Salt stress induced lipid accumulation in heterotrophic culture cells of Chlorella protothecoides: mechanisms based on the multi-level analysis of oxidative response, key enzyme activity and biochemical alteration.

•Salt stress could enhance the lipid synthesis in heterotrophic C. protothecoides.•Effect of salt stress was better than osmotic stress at the same osmolarity level.•ROS production caused by the stress condition may mediate lipid accumulation.•Mechanisms of lipid accumulation were revealed by the multi-level analysis.