Factors Influencing CO2 Biofixation by Microalgae: History
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Subjects: Biotechnology & Applied Microbiology
Contributor:
Chih-Sheng Lin

The production of microalgal biomass is highly influenced by the suitability of microalgae strains, CO2, light, pH, culture system, temperature, and nutrients. The sources of CO2 and nutrients for microalgal cultivation can be flue gas and wastewater, respectively. Therefore, many studies have investigated whether flue gas and wastewater can be integrated with microalgal cultivations, to achieve not only CO2 reduction, but also CO2 reuse for microalgal biomass conversion to produce biofuels. Flue gas and wastewater can also be treated by microalgal cultivations to obtain environmentally friendly and health-friendly effects. In the process of microalgae cultivation, one single factor does not affect the growth of microalgae; it is often the interaction of multiple factors. Therefore, keeping the performance of long-term and stable microalgal cultivation will determine the microalgal growth, especially outdoor cultivation.

  • CO2 biofixation
  • microalgae
  • flue gas
  • wastewater

1. Microalgal Strains

Many studies have indicated highly efficient ways to obtain CO2-tolerant, alkali-tolerant, and/or thermotolerant microalgae with high CO2 fixation efficiency. Microalgal strains could be obtained by screening the environment, by random mutagenesis or by genetic modification (Table 1). Improving the capacity of CO2-tolerant microalgae was good for application in flue gas containing high concentrations of CO2 to reduce the CO2 poisoning effect and increase CO2 fixation productivity [1]. The level of CO2-tolerant microalgae is usually referred to as high, very high, and extremely high, according to ranges of 2–5, 5–20, and 20–100% CO2-tolerant concentrations [2]. As shown in Table 1, these strains not only have the ability to withstand very high CO2 concentrations, but also have better growth performances, to obtain higher CO2 fixation efficiency. Flue gas from steel plants containing approximately 25% CO2, 70–80 ppm nitrogen oxides (NOx) and 80–90 ppm sulfur dioxide (SO2) resulted in up to 90% NOx and SO2, along with 50% CO2 removal efficiency by the cultivation of Chlorella sp. MTF-15 [3][4]. Because CO2 is the main component in boiler flue gas with trace amounts of sulfur oxides (SOx), the resulting biomass after CO2 fixation may be used as an animal additive or feed without the concern of posing biosafety risks [5]. To improve the CO2 fixation efficiency, the screening of alkali-tolerant microalgae has been investigated [6][7][8]. It is known that when the pH of water is above 6.3, dissolved CO2, bicarbonate (HCO3−), and carbonate (CO32−) are the dominant species [9]. Therefore, elevated CO2 dissolution can be utilized in microalgae growth by increasing the pH of the culture medium. An alkali-tolerant Chlorella sp. AT1 was isolated and cultured in alkaline medium (pH = 11) with 10% CO2 aeration [6]. Chlorella sorokiniana SLA-04, which was isolated from alkaline Soap Lake, could adapt to growth in extremely high-pH media (pH > 10) [7][8]. The high biomass productivities of Chlorella sorokiniana SLA-04 were obtained by scavenging CO2 from only the atmosphere at high rates in pH > 10 medium during phototrophic cultivation. Excessive light intensity will cause the internal