation efficiency and effectively inhibiting its oxygenase activity [118]. Furthermore, the dissolution of CO2 forms carbonic acid, which, in turn, affects the pH of the culture medium and also influences CO2 dissolution and DIC conversion. Most microalgae prefer to grow under slightly alkaline conditions (pH 7.5–8.5) [119,120], where HCO3− is the dominant DIC species and dissolved CO2 is rapidly converted to HCO3−. This is favorable for maintaining the CO2 concentration gradient and accelerating mass transfer.
Mass transfer efficiency can be optimized through improvements in PBR design and operation. This includes employing methods such as microporous diffusers, jet spargers, static mixers, or efficient stirring devices to generate fine bubbles, thereby increasing the gas–liquid contact area and contact time, enhancing turbulent mixing, and improving the kLa value [121]. Furthermore, the utilization of hollow fiber membrane bubbleless aeration not only achieves high mass transfer efficiency but also minimizes shear stress-induced damage to cells. Additionally, by monitoring the culture medium pH using an online sensor and utilizing the measured pH as a feedback signal for CO2 supply, the CO2 input rate can be adjusted in real-time via an intelligent aeration system, thus enhancing the carbon utilization efficiency of the algal culture.

4. Microalgal Resource Utilization Methods: The Core of Valorization

Integrating the CO2 fixation of microalgae with the value-added utilization of microalgal biomass, thereby establishing a microalgal biorefinery facility, is crucial for enhancing the economic feasibility and sustainability of microalgal CO2 fixation technology [122].
The utilization pathways for microalgal biomass can be primarily categorized as follows.
(1) Whole microalgal biomass utilization. The direct utilization of microalgal biomass as food, feed, and fertilizer is currently one of the most promising and largest-scale applications. For example, Arthrospira platensis and Chlorella vulgaris are representative examples in this area. These two microalgae have an exceptionally high protein content (typically 40–70% of dry weight), abundant vitamins (e.g., vitamins B and vitamin E), minerals (e.g., iron, zinc), and essential fatty acids and pigments. Hence, some dried Spirulina and Chlorella powder are widely used as health food ingredients, dietary supplements, or in tablet form, holding an important position in the global health food market. Moreover, their high protein content also makes them an excellent choice for vegetarians. As feed, microalgal biomass is extensively applied in aquaculture, livestock farming, and pet food. For example, the global annual production of Spirulina has reached several thousand tons, with major large-scale production concentrated in countries such as China, India, the United States, and Thailand [123]. Its long history of use as human food and animal feed, coupled with high market acceptance and relatively mature technology, makes it one of the most commercially successful directions in the current microalgae industry. The scaled-up application of Chlorella is also steadily increasing, particularly in the Asian market [124].
(2) Extraction of high-value bioactive components. Extracting high-value bioactive components from microalgae is another important category of scaled-up applications. Astaxanthin and phycocyanin are two typical examples. Haematococcus pluvialis is the primary microalgal source for producing natural astaxanthin, which accumulates in large quantities under stress conditions (e.g., high light, nitrogen starvation). Several companies worldwide have achieved large-scale cultivation and astaxanthin extraction from Haematococcus pluvialis, such as Cyanotech in the USA and Algatechnologies in Israel, with annual astaxanthin production ranging from several tons to tens of tons, commanding an extremely high market value [125]. Phycocyanin is the principal blue pigment protein in Arthrospira platensis (Spirulina), possessing various bioactivities such as antioxidant and anti-inflammatory properties, and can be used as a natural food coloring and health supplement ingredient. Due to the scarcity of natural blue color, phycocyanin also has a relatively high market value, and the Spirulina industry in China has achieved a certain scale in phycocyanin extraction [126].
