Acidophilic Microalgae Research Articles

Roles of pH and phosphate in rare earth element biosorption with living acidophilic microalgae

The increasing demand for rare earth elements (REEs) has spurred interest in the development of recovery methods from aqueous waste streams. Acidophilic microalgae have gained attention for REE biosorption as they can withstand high concentrations of transition metals and do not require added organic carbon to grow, potentially allowing simultaneous sorption and self-replication of the sorbent. Here, we assessed the potential of Galdieria sulphuraria for REE biosorption under acidic, nutrient-replete conditions from solutions containing ≤ 15 ppm REEs. Sorption at pH 1.5–2.5 (the growth optimum of G. sulphuraria) was poor but improved up to 24-fold at pH 5.0 in phosphate-free conditions. Metabolic activity had a negative impact on REE sorption, additionally challenging the feasibility of REE biosorption under ideal growth conditions for acidophiles. We further examined the possibility of REE biosorption in the presence of phosphate for biomass growth at elevated pH (pH ≥ 2.5) by assessing aqueous La concentrations in various culture media. Three days after adding La into the media, dissolved La concentrations were up to three orders of magnitude higher than solubility predictions due to supersaturation, though LaPO4 precipitation occurred under all conditions when seed was added. We concluded that biosorption should occur separately from biomass growth to avoid REE phosphate precipitation. Furthermore, we demonstrated the importance of proper control experiments in biosorption studies to assess potential interactions between REEs and matrix ions such as phosphates.Key points• REE biosorption with G. sulphuraria increases significantly when raising pH to 5• Phosphate for biosorbent growth has to be supplied separately from biosorption• Biosorption studies have to assess potential matrix effects on REE behavior

Chromosome-scale genome assembly of an acidophilic microalga Tetratostichococcus sp. P1 isolated from a tropical peatland in Malaysia.

Tetratostichococcus sp. P1 shows an acidophilic phenotype which could allow mass-scale monoculture of this green microalga without severe contamination by environmental microorganisms. In this study, we report a chromosome-scale genome assembly for Tetratostichococcus sp. P1.

Ultrasound-Based Recovery of Anti-Inflammatory and Antimicrobial Extracts of the Acidophilic Microalga Coccomyxa onubensis.

In the present study, the recovery of valuable molecules of proven anti-inflammatory and antimicrobial activity of the acidophilic microalga Coccomyxa onubensis (C. onubensis) were evaluated using green technologies based on ultrasound-assisted extraction (UAE). Using a factorial design (3 × 2) based on response surface methodology and Pareto charts, two types of ultrasonic equipment (bath and probe) were evaluated to recover valuable compounds, including the major terpenoid of C. onubensis, lutein, and the antimicrobial activity of the microalgal extracts obtained under optimal ultrasound conditions (desirability function) was evaluated versus conventional extraction. Significant differences in lutein recovery were observed between ultrasonic bath and ultrasonic probe and conventional extraction. Furthermore, the antimicrobial activity displayed by C. onubensis UAE-based extracts was greater than that obtained in solvent-based extracts, highlighting the effects of the extracts against pathogens such as Enterococcus hirae and Bacillus subtilis, followed by Staphylococcus aureus and Escherichia coli. In addition, gas chromatography-mass spectrometry was performed to detect valuable anti-inflammatory and antimicrobial biomolecules present in the optimal C. onubensis extracts, which revealed that phytol, sterol-like, terpenoid, and even fatty acid structures could also be responsible for the antibacterial activities of the extracts. Moreover, UAE displayed a positive effect on the recovery of valuable molecules, improving biocidal effects. Our study results facilitate the use of green technology as a good tool in algal bioprocess engineering, improving energy consumption and minimizing environmental impacts and process costs, as well as provide a valuable product for applications in the field of biotechnology.

