Enhanced lipid and fatty acid content under photoheterotrophic condition in the mass cultures of Tetraselmis gracilis and Platymonas convolutae

Biomass, lipid, fatty acid methyl ester (FAME) profiles and concentrations in two green marine microalgae Tetraselmis gracilis (T. gracilis) and Platymonas convolutae (P. convolutae) in culture media with different nitrate and carbohydrate supplement concentrations under natural photoperiod were measured under indoor and outdoor conditions; T. gracilis and P. convolutae were cultured in a transparent vertical fiber tank and rectangular white fiber-reinforced plastic (FRP) tank, respectively.The total lipid and FAME concentrations increased under nitrate limitation in both microalgae. The results reflected the dynamic changes in the syntheses of fatty acid (FA) under stress in the studied algae. Between the two algae studied, T. gracilis had the highest percentage of eicosapentaenoic acid (EPA: 14.0%) and eicosatetraenoic acid (ETA: 3.1%) at 0.05gl−1 nitrate concentration. The maximum EPA (4.4%) and ETA (2.0%) concentrations in P. convolutae were observed in the total polyunsaturated fatty acids (PUFAs) grown at a nitrate concentration of 0.1gl−1 in the culture media under outdoor conditions. The major PUFAs detected under different carbon supplements were C20:4 (ω3), C20:5 (ω3) and C20:4 (ω6). T. gracilis had the highest amount of EPA (14.7%) under photoheterotrophic growth with glucose (2gl−1) as the carbon source. The optimal condition for EPA production by P. convolutae (8%) was 1gl−1 glucose at 12-h light photoperiod under indoor mass cultivation and photoheterotrophic growth. These two microalgae had high lipid contents and showed a higher PUFA level under nitrate limitation. This result shows that biomass, lipids, and fatty acids as well as PUFA accumulate and reach elevated levels in these two microalgae species under photoheterotrophic growth conditions.