Testing for terrestrial and freshwater microalgae productivity under elevated CO 2 conditions and nutrient limitation

Background Microalgae CO 2 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 CO 2 efficiently from sources of high CO 2 contents, but it depends on the species. Culture collections of microalgae offer a wide variety of defined strains. However, an inadequate understanding of which groups of microalgae and from which habitats they originate offer high productivity under increased CO 2 concentrations hampers exploiting microalgae as a sustainable source in the bioeconomy. Results A large variety of 81 defined algal strains, including new green algal isolates from various terrestrial environments, were studied for their growth under atmospheres with CO 2 levels of 5–25% in air. They were from a pool of 200 strains that had been pre-selected for phylogenetic diversity and high productivity under ambient CO 2 . Green algae from terrestrial environments exhibited enhanced growth up to 25% CO 2 . In contrast, in unicellular red algae and stramenopile algae, which originated through the endosymbiotic uptake of a red algal cell, growth at CO 2 concentrations above 5% was suppressed. While terrestrial stramenopile algae generally tolerated such CO 2 concentrations, their counterparts from marine phytoplankton did not. The tests of four new strains in liquid culture revealed enhanced biomass and chlorophyll production under elevated CO 2 levels. The 15% CO 2 aeration increased their total carotenoid and fatty acid contents, which were further stimulated when combined with the starvation of macronutrients, i.e., less with phosphate and more with nitrogen-depleted culture media. Conclusion Green algae originating from terrestrial environments, Chlorophyceae and Trebouxiophyceae, exhibit enhanced productivity of carotenoids and fatty acids under elevated CO 2 concentrations. This ability supports the economic and sustainable production of valuable compounds from these microalgae using inexpensive sources of high CO 2 concentrations, such as industrial exhaust fumes.