Biomass Fuel Production through Cultivation of Microalgae Coccomyxa dispar and Scenedesmus parvus in Palm Oil Mill Effluent and Simultaneous Phycoremediation

Palm oil mill effluent (POME) is a potential alternative sources of biomass fuel upon phycoremediation treatment using microorganisms. In this study, Coccomyxa dispar and Scenedesmus parvus, as acidophilic microalgae, were used to investigate growth and the production of biomass fuel from the cultivation of POME, as well the effectiveness of removing contaminants from POME. Individual cultivation was conducted at 26 ± 3 °C for 14 days under three growth modes (mixotrophic, heterotrophic, and autotrophic). To elucidate the potential phycoremediation properties, the characteristics of treated POME were compared, such as optical density (OD), cell dry weight (CDW), calorific energy values (CEV), chemical oxygen demand (COD), biochemical oxygen demand (BOD), carbon, hydrogen, and nitrogen (CHN) elemental analysis, including oil and grease content. S. parvus exhibits an outstanding growth profile for all growth modes compared to C. dispar, with measurements of 228.8, 37.08, and 118.2 mg/L observed at day 14 of cultivation. The highest CEV is 32.30 MJ/kg, which was obtained from S. parvus in the mixotrophic mode. Maximum removal efficiency for COD and BOD was 81% and 19% in the mixotrophic growth mode with S. parvus. These results pinpoint that S. parvus has the potential to be utilized for biomass fuel production with high CEV and effective POME phycoremediation.

Extremophile-based biohybrid micromotors for biomedical operations in harsh acidic environments

The function of robots in extreme environments is regarded as one of the major challenges facing robotics. Here, we demonstrate that acidophilic microalgae biomotors can maintain their swimming behavior over long periods of time in the harsh acidic environment of the stomach, thus enabling them to be applied for gastrointestinal (GI) delivery applications. The biomotors can also be functionalized with a wide range of cargos, ranging from small molecules to nanoparticles, without compromising their ability to self-propel under extreme conditions. Successful GI delivery of model payloads after oral administration of the acidophilic algae motors is confirmed using a murine model. By tuning the surface properties of cargos, it is possible to modulate their precise GI localization. Overall, our findings indicate that multifunctional acidophilic algae-based biomotors offer distinct advantages compared to traditional biohybrid platforms and hold great potential for GI-related biomedical applications.

Methodological Optimization of Supercritical Fluid Extraction of Valuable Bioactive Compounds from the Acidophilic Microalga Coccomyxa onubensis.

Microalgae grow in diverse environments and possess a great biotechnological potential as they contain useful bioactive compounds. These bioactive compounds can be obtained by selective and energy-efficient extraction methods. Various industries are using the supercritical fluid extraction (SFE) method to extract these valuable bioactive compounds. Hence, for the first time, we evaluated the effects of SFE on the recovery of bioactive and antioxidant compounds using Coccomyxa onubensis, a eukaryotic acidophilic microalga of potential relevance which can be used in the field of nutraceutical and functional foods. It was isolated from the Tinto River (Pyritic Belt, Huelva, Spain), a mining region in Spain. Variables such as extraction yield, lutein purity (LP) and recovery (LR), total phenols, and antioxidant capacity (Trolox equivalents antioxidant capacity method) were studied using a Box–Behnken design based on a response surface methodology along with the overall extraction curve fitted to a spline linear model. The effects of temperature (30, 50, and 70 °C), pressure (25, 40, and 55 MPa), and the percentage of co-solvent (0, 25%, and 50% v/v ethanol) on SFE were analyzed, resulting in the co-solvent and temperature as the most significant factors followed by the pressure. Under 70 °C, 40 MPa, and 50% v/v ethanol, C. onubensis reached a maximum of 66.98% of LR. The extracts were richest in total phenols and showed the maximum antioxidant activity (36.08 mg GAEs/g extracts and 2.237 mmol TE/g extracts, respectively) under similar pressure and co-solvent percentage values and different temperatures (30 and 70 °C, respectively). The extracts obtained in this study may have potential applications in the food, nutraceutical, and cosmetic industries. SFE is a highly efficient method to valorize microorganisms living in extreme environments, which are so far unexplored using green extraction methods.

Autotrophic and mixotrophic biomass production of the acidophilic Galdieria sulphuraria ACUF 64

