ИСТИНА

A considerable interest to CO2-tolerant microalgae is fueled by development of CO2-biomitigation methods based on cultivation of microalgae. Still, the mechanisms of CO2-tolerance are scarcely investigated [1,2]. Previously [1,2,3], we described a symbiotic Desmodesmus sp. IPPAS S-2014 from a White Sea hydroid tolerant to extremely high (20-100%) CO2 levels. We also characterized a novel free-living White Sea strain Tetradesmus obliquus 46, closely related to the Desmodesmus sp., as intolerant to high CO2 (unable to survive at 100% CO2 and showing a reduced-growth phenotype when sparged with 20% CO2:air mixture). We compared the ultrastructural and physiological responses to 20% CO2 in the tolerant and the intolerant White Sea chlorophytes using chlorophyll fluorescence measurements and ultrastructural analysis (TEM). A decline in the medium pH to 6.0 was observed in the both microalgae cultures grown at 20% CO2. However, after 6 h of cultivation in the Desmodesmus sp. culture the medium pH increased reaching a steady-state pH of 8.0 by the 9th day of cultivation [3], whereas in T. obliquus it remained as low as 6.0. The TEM analysis revealed a reduction of the photosynthetic membranes in the T. obliquus grown at 20% CO2. This process manifested itself as a considerable decline of the thylakoid stacking and an frequent extrusion of the thylakoid membrane structures, often with formation of the specific epichloroplast membrane structures (EMS), in the vacuolar lumen. This was not the case in the Desmodesmus sp. cells grown under the same conditions [3]. The TEM data were consistent with the results of photosynthetic activity evaluation. The cells of T. obliquus displayed a steady decline in photosynthetic quantum yield, whereas Desmodesmus sp. cells exhibited a rapid transient decline of photosynthetic activity followed by its recovery [1]. According to the TEM survey, excess of photosynthetically fixed carbon in T. obliquus was accumulated as starch grains (SG) and, to a lesser extent, oil bodies (OB) very slowly, likely due to the reduced photosynthetic activity of the cells. Instead of it, Desmodesmus sp. seemingly converted the excess photosynthates to new cell building blocks to support a rapid cell division [3]. To conclude, the features peculiar to the CO2-intolerant chlorophyte include (i) inability to maintain pH homeostasis of the cultural medium; (ii) a steady decline in the photosynthetic activity of the cells; (iii) a reduction of the photosynthetic membranes; and (iv) delayed accumulation of starch (SG) and its subsequent conversion to reserve lipids (OB). Supported by Russian Foundation of Basic Research (projects 15-04-01061 and 15-54-06004). [1] Ptushenko, V., Solovchenko A. (2016) “Tolerance of the photosynthetic apparatus to acidification of the growth medium as a possible determinant of CO2-tolerance of a symbiotic microalga Desmodesmus sp.”. Biochemistry (Moscow), 81(12), 1531–1537. [2] Solovchenko, A., Gorelova, O., Selyakh, I., Semenova, L., Chivkunova, O., Baulina, O., and Lobakova, E. (2014). “Desmodesmus sp. 3Dp86E-1—a novel symbiotic chlorophyte capable of growth on pure CO2”. Marine Biotechnology, 16(5), 495-501. [3] Solovchenko, A., Gorelova, O., Selyakh, I., Pogosyan, S., Baulina, O., Semenova, L., Chivkunova, O., Voronova, E., Konyukhov, I., Scherbakov, P. and Lobakova, E. (2015). " A novel CO2-tolerant symbiotic Desmodesmus (Chlorophyceae, Desmodesmaceae): Acclimation to and performance at a high carbon dioxide level." Algal Research 11: 399-410.