Fermentation CO2 biosequestration by microalgae for the production of health beneficial natural compounds

Abstract

In this work, three different microalgal strains were investigated for effective biosequestration of CO2 generated by beer (yeast) fermentation. They were a culture collection Chlamydomonas reinhardtii and two bioprospected strains, Coccomyxa sp. (P-918) obtained from a polishing pond at an operational smelter (pH 2.8) and Chlamydomonas sp. (M-23) obtained from a low pH abandoned mine site (pH 3). The three strains were investigated for use in beer fermentation CO2 fixation and their production of both lipid and protein. The pH of beer fermentation CO2-enriched Chlamydomonas reinhardtii cultures varied between 4.5 and 7.3 throughout the experiment. For the Chlamydomonas sp., pH varied between 4.6 and 7.1 and for Coccomyxa sp. between 5.3 and 7.4. As expected, during higher fermentation activity (day 1 to 3 for each beer kit), more CO2 was released to the microalgal cultures causing a drop in pH. When the rate of fermentation slowed in day 4, there was an increase in pH. For all three microalgal cultures which were grown only under atmospheric CO2 (controls), the pH increased continuously along with microalgal growth over 16 days of experiment. Chlamydomonas reinhardtii control culture pH was 6.7 at the start of the experiment and reached 8.7 at day 16. For the Chlamydomonas sp. control culture, pH increased steadily from 6.6 to 8.4, and for Coccomyxa sp. from 7 to 8.6. Experimental results indicated that the bioprospected Coccomyxa sp. adapted well to the low pH created by sparging in beer fermentation CO2. Its volumetric biomass productivity was 0.124 gdwL-1 day-1, which was higher than both Chlamydomonas reinhardtii (0.072 gdwL-1day-1) and bioprospected Chlamydomonassp. (0.086 gdwL-1day-1). The Coccomyxa sp. when exposed to fermentation CO2 reached a maximum specific growth rate of 0.167 day−1, which was 29% higher than achieved without sparging in fermentation CO2. Moreover, its carbon fixation rate increased from 122.1 to 227.9 mgCO2 L1day-1 with fermentation CO2. However, lipid synthesis occurred more rapidly and efficiently in Chlamydomonas sp. and Chlamydomonas reinhardtii rather than Coccomyxa sp., reaching 39% and 35% of biomass dry weight after 16 days of beer fermentation CO2 exposure. Whereas the amount of lipid in Coccomyxa sp. was 26% of the biomass dry weight at 16 days. This would indicate that the bioprospected Chlamydomonas sp. was a better candidate for biofuel production as its dry weight lipid content increased from 20% to 39% when exposed fermentation CO2. While the lipid content of Chlamydomonas sp. culture that grew under atmospheric CO2 reached 24% of biomass dry weight at the end of experiment (day 16) from its initial 20%. It was found that protein content with fermentation CO2 was 42.5% of Coccomyxa sp. biomass dry weight. Protein content of Chlamydomonas reinhardtii and Chlamydomonas sp. dry weight were 30.7% and 27.4%, respectively.