Abstract Galdieria sulphuraria is an acidophilic microalga isolated in proximity of sulfuric ponds where pH is below 3 and most organisms cannot grow. We cultivated G. sulphuraria ACUF 64 free of contamination for over 2 months in a medium containing organic carbon at pH 1.7 with continuous, high intensity, lighting. We compared biomass productivity of chemostat and repeated batch cultivations. The optimal biomass density in autotrophic and mixotrophic cultures was identified. In autotrophy biomass productivity was 28.3 g x ·m −2 ·day −1 , 1.8 to 7.7-fold higher than previously reported. Autotrophy was compared to ‘oxygen balanced’ mixotrophy where intracellular recirculation of O 2 and CO 2 take place. Aeration was not needed and 92% of the substrate carbon was converted into biomass. In mixotrophy biomass productivity was 1.8 times higher than autotrophic culture and linear growth was maintained at high biomass concentration (9.7 g x ·L −1 ). Light tolerance and high productivity in dense culture make our strain promising for mixotrophic outdoor cultivation. • Highest Galdieria sulphuraria autotrophic productivity (29 g·m −2 ·day −1 ) • Highest Galdieria sulphuraria mixotrophic productivity (51 g·m −2 ·day −1 ) • Oxygen balanced mixotrophy does not need aeration and 92% of substrate converted into biomass. • Galdieria sulphuraria can be cultivated at high light intensity. • In mixotrophy linear growth maintained at high biomass concentration (340 g x ·m −2 ).

Bio-removal of PtCl62− complex by Galdieria sulphuraria

Bio-removal of negative charged platinum complex is of great challenge owing to electrostatic repulsions between PtCl62− and general extracellular polymeric substance (EPS) of microorganism. Galdieria sulphuraria (GS) are thermophilic and acidophilic microalga with specific metabolism, which subsequently lead to their unique cellular compositions such as EPS and phycocyanin, possibly providing a strategy to deal with negative charged metal complex. Accordingly, G. sulphuraria are employed to remove negative charged PtCl62− complex with initial concentrations ranging from 0, 10, 20, 30, to 45 ppm. The growth rates of G. sulphuraria with microalgae named as GS-0, GS-10, GS-20, GS-30, and GS-45, respectively, and simultaneously bio-removal efficiencies of PtCl62− are investigated. G. sulphuraria are independent to PtCl62− within 0–30 ppm, while they are inhibited within 45 ppm of PtCl62−. The PtCl62− removal efficiencies of GS-10, GS-20, and GS-30 increase from 94.58%, 95.52%, to 95.92%, while decrease to 71.81% of GS-45. About 92.39%, 93.77%, 94.29%, and 75.21% of PtCl62− adsorbed are accumulated within GS-10, GS-20, GS-30, GS-45, with few in EPS. The PtCl62− complexes accumulated in EPS and algae cells are possibly decomposed to PtCl4 according to the increasing zeta potentials of EPS and algae cells. The results indicate that PtCl62− is efficiently removed by G. sulphuraria, achieving bio-removal of negative charged PtCl62− complex from wastewater.

Cultivation of the Acidophilic Microalgae Galdieria phlegrea with Wastewater: Process Yields

Algal based wastewater treatment offers the opportunity to recover, in the form of biomass, the nutrients and internal chemical energy of wastewater. Recently, there has been a growing interest in the use of extremophilic microalgae, as they can easily adapt to difficult and often pollutant-rich environments. The thermo-acidophilic microalga Galdieria phlegrea is a species of recent discovery and great metabolic versatility, but it has still been poorly studied. Here, G. phlegrea was cultivated using raw municipal wastewater in 1 L Erlenmeyer flasks with 700 mL working volume at 37 °C for up to nine days. During the cultivation phase, biomass growth, phycocyanin content, ammonium and phosphate removal from the wastewater, lipid fraction, total carbon and nitrogen in the biomass, and variation in δ13C and δ15N isotopic ratios (a novel analytical contribution in these experiments) were monitored. Results indicated that G. phlegrea was able to grow in raw effluent, where it removed more than 50% ammonium and 20% phosphate in 24 h; total lipid content was in the range of 11–22%, while average C-N content was of 45% and 6%, respectively; isotopic analyses proved to be a useful support in identifying C and N metabolic pathways from effluent to biomass. Overall, G. phlegrea showed consistent performance with similar Cyanidiophyceae and is a potentially viable candidate for municipal wastewater valorization from a circular economy perspective.