(3) Conversion to bioenergy. Although microalgae were once considered to hold great promise for bioenergy production, for example, utilizing lipid-rich microalgae species for biodiesel production or subjecting biomass to anaerobic digestion to produce biomethane, the economic feasibility of directly converting unextracted microalgal biomass into fuel on a large scale is currently difficult compared to using fossil fuels or other biofuels (such as corn ethanol and soy biodiesel). The main challenges lie in the high cost of microalgal biomass production, the high energy consumption for harvesting and dewatering, and the lipid content often failing to reach desired high levels. Consequently, bioenergy production is currently viewed more as a secondary or supplementary route within the microalgal biorefinery process, such as anaerobically digesting the residual biomass after the extraction of high-value components to recover energy [127,128]. However, with technological advancements and cost reductions, microalgal biofuels still hold potential for future development.
(4) Synthesis of microalgae-based biomaterials. The production of PHAs by microalgae has emerged as a research hotspot in the field of biomaterials in recent years. PHAs are a class of natural biodegradable plastics characterized by excellent biocompatibility and biodegradability, and they are widely used in packaging, medical materials, and other applications. Certain cyanobacteria (such as Synechocystis and Synechococcus) and green algae have been reported to synthesize measurable amounts of PHAs. PHAs production using microalgae not only helps mitigate plastic pollution but also contributes to carbon neutrality through CO2 fixation, aligning with the goals of sustainable development [129].
(5) Microalgae-based environmental remediation. In addition, microalgae can also be applied in environmental remediation. Microalgae absorb and utilize large amounts of nitrogen and phosphorus from wastewater for growth, which reduces the nitrogen and phosphorus levels in wastewater. In addition, microalgae simultaneously produce oxygen through photosynthesis, which can provide oxygen for aerobic bacteria in wastewater, promoting the degradation of organic matter [130]. The application of microalgae in wastewater treatment is also increasingly receiving attention and is gradually being scaled up. This approach both purifies water quality and produces biomass, forming a coupled system. Large-scale microalgae pond systems have been used for the secondary or tertiary treatment of municipal or industrial wastewater and are particularly effective in phosphorus removal. For instance, microalgae ponds have been used in some municipalities in California, USA [131,132]. This application provides both environmental services and feedstock for subsequent biomass utilization, becoming a sustainable wastewater treatment and resource recovery technology.
In summary, the value-added utilization pathways for microalgal are diverse, but the most mature and scaled-up applications are currently concentrated in their use as high-nutritional-value food and feed and for the extraction of high-value compounds. These applications benefit from their higher product added value, which can partially compensate for the high cost of microalgal production. Applications such as bioenergy production and PHAs production, while having significant potential, are currently at a relatively small commercial scale and are more in the research and experimental development or pilot stages. In the future, developing integrated microalgal biorefinery models that enable the co-production of multiple products is expected to further enhance the economic feasibility of microalgal CO2 and valorization.

