A novel chlorophyte Desmodesmus sp. 3Dp86E-1 isolated from a White Sea hydroid Dynamena pumila was cultivated at CO2 levels from atmospheric (the 'low-CO2' conditions) to pure carbon dioxide (the 5, 20, and 100 % CO2 conditions) under high (480 μE/(m(2) s) PAR) light. After 7 days of cultivation, the '100 % CO2' (but not 5 or 20 % CO2) cells possessed ca. four times higher chlorophyll content per dry weight (DW) unit than the low-CO2 culture. The rate of CO2 fixation under 100 % CO2 comprised ca. 1.5 L/day per L culture volume. After a lag period which depended on the CO2 level, biomass accumulation and volumetric fatty acid (FA) content of the Desmodesmus sp. 3Dp86E-1 bubbled with CO2-enriched gas mixtures increased and was comparable to that of the culture continuously bubbled with air. Under the low-to-moderate CO2 conditions, the FA percentage of the algal cells increased (to 40 % DW) whereas under high-CO2 conditions, FA percentage did not exceed 15 % DW. A strong increase in oleate (18:1) proportion of total FA at the expense of linolenate (18:3) was recorded in the '100 % CO2' cells. Electron microscopy and pulse-amplitude-modulated chlorophyll fluorescence investigation revealed no damage to or significant downregulation of the photosynthetic apparatus in '100 % CO2' cells grown at the high-PAR irradiance. Possible mechanisms of high-CO2 tolerance of Desmodesmus sp. 3Dp86E-1 are discussed in view of its symbiotic origin and possible application for CO2 biomitigation.