Elevated CO2 concentration impacts cell wall polysaccharide composition of green microalgae of the genus Chlorella

The effect of CO₂concentration on the relative content of starch, lipid and cell wall carbohydrates in microalgal biomass was investigated for the four following Chlorella strains: C. vulgaris (UTEX 259), C. sorokiniana (UTEX 2805), C. minutissima (UTEX 2341) and C. variabilis (NC64A). Each strain had a different response to CO₂concentration. The starch content was higher in UTEX259 and NC64A cultured with 2% CO₂in the air supply than in cells cultured with ca. 0·04% CO₂(ambient air), while starch content was not affected for UTEX 2805 and UTEX 2341. The lipid content was higher in Chlorella minutissima UTEX 2341 cultured in 2% CO₂than in cells cultured in ambient air, but was unchanged for the other three strains. All four Chlorella strains tended to have a higher percentage of uronic acids and lower percentage of neutral sugars in their cell wall polysaccharide complement when grown with 2% CO₂supply. Although the percentage of neutral sugars in the cell walls varied with CO₂concentration, the relative proportions of different neutral sugar constituents remained constant for both CO₂conditions. The results demonstrate the importance of considering the effects of CO₂on the cell wall carbohydrate composition of microalgae. SIGNIFICANCE AND IMPACT OF THE STUDY: Microalgae have the potential to produce products that will reduce society's reliance on fossil fuels and address challenges related to food and feed production. An overlooked yet industrially relevant component of microalgae are their cell walls. Cell wall composition affects cell flocculation and the recovery of intracellular products. In this study, we show that increasing CO₂level results in greater cell wall polysaccharide and uronic acid content in the cell walls of three strains of microalgae. The results have implications on the management of systems for the capture of CO₂and production of fuels, chemicals and food from microalgae.