temperature of the cultivation system to rise, causing the growth of microalgae to be inhibited. Two effective thermotolerant mutants, M18, and M24 of Chlorella pyrenoidosa obtained by mutagen treatment, were capable of surviving at temperatures up to 47 °C, and showed optimal growth at 37 °C [10]. The research on screening specific algae strains in Table 1 is mainly in Taiwan, including the characteristics of CO2, alkali, and thermo-tolerance. However, in subtropical zones, the temperature of microalgal culture broth in PBRs can go up to about 40 °C by irradiation of sunlight [3], showing that the screening of thermotolerant strains is very important. The thermotolerance of Chlorella sp. M4, which was obtained by mutagenesis treatment from Chlorella sp. GD, was capable of overcoming high-temperature inhibition during outdoor culture due to high photosynthetic efficiency and biomass productivity at 40 °C with high-concentration CO2 aeration [11]. Thermotolerant microalgal strains can also be screened from high-temperature zones, such as the effluent of steel-making, power generation plants, and hot springs [12]. Thermotolerant microalgae are excellent candidates for large-scale outdoor cultivation, especially in subtropical and tropical countries [13]. Dual CO2 and thermotolerant Chlorella sp. strains 283 and 359 were isolated from their original strain of Chlorella vulgaris ESP-31 by N-methyl-N-nitro-N-nitrosoguanidine (NTG) mutagenesis [14]. The microalgal strain grew well at 40 °C and had high biomass productivity, 0.73–0.89 g L−1 d−1, for a 4-day culture.
Table 1. Growth performance and CO2 fixation efficiency of microalgal Chlorella with different tolerant characteristics.
Tolerance Characteristics Microalgae Gas Aeration Temp. (°C) Maximum Biomass Conc. (g L−1) Biomass Productivity
(g L−1 d−1)		 CO2 Fixation Efficiency 1 (g L−1 d−1) Country 2 References
High-CO2 tolerant Chlorella sp. MTF-15 Flue gas 4 26 2.52 0.515 0.942 TW [4]
Chlorella sp. AE20 10% CO2 28 3.22 0.293 0.536 CN [15]
20% CO2 3.13 0.285 0.522
30% CO2 3.02 0.275 0.503
Chlorella vulgaris NIOCCV 5% CO2 28 0.674 0.111 0.203 IN [16]
10% CO2 1.58 0.265 0.485
20% CO2 0.976 0.163 0.298
High-CO2 and CH4 tolerant Chlorella sp. MB-9 20% CO2 and 80% CH4 26 2.35 0.243 0.445 TW [17]
CO2 tolerant Chlorella sp. GD Boiler flue gas 3 26 6.54 0.892 1.632 TW [5]
High-CO2 tolerant Chlorella sp. LAMB 31 40% CO2 26 ~0.9 0.079 0.144 CN [18]
High-CO2 and thermotolerant Chlorella vulgaris ESP-31, 283 and 359 Simulated flue gas (25% CO2, 80–90 ppm SO2, 90–100 ppm NO) 40 1.91 (283)/1.99 (359) 0.73 (283)/0.89 (359) 1.336 (283)/1.629 (359) TW [14]
Alkali-tolerant
(pH 6–10)		 Chlorella sp. AT1 10% CO2 26 5.08 1.010 1.848 TW [19]
Alkali-tolerant (pH > 10) Chlorella sorokiniana
SLA-04		 Air 20 0.9 0.059 0.108 US [7]
Air 20–25 0.74 0.046 0.078 [8]
Thermotolerant Chlorella pyrenoidosa M18 Air 37 4.65 0.931 1.702 IN [10]
Chlorella pyrenoidosa M24 4.11 0.822 1.504
Chlorella sp. M4 6% CO2 40 4.2 1.05 1.922 TW [11]
Chlorella pyrenoidosa M18 Air 45 1.69 0.338 0.619 IN [13]
Chlorella sorokiniana 10% CO2 37 1.16 0.232 0.425 IN [12]
15% CO2 1.05 0.211 0.384
5% CO2 and 80 ppm NO 1.27 0.254 0.465
1 CO2 fixation efficiency (g L−1 d−1) was calculated by 1.83-fold of biomass productivity. 2 Country abbreviation: Taiwan (TW), China (CN), and India (IN). 3,4 Concentration of CO2 in the flue gas and boiler gas was 25% and 8%, respectively.

