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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. [ABSTRACT FROM AUTHOR]

The production of low-value products such as biodiesel from microalgae is still too expensive. An alternative to commonly used species is the alga Botryococcus braunii, which excretes high quantities of hydrocarbons into the extracellular matrix. Because of its low growth rate, the common processing procedure (cultivation, harvesting, dewatering, cell disruption, lipid extraction) is not feasible for this alga, and moreover, at present, there is no existing process for the continuous extraction of hydrocarbons from B. braunii during cultivation without an inhibition of cell activity over a longer time period. Therefore, we attempted to develop an in situ extraction system which allows the continuous non-destructive extraction of the extracellular hydrocarbons during the cultivation. With the new extraction system, we were able to show that an average lipid yield of 19.15, 15.32, and 3.80 mg g dry weight (DW) day by the strains SCCAP 1761, CCAP 807/2, and SAG 807/1, respectively, could be achieved during a 5-day experiment. As only B. braunii SAG 807/1 showed a positive biomass yield, the optimization of the new system was conducted with this strain. We were able to show that a continuous extraction was possible for at least 6 weeks. The optimal effective extraction was in the range of 30 to 36 s day with a lipid yield between 1.22 and 2.20 mg g DW day. [ABSTRACT FROM AUTHOR]