Testing for terrestrial microalgae productivity under elevated CO2 conditions and nutrient limitation

Abstract Background: Microalgae CO2 fixation results in the production of biomass rich in high-valuable products, such as fatty acids and carotenoids. Enhanced productivity of valuable compounds can be achieved through the microalgae's ability to capture CO2 efficiently from sources of high CO2 contents but is dependent on the species. Although culture collections of microalgae provide a large variety of defined strains, inadequate understanding of which groups of microalgae and strains' origins from habitat offer high productivity under increased CO2 concentrations hampers exploiting microalgae as a sustainable source in the bioeconomy. Results: Growth under atmospheres of CO2 levels of 5 - 25 % in air was examined for 12 new microalgae isolates from adverse terrestrial environments and 69 strains from the culture collection of algae of Göttingen University (SAG), preselected from about 200 strains for their phylogenetic diversity and high productivity under ambient CO2. Green algae from terrestrial environments, including the new isolates, exhibited enhanced growth up to 25% CO2 atmosphere. In contrast, in unicellular red algae and stramenopile algae, which originated through the endosymbiotic uptake of a red algal cell, growth at CO2 concentrations above 5% was suppressed or ceased. While terrestrial stramenopile algae generally tolerated such CO2 concentrations, their counterparts from marine phytoplankton did not. The tests of four new terrestrial strains in submersed culture revealed enhanced biomass and chlorophyll production under elevated CO2 levels. The 15% CO2 aeration increased their total carotenoid and fatty acid contents, which were further stimulated when combined with starvation in macronutrients, i.e., less with phosphate and significantly more with nitrogen-depleted culture media. Conclusion: Isolates of green algae from terrestrial environments, Chlorophyceae and Trebouxiophyceae, exhibit enhanced productivity of carotenoids and fatty acids under elevated CO2 concentrations. This ability supports the economic and sustainable production of valuable compounds of these microalgae using inexpensive sources of high CO2 concentrations such as industrial exhaust fumes.