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

Background: Microalgae CO 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 efficiently from sources of high CO 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 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 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. Green algae from terrestrial environments exhibited enhanced growth up to 25% CO. In contrast, in unicellular red algae and stramenopile algae, which originated through the endosymbiotic uptake of a red algal cell, growth at CO concentrations above 5% was suppressed. While terrestrial stramenopile algae generally tolerated such CO 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 levels. The 15% CO 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 concentrations. This ability supports the economic and sustainable production of valuable compounds from these microalgae using inexpensive sources of high CO concentrations, such as industrial exhaust fumes.