Salinity stress increases lipid, secondary metabolites and enzyme activity in Amphora subtropica and Dunaliella sp. for biodiesel production

Amphora subtropica and Dunaliella sp. isolated from Tunisian biotopes were retained for their high lipid contents. Respective optimized parameters for rapid growth were: pH 9 and 10, light period 21 and 24 h and temperature 31 and 34 °C, respectively. After optimization, Amphora subtropica growth rate increased from 0.2 to 0.5 day−1 and Dunaliella sp. growth rate increased from 0.38 to 0.7 day−1. Amphora subtropica biomass production, productivity and lipid content increased from 0.3 to 0.7 g L−1 (dw), 69-100 mg L−1 d−1 (dw) and 150-190 g kg−1 (dw), respectively, and Dunaliella sp. from 0.5 to 1.4 g L−1 (dw), 124-200 mg L−1 d−1 (dw) and 190-280 g kg−1 (dw), respectively. Often to overcome trade-off between microalgae rapid growth and high lipid content which are often conflicting and very difficult to obtain at the same time, separation in a growth stage and a lipid accumulation stage is obvious. Salinity stress in a single stage of culture was studied. Compared to the optimal concentration of growth, excess or deficiency of NaCl engendered the same cellular responses by implication of oxidative stress systems and reactivation of defense and storage systems. Indeed, increasing salinity from 1 M to 2 M for Amphora subtropica or decreasing salinity from 3 M to 2 M for Dunaliella sp. have both increased lipids content from (220 and 280) to (350 and 430) g kg−1, carotenoids from (1.8 and 2.4) to (2.3 and 3.7) pg cell−1, TBARS amount from (10.4 and 5.3) to (12.1 and 10.7) nmol mg−1 proteins and SOD activity from of (46.6 and 61.8) to (71.6 and 79.4) U mg−1 proteins, respectively. With further improved fatty acids profile, the microalgae strains could be potent candidates for biofuel production.