5. Future Outlook: Pioneering the Next Wave of Microalgal Biotechnology

The trajectory of microalgal biotechnology worldwide is defined by a significant paradox: While the field is marked by advanced research and development in many regions, large-scale commercial implementation remains economically constrained. The production cost of microalgal biomass is estimated to range from EUR 290 to EUR 570 per kilogram of dry weight, depending on the cultivation method employed. In addition [133], the costs associated with harvesting and extraction account for up to one-third of the final biomass cost, representing one of the main bottlenecks in the process [134]. The economic feasibility of the microalgae industry depends on multiple factors, including crude oil prices, microalgal biomass productivity, and lipid content. Studies have indicated that the cost of CO2 emission reduction by cultivating microalgae with carbon dioxide is CNY 219 per ton of CO2, with a revenue per ton of CO2 reduced ranging from CNY 36 to 164 and a product gross profit margin between 14% and 43%. These figures suggest that this technology also offers certain economic advantages while simultaneously achieving emission reduction [135]. Accelerated synergistic innovation between fundamental research and engineering technology could pave the way for a more promising development outlook in the application of microalgae in environmental remediation and bioenergy production.

5.1. Strain Improvement and Synthetic Biology Driven Innovation: Engineering ‘Super Algae’

The essence of microalgae’s high carbon fixation efficiency originates from their unique genetic regulation and metabolic networks. Optimizing strain performance has consistently been central to upgrading carbon fixation systems [136]. Traditional screening and mutagenesis breeding can enhance the tolerance and fixation rates of some wild algal strains to CO2 and adverse environmental stresses [137]. In recent years, targeted gene editing (e.g., CRISPR/Cas9, TALEN, etc.) and synthetic biology have emerged as leading drivers in this field [125,126]. By site-specifically enhancing the expression of key carbon-fixing enzymes such as RuBisCO and PEP carboxylase, and reconstructing exogenous carbon fixation modules, CO2 utilization efficiency has been significantly improved [138,139].
Synthetic biology can also enable “designer algal strain” creation; it allows for the effective linkage of the CO2 fixation pathway with the biosynthesis pathways of high-value products, transforming the carbon fixation process from merely an emission reduction activity into a sustainable biomanufacturing factory [126,137]. Furthermore, cultivating “super algae” capable of tolerating industrial waste gas conditions such as flue gas, high heavy metal concentrations, high salinity, and extreme temperature variations has become a practical necessity for achieving the direct coupling of microalgal carbon fixation with large-scale industrial emissions. These strategies are progressively moving from laboratory research towards pilot-scale trials and collaborative industrial R&D, promoting a deeper integration of “strain–process–product”.

5.2. Cultivation System and Engineering Process Collaborative Upgrades: Towards Intelligent and Scalable Bioproduction

Efficient, low-energy, and easily scalable cultivation systems are fundamental to the industrialization of microalgal carbon fixation. In recent years, reactors integrating innovative designs for efficient light guiding, smart mixing, and efficient gas–liquid mass transfer have achieved significant breakthroughs in increasing volumetric carbon fixation rates [95,140]. Furthermore, the introduction of computational biology [141] and digital twin PBRs [142] provides theoretical and practical foundations for scenario-based engineering scale-up and smart operation and maintenance for different climates and carbon sources [143,144].

5.3. Biorefinery and Full Industry Chain Synergy: Maximizing Value and Sustainability

The true realization of ‘cost reduction and efficiency improvement’ for microalgal carbon fixation technology depends on the comprehensive utilization of downstream biomass resources [145]. Currently, removal, dewatering, and cell disruption are high-energy-consuming steps [146,147,148]. Novel technologies such as electroflocculation, bioflocculation, and membrane separation can effectively reduce energy consumption while preserving the quality of active products [149,150]. Multi-step biorefining enables multi-level separation modes for lipids, proteins, carbohydrates, pigments, antioxidants, pharmaceutical components, etc., significantly increasing overall added value [151,152,153].
Breakthroughs in microalgal carbon fixation will stem from interdisciplinary innovation across biology, engineering, information science, environmental science, policy, and other fields. In the future, artificial intelligence and high-throughput omics will be used for targeted screening of super carbon-fixing strains, while next-generation reactors will be deeply integrated with carbon trading and carbon management platforms. Customized solutions for scenarios such as industrial flue gas, agricultural waste, and marine engineering will drive the synergy of multiple microalgal systems, achieving carbon emission reduction and green value addition across different regions and industries.

6. Conclusions

In the context of escalating global climate change and diminishing fossil fuel reserves, microalgae-driven carbon dioxide valorization emerges as a crucial component of green and low-carbon technology systems. It offers a distinct and expansive pathway for future energy structure transformation, the achievement of carbon neutrality goals, and the advancement of sustainable circular economies. Currently, microalgal carbon fixation systems have achieved a critical transition from laboratory to demonstration scale. However, widespread large-scale implementation still faces several practical challenges, including the development and genetic robustness of high-efficiency, low-cost strains; the optimization of reactor systems concerning light energy and gas–liquid mass transfer; the control of energy consumption during biomass harvesting and separation; and comprehensive techno-economic and life cycle assessments. In the future, through the targeted modification of core carbon fixation enzyme systems and metabolic networks using genetic engineering and synthetic biology approaches, combined with deep coupling of CO2 bioconversion and high-value product synthesis, it is anticipated to enable the creation of ‘super carbon capture factories’ tailored for industrial emission scenarios. Concurrently, engineering innovations such as smart PBRs, digital twin modeling, and AI data-driven cultivation, coupled with industrial models deeply integrated with waste valorization, are expected to substantially enhance carbon fixation efficiency, system energy efficiency, and economic viability.
More importantly, the microalgae-centric platform for carbon capture and valorization possesses a highly flexible product portfolio and significant scenario adaptability. This holds potential not only for driving commercial breakthroughs in renewable energy like biofuels but also for achieving synergistic win–win outcomes of carbon reduction and green value addition across diverse sectors such as green chemistry, nutrition and health, and environmental remediation. In the future, sustained efforts in strengthening interdisciplinary fundamental research, process integration, and policy support, coupled with promoting the synergistic development of microalgal carbon cycling platforms integrated with carbon trading, ecological restoration, and green manufacturing, will provide robust support and a technological engine for achieving the goals of low-carbon, efficient, and sustainable development for human society.

