A new subarctic strain of Tetradesmus obliquus. Part II: comparative studies of CO2-stress tolerance

A huge interest in CO2-tolerant microalgae is fueled by development of CO2-biomitigation methods based on intensive cultivation of microalgae. Still, the mechanisms of CO2-tolerance are scarcely investigated. Previously, we described a symbiotic Desmodesmus sp. IPPAS S-2014 from a White Sea hydroid tolerant to extremely high (20–100%) CO2 levels. In the present work, we compared its ultrastructural and physiological responses to those of a novel free-living White Sea strain of Tetradesmus obliquus IPPAS S-2023 characterized in the companion paper. The strain S-2023 is closely related to Desmodesmus sp. IPPAS S-2014 but lacks its tolerance to extremely high CO2 (it is unable to survive at 100% CO2 and exhibits a reduced-growth phenotype when sparged with 20% CO2: air mixture). We compared the responses of the cell organization and photosynthetic activity to 20% CO2 in the tolerant and the intolerant White Sea chlorophytes using chlorophyll fluorescence measurements and ultrastructural analysis (transmission electron microscopy). The features peculiar to the CO2-intolerant chlorophyte include (i) inability to maintain pH homeostasis, (ii) a steady decline in the photosynthetic activity of the cells, (iii) a reduction of the photosynthetic membranes, and (iv) delayed accumulation of starch (starch grains) and its subsequent conversion to reserve lipids (oil bodies). Nitrogen starvation enhances the effects of high-CO2 stress in the CO2-intolerant microalga. The results of this work are discussed in the context of selection of tolerant algal strains for CO2 biomitigation applications.