2. CO2 from Flue Gas

Flue gas is the main source of CO2 emissions on Earth. CO2 in flue gas has been used as a carbon source for microalgae cultivation in most studies (Table 2). However, CO2 fixation by microalgae still has many problems that need to be overcome. For example, high-CO2 tolerance in microalgae is insufficient and directly discharges flue gas into microalgal culture ponds, which might lead to rapid changes in the pH of the culture broth [4][20][21]. When microalgae cannot adapt to extreme culture conditions, death of the microalgae will occur. Therefore, it is necessary to screen microalgae for pH tolerance. In general, the main component of flue gas is CO2, which presents a variety of CO2 concentrations, depending on the fuel source and the design of the plant. Chlorella sp. MTF-15 was cultured with flue gas aeration from a hot stove (26% CO2), coke oven (25% CO2), or power plant (24% CO2) at the China Steel Corporation, the largest steel plant in Taiwan. The biomass productivity of the microalgae cultured with flue gases from coke ovens, hot stoves, and power plants was 0.515, 0.314, and 0.342 g L−1 d−1, respectively [4]. Chlorella sp. was cultured in medium, with a controlled pH of 6, by aerating with synthetic flue gas (30% CO2) obtained from the African Oxygen Company in South Africa, and the maximum biomass concentration and biomass productivity were 3.42 g L−1 and 0.145 g L−1 d−1, respectively [22]. When Chlorella sorokiniana was aerated with flue gas (16% CO2) from the oil-producing industry of India, the maximum CO2 sequestration was 3.07 g L−1 [23]. The maximum biomass concentration and biomass productivity of Chlorella sp. KR-1 aerated with flue gas from a coal-burning power plant in Korea were 2.81 g L−1 and 0.561 g L−1 d−1, respectively, and the CO2 removal efficiency was approximately 13% [24]. The maximum specific growth rate and biomass concentration of Chlorella fusca LEB111 aerated flue gas (10% CO2) from coal power plants in Brazil were 0.181 d−1 and 1.24 g L−1, respectively [25]. The efficient biomitigation of CO2 (12–15%), NOx (0.01–0.08%), and SOx (0.006–0.06%) of flue gas from a power plant was obtained by the cultivation of Chlorella vulgaris [26][27]. The biomass concentration and amounts of CO2 sequestration of Chlorella sp. aerated with flue gas produced from the burning of coal were 1.92 g L−1 and 0.974 g L−1, respectively [28]. When integrated with sewage and flue gas in microalgal cultivation, the biomass concentration and CO2 removal efficiency of Chlorella vulgaris aerated with a coal-burning boiler (6% CO2) in India were 1.72 g L−1 and 90%, respectively [29]. A microalga Chlorella sp. Cv could tolerate the full-simulated flue gas, 10% CO2 + 200 ppm NOx + 100 ppm SOx. Under optimal conditions, the microalga could tolerate the simulated flue gas, and the maximum specific growth rate was 0.9824 d−1 [30]. It was proposed that the upregulation of several genes related to photosynthesis, oxidative phosphorylation, CO2 fixation, sulfur metabolism, and nitrogen metabolism was beneficial for the evolved microalga strain to tolerate the simulated flue gas [21]. Countries with high dependence on coal, such as China and India, are also actively engaged in CO2 carbon reduction research, using CO2 from the exhaust gas in microalgal cultivation to achieve carbon reduction, and use the produced microalgae biomass as a feedstock of biofuels. It has the opportunity to achieve economic and environmental sustainability by integrating the CO2 reutilization of exhaust gas and the effective development of biofuels.
Table 2. Growth, CO2 fixation efficiency, and lipid productivity of the microalgae Chlorella cultures using flue gas.
Microalgae Flue Gas Source CO2 (%) Biomass Productivity (g L−1 d−1) CO2 Fixation Efficiency 1 (g L−1 d−1) Lipid (%) Lipid Productivity 2 (g L−1 d−1) Country 3 References
Chlorella sp. MTF-15 Coke oven 13 0.528 0.966 21.5 0.614 TW [4]
25 0.515 0.942 26.4 0.666
Hot stove 13 0.449 0.822 33.8 0.866
26 0.314 0.575 35.2 0.591
Power plant 12 0.423 0.774 36.3 0.792
24 0.342 0.626 41.6 0.633
Chlorella sorokiniana Industrial flue gas 16 0.231 0.423 21.1 0.049 IN [23]
Chlorella sp. KR-1 Coal-fired flue gas 13 0.561 1.027 29.9 0.168 KR [24]
Chlorella sp. Coal burning 5 0.273 0.500 8.69 0.024 IN [28]
Chlorella fusca LEB 111 Coal power plant 10 0.111 0.203 15.5 0.017 BR [25]
Chlorella vulgaris Coal burning boiler 6 0.312 0.571 23.2 0.074 IN [29]
Chlorella sp. GD Boiler flue gas 8 1.296 2.372 21.7 0.214 TW [5]
Chlorella sp. Flue gas 30 0.145 0.265 24.7 0.036 ZA [22]
Chlorella sp. Cv Simulated flue gas 15 0.53 0.969 ND ND CN [21]
Chlorella vulgaris Power plant 12 0.502 0.919 40.1 0.201 ES [26]
Chlorella sp. C2 Power plant 3 0.314 0.575 31.5 0.099 CN [31]
1 CO2 fixation efficiency (g L−1 d−1) was calculated by 1.83-fold of biomass productivity. 2 Lipid productivity (g L−1 d−1) = (biomass productivity × lipid content)/100. 3 Country abbreviation: Taiwan (TW), India (IN), Korea (KR), Brazil (BR), South Africa (ZA), China (CN), Spain (ES).