The acidophilic microalga Coccomyxa onubensis and atorvastatin equally improve antihyperglycemic and antihyperlipidemic protective effects on rats fed on high-fat diets

Biomass of the acidophilic green alga Coccomyxa onubensis may be used as a food source for animals without collateral toxic effects, as diet supplemented the microalga has significant hypoglycemic and hypocholesterolemic effects on healthy animals. Rats were fed for 108 days with a high-fat diet, and at the end of the experiment, they were overweight and had significantly increased serum levels of glucose (2.0-fold), total cholesterol (1.6-fold), and low-density lipoprotein (LDL)-cholesterol (7.7-fold). The supplement of C. onubensis powder (6.25% w/w dry weight) in the high-fat diet significantly protected the rats against cardiovascular risks by reducing the serum levels of glucose (38.47%), total cholesterol (22.65%), and LDL-cholesterol (26.70%). The protective effects of the microalga were comparable with that of 10 mg/kg body weight per day of atorvastatin. The high-fat diet decreased both ω–3 eicosapentaenoic and docosahexaenoic acids in the brain tissue of rats; however, C. onubensis powder could not restrict these changes. Simultaneously, the high-fat diet increased the levels of both palmitic and arachidonic (ω–6) acids in the telencephalon tissue of rats; this was prevented when microalga biomass was used in the diet of rats.

Outdoor Large-Scale Cultivation of the Acidophilic Microalga Coccomyxa onubensis in a Vertical Close Photobioreactor for Lutein Production

The large-scale biomass production is an essential step in the biotechnological applications of microalgae. Coccomyxa onubensis is an acidophilic microalga isolated from the highly acidic waters of Río Tinto (province of Huelva, Spain) and has been shown to accumulate a high concentration of lutein (9.7 mg g−1dw), a valuable antioxidant, when grown at laboratory-scale. A productivity of 0.14 g L−1 d−1 was obtained by growing the microalga under outdoor conditions in an 800 L tubular photobioreactor. The results show a stable biomass production for at least one month and with a lutein content of 10 mg g−1dw, at pH values in the range 2.5–3.0 and temperature in the range 10–25 °C. Culture density, temperature, and CO2 availability in highly acidic medium are rate-limiting conditions for the microalgal growth. These aspects are discussed in this paper in order to improve the outdoor culture conditions for competitive applications of C. onubensis.

Isolation and characterization of novel acidophilic microalgae from abandoned mining site area for carbohydrate biosynthesis and its kinetic growth study in photobioreactor

Microalgae have gained attention as an alternative feedstock for biofuel production due to its potential to accumulate carbohydrate, protein and lipid. However, current microalgae culture technology has been impeded by low biomass production, carbohydrate accumulation and contamination issue. In order to overcome this limitation, this study involves the isolation, screening and identification of acidophilic microalgae from Frog Hill. Then, the effect of different initial pH on growth and carbohydrate productivity was studied toward the selected microalgae. Further cultivation was done in indoor and outdoor using 15L fabricated PBR under optimize pH condition. At the end of experiment, the growth kinetic and carbohydrate productivity for both conditions were evaluated. Total of 4 native acidophilic microalgae were successfully isolated and was identified using 18s RNA. The Scenedesmus parvus was exhibited highest biomass production and carbohydrate productivity up to 1.2640 ± 0.001 g L−1 and 15.47 ± 0.048 mg L−1 d−1 under highly acidic condition. It also showed a better performance under outdoor cultivation up to 9.54% compared to indoor cultivation. Hence, this research paper showed that the potential of novel acidophilic microalgae survives under extremely condition with high carbohydrate biosynthesis, which could be useful for industrial application.

The effects of contemporary selection and dispersal limitation on the community assembly of acidophilic microalgae.

Extremophilic microalgae are primary producers in acidic habitats, such as volcanic sites and acid mine drainages, and play a central role in biogeochemical cycles. Yet, basic knowledge about their species composition and community assembly is lacking. Here, we begin to fill this knowledge gap by performing the first large-scale survey of microalgal diversity in acidic geothermal sites across the West Pacific Island Chain. We collected 72 environmental samples in 12 geothermal sites, measured temperature and pH, and performed rbcL amplicon-based 454 pyrosequencing. Using these data, we estimated the diversity of microalgal species, and then examined the relative contribution of contemporary selection (i.e., local environmental variables) and dispersal limitation on the assembly of these communities. A species delimitation analysis uncovered seven major microalgae (fourred, two green, and one diatom) and higher species diversity than previously appreciated. A distance-based redundancy analysis with variation partitioning revealed that dispersal limitation has a greater influence on the community assembly of microalgae than contemporary selection. Consistent with this finding, community similarity among the sampled sites decayed more quickly over geographical distance than differences in environmental factors. Our work paves the way for future studies to understand the ecology and biogeography of microalgae in extreme habitats.

