Mixotrophic cultivation promotes growth, lipid productivity, and PUFA production of a thermophilic Chlorophyta strain related to the genus Graesiella

The effects of trophic modes autotrophy, heterotrophy, and mixotrophy on growth, photosynthetic activity, lipid content, and fatty acids (FA) profile were investigated for the thermophilic Chlorophyta strain, Graesiella sp., isolated from a mats community colonizing geothermal springs in the north of Tunisia. Mixotrophic mode enhanced significantly both biomass productivity and lipid content compared to autotrophic and heterotrophic cultures. Indeed, the highest biomass productivity 0.17 g dw L −1 day −1 was obtained for mixotrophic cultures while it was 0.12 g dw L −1 day −1 in autotrophic cultures and did not exceed 0.05 g dw L −1 day −1 in heterotrophic ones. Furthermore, the maximum lipid productivity (0.068 g L L −1 day −1 ) was observed during mixotrophic stationary phase, and it was more than five and nine fold higher than that obtained in heterotrophic and autotrophic cultures. High lipid production was obtained in photoheterotrophic conditions since it was correlated concomitantly with the increase of the glucose uptake and the decrease of the maximal quantum yield of PSII. FA analysis revealed the predominance of unsaturated fatty acids (UFA) 16:1, 18:1n9, 18:2n6, and 18:3n3, which represented more than 55 % of the total FA. FA profile variation was observed with respect to trophic modes and growth phases. The ratio UFA to SFA (saturated FA) reached its highest level (3.5) in exponential growth phase for both heterotrophic and mixotrophic modes mainly due to the increase in the proportion of 16:1.The stationary growth phases of mixotrophic and heterotrophic cultures were particularly marked by a substantial increase of the 18 polyunsaturated carbon chains. In the first mode of culture, the proportion of 18:2n6 exceeded 14 % dw, while lipids produced in the second mode of cultures were particularly rich in 18:3n3, representing more than 3.5 % dw.