3. Nutrients from Wastewater

In 2015, the United Nations World Water Development Report noted that the available freshwater resources globally will decrease by 40% by 2030. However, more than 80% of the world’s wastewater is discharged into the environment without treatment. The management model for wastewater should be changed from “treatment and disposal” to “reuse, recycle, and resource recovery”. Therefore, the use of wastewater for microalgae cultivation is a technological development trend [32][33][34]. The source of wastewater can be mainly divided into three categories: agricultural, municipal wastewater, and industrial wastewater. As illustrated in Table 3, the growth performance and biomass productivity of microalgae cultured in different types of wastewater were different because the contents of COD, total nitrogen (TN), total phosphorus (TP), and specific inorganic substances in wastewater were obviously different [35][36][5][37].

3.1. Agriculture Wastewater

The main source of agricultural wastewater was large livestock and poultry operations, and the main components in this wastewater were ammonium and organic nitrogen, which are good for microalgal growth. Piggery wastewater is commonly used in microalgal cultivation because this wastewater is rich in nutrient sources [38][39][40][41][42]. Additionally, aquaculture is a fast-growing industry because it has significantly increased the global demand for fish and seafood. Novel aquaculture systems incorporating wastewater treatment and effluent reuse have been rapidly developed for compliant wastewater discharge. Although the nutrient content of aquaculture wastewater is significantly lower than that of piggery wastewater, the content of pathogenic microorganisms and heavy metals contained in aquaculture wastewater is relatively low [43][44]. Therefore, aquaculture wastewater can be used as a large amount of water needed for microalgal cultivation, and the resulting microalgae biomass can be applied not only to a feedstock of biofuels, but also to animal additives or feed, which is a more minimal biosafety issue [5]. In Taiwan, most livestock wastewater is produced from pig farming. Therefore, it can be seen that the state has actively invested in research on the treatment of piggery wastewater. The raw piggery wastewater without pre-treatment could also be applied in microalgal cultivation. The produced microalgal biomass has about 20% lipids and is suitable for use as a feedstock of biodiesel [35][5][39][45].

3.2. Municipal Wastewater

At present, a large amount of municipal wastewater is being produced due to an increase in urban population growth. The composition of municipal wastewater varies greatly because of the substances from various families, businesses, and institutions. For example, the COD and TN in a municipal sludge digestate were 2175 mg L−1 and 840 mg L−1, and 164 mg L−1 and 43.2 mg L−1 [46], in municipalities with reserve osmosis concentrate [47], respectively. Generally, the COD, TN, and TP utilization efficiencies of municipal wastewater in microalgal Chlorella cultivation were approximately 85–100%, 80–100%, and 90–100%, respectively (Table 3). However, growth and biomass productivity are low because municipal wastewater lacks nutrients for microalgae utilization [48][49][50]. Research on the reutilization of municipal wastewater in microalgae cultivation is commonly seen in many countries, such as United Kingdom (GB), USA (US), Australia (AU), etc. Due to the difference in the compositions of wastewater, to apply the technology of microalgal cultivation to cities, the culture process needs to be modified depending on the region to achieve stable growth of microalgae, and further, to achieve the dual advantages of wastewater purification and CO2 reduction.

