Examining the impact of carbon dioxide levels and modulation of resulting hydrogen peroxide in Chlorella vulgaris

The eukaryotic green alga Chlorella vulgaris UTEX 395 was cultured under carbon dioxide (CO2) concentrations ranging from 0.04% to 15% in order to examine the effect of CO2 on algal growth, biomass composition and reactive oxygen species (ROS) accumulation in the culture medium. Supplying 5% CO2 yielded the highest biomass growth rate (μ = 0.35 day−1) compared with 0.04% (μ = 0.15 day−1) and 15% (μ = 0.19 day−1) CO2 conditions. Experimental evidence showed that increasing CO2 levels from 0.04% to 2% and above did not alter overall protein content significantly but did enhance C16:1 and C18:1 monounsaturated fatty acid (MUFA) composition by 3.5 and 2 fold, respectively, reducing C18:3 polyunsaturated fatty acid (PUFA) levels. Interestingly, bubbling 5% and 15% CO2 increased one type of ROS, H2O2 levels, in sterile medium by 1.8 to 2 μM while growing C. vulgaris substantially lowered these H2O2 levels. The ability to lower H2O2 levels, which was reduced for non-viable algal cells, was also observed with C. protothecoides UTEX 29 and C. sorokiniana UTEX 1230. In order to understand the impact of H2O2 directly, 10 μM and 25 μM H2O2 were added daily to 0.04% CO2-bubbled C. vulgaris cultures. Periodic H2O2 addition did not affect the growth of C. vulgaris or change its biomass composition. These findings demonstrate C. vulgaris can thrive at elevated concentrations of CO2 and also showed the capacity of microalgae to reduce the ROS level, specifically H2O2, present in a CO2 bubbling environment. •Increasing CO2 levels from 0.04% to over 2% increases MUFA and decreases PUFA.•Elevating CO2 levels from 0.04% to 15% increases H2O2 level from 0.1 μM to 1.8 μM.•C. vulgaris, C. protothecoides and C. sorokiniana suppress H2O2 levels.•Live C. vulgaris reduced H2O2 level by nearly 100% under 5% CO2 conditions.