Carbon biofixation and lipid composition of an acidophilic microalga cultivated on treated wastewater supplied with different CO2 levels

ABSTRACTThis study evaluated productivity, CO2 biofixation, and lipid content in biomass of the acidophilic microalga Chlamydomonas acidophila LAFIC-004 cultivated with five different carbon dioxide concentrations. The influence of carbon dioxide concentration on nutrient removal and pH was also investigated. Treated wastewater (secondary effluent) was used as culture medium. Five experimental setups were tested: T-0% – injection of atmospheric air (0.038% CO2), T-5% (5% CO2), T-10% (10% CO2), T-15% (15% CO2) and T-20% (20% CO2). The T-5% and T-10% experiments showed the highest values of productivity and CO2 biofixation, and maximum biomass dry weight was 0.48 ± 0.02 and 0.51 ± 0.03 g L−1, respectively. This acidophilic microalga proved to be suitable for carbon biofixation and removal of nutrients from secondary effluent of wastewater treatment plants with high CO2 concentration. All assays were performed without pH control. This microalga species presented high lipid content. However, fatty acid methyl esters (FAME) are not suitable for biodiesel use.

The Use of Algal Biomass to Sustain Sulfidogenic Bioreactors for Remediating Acidic Metal-Rich Waste Waters

Two different species of acidophilic micro-algae were grown in axenic culture, biomass harvested and injected into a low pH sulfate-reducing bioreactor, to act as a substrate for biosulfidogenesis. The hydrogen sulfide generated was used to precipitate copper in an off-line vessel, and the bioreactor pH was maintained by automated addition of a pH 2.5 feed liquor, to compensate for protons consumed by biosulfidogenesis. Results demonstrated the potential for using algal biomass for this purpose, precipitating about 1.5 mg Cu2+ L-1h-1, though rates of sulfidogenesis were considerably slower that when glycerol and yeast extract were used as organic feed-stocks.

Antimicrobial activity of the acidophilic eukaryotic microalga Coccomyxa onubensis

SUMMARYExtremophilic microalgae are unexplored as a source of pharmaceuticals despite the fact that its biomass can be produced at large scale with low risk of contamination. A significant amount of antimicrobial activity was produced by extracts obtained from the eukaryotic acidophilic microalgae Coccomyxa onubensis in non‐polar solvents, such as hexane, diethyl ether, and chloroform or in weakly polar solvents, such as dichloromethane, against Gram‐negative and Gram‐positive bacteria, and also the yeast Candida albicans. The most effective activity was shown by chloroform extract against Escherichia coli S, Salmonella enterica, and Proteus mirabilis; hexane extract against P. mirabilis, Sa. enterica, and Ca. albicans; dichloromethane extract against Sa. enterica or diethyl ether extract against E. coli S and the Gram‐positive Staphylococcus aureus MB. The lowest minimum inhibitory concentration values were recorded against E. coli S (305 μg mL −1) and P. mirabilis (153 μg mL −1) (using chloroform extract) and against P. mirabilis (106 μg mL−1) (using hexane extract). Fatty acids, but not carotenoids, seem to be involved in the antimicrobial activity of this microalga. However, further biochemical and biotechnological studies must be conducted in order to characterize and purify the bioactive principles from Co. onubensis for assessing its potential as a pharmaceutical source and feasibility of production.

Microalgae as a safe food source for animals: nutritional characteristics of the acidophilic microalga Coccomyxa onubensis