3.3. Industrial Wastewater

Some small- and medium-sized enterprises and informal industries often discharge wastewater into municipal pipelines or directly discharge it into the environment. Compared with the hazards caused by agricultural and municipal wastewater, industrial wastewater could be more harmful to water resources and the environment due to the contents of toxic heavy metal components. There are also studies on diluting the wastewater to reduce the sensitivity of the microalgal strain towards the toxicity of wastewater, and increase the wastewater utilization effectivity to obtain the microalgal growth [35][51]. However, wastewater from food processing is usually regarded as a safety resource and is suitable for the production of microalgal biomass for feed or food uses [52]. Because the sources of industrial wastewater were obviously different, the ranges of COD, TN, and TP utilization efficiencies of industrial wastewater in microalgal Chlorella cultivation were approximately 25–95%, 30–100%, and 50–100%, respectively [46][53][54][55] (Table 3). The COD, TN, and TP contents of the food industry wastewater is relatively rich, which is very suitable for use as nutrient sources for microalgae cultivation. Therefore, the better growth of microalgae can be obtained. However, the problem of bacterial contamination is more likely to occur because of the higher nutrient contents. This will affect the long-term stable performance of the microalgal cultivation technology.
Table 3. Biomass and lipid production and productivity of the microalgae Chlorella cultures using wastewater.
Wastewater Source Microalgae COD 1 (mg L−1) TN 1 (mg L−1) TP 1 (mg L−1) Biomass Productivity (g L−1 d−1) CO2 Fixation Efficiency 2 (g L−1 d−1) Lipid (%) Lipid Productivity 3 (g L−1 d−1) Country 4 References
Agricultural wastewater                  
Raw dairy Chlorella sp. 2593 283 116 0.261 0.478 - - CN [38]
Anaerobically treated piggery Chlorella vulgaris CY5 377 287 28 0.281 0.514 19.6 0.055 TW [39]
Piggery Chlorella sp. GD 490 550 20 0.681 1.246 21.8 0.148 TW [35]
Aquaculture 121 234 15 1.296 2.372 21.3 0.276 TW [5]
Swine Chlorella vulgaris UTEX-265 1481 307 4.3 0.247 0.452 27.1 0.067 KR [40]
Piggery Chlorella sorokiniana AK-1 1500–4500 500–700 150–250 0.55 1.006 - - TW [45]
Livestock waste Chlorella sp. 2000 222 103 0.289 0.529 36.3 0.105 CN [41]
Municipal wastewater                  
Centrate Chlorella sorokiniana UTEX1230 - 53 9.4 0.083 0.152 9.4 0.008 GB [48]
Domestic Chlorella vulgaris 142 56 9 0.054 0.099 21.5 0.012 US [56]
Chlorella minutissima 0.049 0.090 22.9 0.011
Municipal Chlorella vulgaris SAG 211-11b 2175 840 10 0.144 0.264 23 0.033 FI [46]
Secondary Chlorella vulgaris UTEX 26 131 112 35 0.078 0.143 8.7 0.021 MX [49]
Chlorella vulgaris CICESE 0.105 0.192 20.2 0.025
Centrate Chlorella vulgaris 513 803 32 0.071 0.130 29.6 0.021 CN [50]
Municipal
(osmosis concentrate)		 Chlorella vulgaris 164 43.2 13.1 0.32 0.585 - - AU [47]
Industrial wastewater                  
Meat processing Chlorella sp. UM6151 2100 212 54 0.171 0.313 17.5 0.029 US [52]
Food Chlorella vulgaris 341 - - 0.207 0.379 31 0.064 CN [53]
Pulp and paper Chlorella vulgaris SAG 211-11b 905 350 28 0.208 0.381 21.7 0.045 FI [46]
Alcohol and starch processing Chlorella pyrenoidosa 3599 334 39 0.376 0.688 19.7 0.074 CN [54]
Tofu whey Chlorella pyrenoidosa FACHB-9 - 592 49 0.283 0.518 17.5 0.049 CN [55]
1 COD, TN, and TP: chemical oxygen demand, total nitrogen, and total phosphorus of wastewater. 2 CO2 fixation efficiency (g L−1 d−1) was calculated by 1.83-fold of biomass productivity. 3 Lipid productivity (g L−1 d−1) = (biomass productivity × lipid content)/100. 4 Country abbreviation: China (CN), Taiwan (TW), Korea (KR), United Kingdom (GB), USA (US), Finland (FI), Mexico (MX), Australia (AU). -: Data not shown.