Author Contributions

Conceptualization, L.Z.; resources, L.Z.; writing—original draft preparation, S.W.; writing—review and editing, S.W. and X.Z.; supervision, S.W. and K.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Nos. U22A20443); the Heilongjiang Provincial Natural Science Foundation of Excellent Young Scholars (Grant No. YQ2023C031); State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2024TS22).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Fermentation 11 00371 g001
Figure 1. Four generations of biofuel production: from agricultural products to algae. (R&D, research and development).
Figure 1. Four generations of biofuel production: from agricultural products to algae. (R&D, research and development).
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Figure 2. Mechanism of photosynthetic carbon fixation in microalgae. (CAe, extracellular carbonic anhydrase; CAi, internal carbonic anhydrase; RuBP, ribulose-1,5-bisphosphate; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; CBB, Calvin–Benson–Bassham cycle; GAP, glyceraldehyde 3-phosphate; 3-PGA, 3-phosphoglycerate).
Figure 2. Mechanism of photosynthetic carbon fixation in microalgae. (CAe, extracellular carbonic anhydrase; CAi, internal carbonic anhydrase; RuBP, ribulose-1,5-bisphosphate; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; CBB, Calvin–Benson–Bassham cycle; GAP, glyceraldehyde 3-phosphate; 3-PGA, 3-phosphoglycerate).
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Table 2. Comparative analysis of photobioreactor systems.
Table 2. Comparative analysis of photobioreactor systems.
Types of PBRMicroalgal SpeciesProductionStrengthsLimitationsReferences
OpenRaceway pondGraesiella sp. WBG-112.5 g/(m2∙d)Simple technology, low investment, large-scale cultivationLarge footprint, unstable culture conditions, susceptible to contamination[74]
Haematococcus lacustris2.2% DW[75]
Nannochloropsis salina24.5 g/(L∙d)[76]
ClosedTubularSpirulina platensis LEB-50.86 g/LHigh light surface area to volume ratio, easy amplificationDissolved oxygen accumulation, easy to stick to the wall[77]
Neochloris oleoabundans7.4 g/(dw2∙d)[78]
Flat panelChlorella sp.49.79 g/(m2∙d)Large illumination area, simple structure, easy to clean, easy to operateShort optical range, difficult to amplify, some degree of wall attachment[79]
Chlorella sorokinina Pa.910.85 g/(L∙d)[80]
Thermosynechococcus CL–11.61 g/(L∙d)[81]
AirliftChlorella sorokiniana0.83 g/(L∙d)Compact structure, high mass transfer efficiency, no bacterial contamination,
algae growth is fast		High cost of construction, large operation and maintenance costs, complex operation[82]
Chlorella sorokiniana15.93 g/(L∙d)[83]
Haematococcus pluvialis0.56 g/(L∙d)[84]
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Wu, S.; Ye, K.; Zheng, X.; Zhao, L. Microalgal Valorization of CO2: A Sustainable Pathway to Biofuels and High-Value Chemicals. Fermentation 2025, 11, 371. https://doi.org/10.3390/fermentation11070371

AMA Style

Wu S, Ye K, Zheng X, Zhao L. Microalgal Valorization of CO2: A Sustainable Pathway to Biofuels and High-Value Chemicals. Fermentation. 2025; 11(7):371. https://doi.org/10.3390/fermentation11070371

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Wu, Shutong, Kaiyin Ye, Xiaochuan Zheng, and Lei Zhao. 2025. "Microalgal Valorization of CO2: A Sustainable Pathway to Biofuels and High-Value Chemicals" Fermentation 11, no. 7: 371. https://doi.org/10.3390/fermentation11070371

APA Style

Wu, S., Ye, K., Zheng, X., & Zhao, L. (2025). Microalgal Valorization of CO2: A Sustainable Pathway to Biofuels and High-Value Chemicals. Fermentation, 11(7), 371. https://doi.org/10.3390/fermentation11070371

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