BackgroundEdible microalgae are marine or fresh water mesophilic species. Although the harvesting of microalgae offers an abundance of opportunities to the food and pharmaceutical industries, the possibility to use extremophilic microalgae as a food source for animals is not well-documented.ObjectiveWe studied the effects of dietary supplementation of a powdered form of the acidophilic microalga Coccomyxa onubensis on growth and health parameters of laboratory rats.MethodFour randomly organized groups of rats (n=6) were fed a standard diet (Diet 1, control) or with a diet in which 0.4% (Diet 2), 1.25% (Diet 3), or 6.25% (Diet 4) (w/w) of the standard diet weight was substituted with dried microalgae powder, respectively. The four groups of animals were provided ad libitum access to feed for 45 days.ResultsC. onubensis biomass is rich in protein (44.60% of dry weight) and dietary fiber (15.73%), and has a moderate carbohydrate content (24.8%) and a low lipid content (5.4%) in which polyunsaturated fatty acids represent 65% of the total fatty acid. Nucleic acids are present at 4.8%. No significant difference was found in growth rates or feed efficiency ratios of the four groups of rats. Histological studies of liver and kidney tissue revealed healthy organs in control and C. onubensis-fed animals, while plasma hematological and biochemical parameters were within healthy ranges for all animals. Furthermore, animals fed a microalgae-enriched diet exhibited a statistically significant decrease in both blood cholesterol and triglyceride levels. The blood triglyceride content and very low density lipoprotein-cholesterol levels decreased by about 50% in rats fed Diet 4.ConclusionsThese data suggest that C. onubensis may be useful as a food supplement for laboratory animals and may also serve as a nutraceutical in functional foods. In addition, microalgae powder-supplemented diets exerted a significant hypocholesterolemic and hypotriglyceridemic effect in animals.

Significance of Microbial Communities and Interactions in Safeguarding Reactive Mine Tailings by Ecological Engineering

Pyritic mine tailings (mineral waste generated by metal mining) pose significant risk to the environment as point sources of acidic, metal-rich effluents (acid mine drainage [AMD]). While the accelerated oxidative dissolution of pyrite and other sulfide minerals in tailings by acidophilic chemolithotrophic prokaryotes has been widely reported, other acidophiles (heterotrophic bacteria that catalyze the dissimilatory reduction of iron and sulfur) can reverse the reactions involved in AMD genesis, and these have been implicated in the "natural attenuation" of mine waters. We have investigated whether by manipulating microbial communities in tailings (inoculating with iron- and sulfur-reducing acidophilic bacteria and phototrophic acidophilic microalgae) it is possible to mitigate the impact of the acid-generating and metal-mobilizing chemolithotrophic prokaryotes that are indigenous to tailing deposits. Sixty tailings mesocosms were set up, using five different microbial inoculation variants, and analyzed at regular intervals for changes in physicochemical and microbiological parameters for up to 1 year. Differences between treatment protocols were most apparent between tailings that had been inoculated with acidophilic algae in addition to aerobic and anaerobic heterotrophic bacteria and those that had been inoculated with only pyrite-oxidizing chemolithotrophs; these differences included higher pH values, lower redox potentials, and smaller concentrations of soluble copper and zinc. The results suggest that empirical ecological engineering of tailing lagoons to promote the growth and activities of iron- and sulfate-reducing bacteria could minimize their risk of AMD production and that the heterotrophic populations could be sustained by facilitating the growth of microalgae to provide continuous inputs of organic carbon.

Growth and photosynthetic utilization of inorganic carbon of the microalga Chlamydomonas acidophila isolated from Tinto river

Growth and utilization of inorganic carbon were investigated in the acidophilic microalga Chlamydomonas acidophila, isolated from Tinto river, in the spanish southwest mining area, in the province of Huelva. According to the chemical composition of its natural habitat, an artificial culture medium containing 4 g l −1 SO 4K 2 was designed at pH 2.5 and used to grow the acidophilic microalgae in batch cultures. The cultures were bubbled with air enriched with 5% (v/v) CO 2 as unique carbon source. The culture reached the stationary growth phase 250 h after the experiment started up. The initial chlorophyll content was 5 μg ml −1 (initial value, t = 0) and it increased up to 186 μg ml −1 (maximum value). The growth rate was 0.52 d −1, low value when compared to other non-extremophilic microalgae. Carotenoids to chlorophyll ratio continuously increased from 0.1 to 0.2 due to fast production of carotenoids. HPLC analysis of the accumulated carotenoids showed lutein to play a significant role in the antioxidant response of the microalgae. The photosynthetic utilization of inorganic carbon by C. acidophila was studied in liquid medium cultures growing with air enriched with 5% (v/v) CO 2. The maximum photosynthetic activity increased when increasing pH from 2.5 to 6. The apparent photosynthetic affinity constant for dissolved inorganic carbon ( K DIC) was 1.7 μM at pH 2.5 and it hardly changed at pH 4.0 and 6.0. These results show the acidophilic microalgae to have high affinity by CO 2 which seems not to depend on the presence of HCO 3 −. The growth of the acidophilic microalgae was found to be dependent on CO 2.