4. Light

Because of photosynthesis for microalgal growth, light is the most important parameter in microalgal cultivation. Lighting in microalgal cultivation contains two main factors: light intensity and the wavelength of light. In general, the growth rate of microalgae can be greatly increased along with an increase in light intensity; however, when the light intensity exceeds the saturation light that can be tolerated by microalgae, the growth rate of microalgae will be significantly decreased [57]. Therefore, to achieve the maximum growth rate of microalgae, the light intensity is usually controlled to “light saturation”. Because microalgae itself will block light from passing, the light intensity decreases sharply with distance through the surface, causing a decrease in the growth rate of microalgae [58]. Under 400-μmol m−2 s−1 specific light intensity, the microalgal biomass productivity of a Chlorella sp. strain in photobioreactors (PBRs) was approximately 2-fold higher at 0.518 g L−1 d−1 than that grown in outdoor raceway open ponds [59]. The growth of microalgae Chlorella increased by continuous illumination using a light-emitting diode (LED) at the optimal light intensity without a shortage in light energy [60][61]. The incremental light intensity strategy was also an efficient way to improve microalgae growth because photoinhibition at the initial culture phase and insufficient light intensity at the latter culture phase could be avoided [62]. In terms of the wavelength of light, a wavelength range of 400 to 750 nm is absorbed during photosynthesis by most microalgae. The light source for the autotrophic cultivation of Chlorella vulgaris was investigated, and the results showed that red LED light (630–665 nm) resulted in small cells with active divisions, while blue light (430–465 nm) LED illumination led to a significant increase in cell size [63]. The mixed LED light wavelength with red and blue LED light (e.g., red:blue is 5:5) also affects and enhances microalgal growth, including Scenedesmus obliquus, Neochloris oleoabundans, and Chlorella vulgaris [64].

5. pH

The pH of culture broth affects the enzyme activity related to the metabolism of microalgae and the ion absorption efficiency of microalgae, which in turn affects the growth and carbon fixation efficiency of microalgae [6][65]. The optimal pH for growth varies among microalgal species, and in general, the optimum pH is neutral for most microalgae [66]. Flue gas usually contains high concentrations of CO2, NOx, and SO2 [4]. When microalgae were directly aerated with flue gas containing 10–30% CO2, the pH of the culture broth might be reduced to 5.5 [67]. When the microalgae were aerated with flue gas containing SO2 at 100 to 250 ppm, the pH of the culture broth decreased to pH 2.5 to 3.5 to generate bisulfite (HSO3−), sulfite (SO32−), and sulfate (SO42−) [67]. If the flue gas is directly aerated into the culture broth of microalgae without dilution, the excess CO2 of flue gas will be discharged back to the atmosphere. To reduce the CO2 discharged back to the atmosphere, the CO2 captured from flue gas aerated into alkaline medium is easily converted into HCO3−, which is dissolved in water and used for microalgal growth. The solubility of CO2 in water is low, but the CO2 content in the culture broth can be increased under alkaline conditions to further increase the CO2 utilization efficiency of microalgae [68]. In addition, gradually increasing the pH in a microalgal culture is desirable for reducing microbial diversity and is good for outdoor cultivation of microalgae [69].

