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

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 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 CO₂ 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 CO₂ 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 CO₂. Green algae from terrestrial environments exhibited enhanced growth up to 25% CO₂. 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. While terrestrial stramenopile algae generally tolerated such CO₂ 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 CO₂ levels. The 15% CO₂ 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 CO₂ concentrations. This ability supports the economic and sustainable production of valuable compounds from these microalgae using inexpensive sources of high CO₂ concentrations, such as industrial exhaust fumes.