6. Temperature

The optimal temperature range for microalgae growth is generally 15–26 °C [70]. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity may be a primary site of damage by elevated temperature to cause a decrease in photosynthesis efficiency [71]. In contrast, there was not only a decrease in the metabolic rate of microalgae, but also a decrease in CO2 solubility in culture broth. Therefore, the optimal temperature for growth varies among microalgal species. The temperature of the flue gas will generally be as high as 120 °C or even higher [4]. Flue gas usually needs cooling to be aerated into the culture broth because the temperature of flue gas is too high. If the thermal-tolerant potential of microalgae is good, the cost of flue gas cooling can be reduced. In addition, when sunlight is used outdoors as a light source, the temperature of the culture broth easily changes with the surrounding environment. Béchet et al. [72] indicated that 18,000 GJ year−1 ha−1 of heat energy must be removed to maintain the broth temperature of column PBRs at or below 25 °C. Considering the cost of temperature control, thermotolerant microalgal strains are needed, especially in large-scale outdoor cultivation. When Chlorella sorokiniana was cultivated in outdoor 51-L column PBRs, the culture broth temperature reached 41 °C without growth inhibition [73], and similar results showed better growth performance under uncontrolled temperature in outdoor conditions [11][13].

7. Microalgal Cultivation System

Open (raceway) ponds and PBRs of microalgal cultivation systems are usually—and primarily—adopted. Studies on CO2 fixation by microalgae used in open ponds and PBRs are outlined in Table 4. It has been reported that microalgal biomass production produced from open ponds is more efficient than 90% of worldwide biomass production [74]. The most prominent features of open ponds include simple construction, low cost and easy operation [75][76]. However, disadvantages of open ponds are also obvious, such as a large footprint, difficulties in operation control, unstable culture conditions, high evaporation loss, easy contamination, and the decay of light intensity with medium depth. Compared with open ponds, PBRs have many advantages, such as the most efficient mixing, the best growth conditions, high volumetric mass transfer rates, low risk of contamination, lowest losses of CO2, low shear stress and relatively low energy consumption [77][37][78]. However, the limitations of PBRs are construction cost and scale-up [79]. Overcoming the above shortcomings of cultivation systems is a future research direction for developing advanced cultivation systems. The two cultivation systems still have many challenges in practical operation [80]. Closed cultivation systems, e.g., PBRs, are still not widely applied in industry because the operation cost and construction costs of the systems are too high despite the high microalgal biomass productivity [81]. To solve the limitations of large-scale outdoor microalgae cultivation systems, from an engineering perspective, how to increase the efficiency of gas aeration and mixing should be considered. Low cost and energy consumption can both be achieved by the design of air mixing with flue gas CO2 aeration to improve microalgal growth by sufficient CO2 utilization. Therefore, outdoor large-scale microalgae cultivation systems can become closer to the industrialization process and commercial application by improving the efficiency of gas aeration and mixing. Suitable microalgal cultivation systems usually depend on factors such as cost, CO2 capture source, nutrient sources, and the type of target products. At present, most studies on CO2 fixation by microalgae are used in open ponds or PBRs, and few studies have integrated both microalgal cultivation systems to enhance biomass productivity [59][82]. In our previous study [59], an efficient PBRs/raceway circulating (PsRC) system integrated with the advantages of PBRs and paddlewheel-driven raceway ponds had great potential for the mass cultivation of microalgae. The total amount of CO2 fixation of the PsRC system was approximately 1.2 kg d−1 with 50% CO2 utilization efficiency, as simultaneous microalgal biomass production and CO2 fixation occurred by cultivating alkali-tolerant Chlorella sp. AT1 with alkaline-CO2 capturing operation in the PsRC system. Long-term cultivation for 40 days in a novel membrane photobioreactor, the steadily growth of Chlorella vulgaris were obtained and the maximum removal efficiency of CO2 was 80%. Because the self-forming dynamic membrane from microalgae was easy to harvest, the potential of achieving a sustainable CO2 fixation technology [83]. To investigate the carbon fixation effectivity of microalgae in outdoor cultivation, many studies have used the design of the cultivation system to scale up to pilot scale and industrial scale. The pilot scale is mainly used for research, because the expansion of the outdoor cultivation system may increase the cost of construction, the risk of microorganism pollution, and the release large amounts of CO2. In Table 4, the research in China and Taiwan has reached a ton scale, and it can be combined with waste gas for microalgae cultivation. The cultivation system combination the strategy of an increase of the CO2 content in the water for the microalgal growth and enhance the CO2 carbon fixation efficiency. One is to couple with spraying absorption tower to increase the CO2 content in the water [84], another is to use alkali-tolerant mutant strain combined with alkaline-CO2 capturing medium [59].
Table 4. Biomass productivity and CO2 fixation efficiency of microalgae Chlorella in different cultivation systems.
Microalgae Cultivation System Cultivation Scale (L) CO2 (%) Maximum Biomass Conc. (g L−1) Biomass Productivity (g L−1 d−1) CO2 Fixation Efficiency 1 (g L−1 d−1) Country 2 References
Chlorella sp. MTF-15 Column-type PBR 1 12.5 (1/2 flue gas) 2.855 0.528 0.966 TW [4]
1200 1.555 0.197 0.361
Chlorella vulgaris Porous air-lift PBR 16 0.03 (air) 0.095 0.004 0.174 HK [85]
Loop air-lift PBR 0.126 0.007 0.231
Bubbling PBR 0.783 0.054 1.433
Chlorella sp. GD Column-type PBR 1 2 4.813 0.870 1.592 TW [35]
8 (boiler flue gas) 4.921 1.296 2.333 [5]
Chlorella vulgaris Plastic bottle 15 4 3.151 0.378 0.711 PL [86]
Chlorella vulgaris Flat-plate PBR 1.6 5 2.303 0.551 1.008 CN [57]
Chlorella vulgaris Bubble column PBR 56 0.03 (air) 0.962 0.043 0.079 MY [87]
Chlorella pyrenoidosa Open raceway pond 8000 99.5 0.927 0.114 0.214 CN [84]
Chlorella vulgaris Coiled tubular tree PBR 1.2 0.03 (air) 0.552 0.084 0.153 CA [88]
Chlorella sorokiniana Flat panel PBR 90 5 1.913 0.091 0.167 US [89]
Chlorella vulgaris Pilot-scale PBR 150 Without aeration 2.211 0.198 0.362 CN [90]
Chlorella sp. AT1 Column-type PBR 1 10 7.372 1.011 1.851 TW [19]
PBRs/Raceway circulating system 288 2 2.561 0.321 0.588 TW [59]
528 1.963 0.237 0.434
1008 1.052 0.107 0.195
3600 1.686 0.150 0.275
6600 1.257 0.109 0.199
Chlorella sp. HS2 Flat panel PBR 2 1 3.811 0.543 1.021 KR [91]
Chlorella vulgaris UTEX 26 Raceway 1100 0.03 (air) 0.25 20–26 (g m−2 d−1 for 65 days culture) - MX [75]
Chlorella pyrenoidosa PY-ZU1 Pond-tubular hybrid PBR <5 (a model system) 15 2.3 0.770 1.409 CN [82]
Chlorella vulgaris Raceway with computational fluid dynamics 20 50 (mix with air and pure CO2 gas) 5.2 11.89 (g m−2 d−1, 14 cm depth of raceway) - TW [76]
Chlorella vulgaris CCAP 211/11B Membrane photobioreactor 40 15 1.01 0.166 0.704 IT [83]
1 CO2 fixation efficiency (g L−1 d−1) was calculated by 1.83-fold of biomass productivity. 2 Country abbreviation: Taiwan (TW), Hong Kong (HK), Poland (PL), China (CN), Malaysia (MY), USA (US), Korea (KR), Mexico (MX), Italy (IT). -: Data not shown.

This entry is adapted from the peer-reviewed paper 10.3390/su132313480

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