Metabolic interpretation of NaCl stress-induced lipid accumulation in microalgae for promising biodiesel production with saline wastewater

The rising use of saline wastewater for biodiesel production through oleaginous microalgae demands fundamental studies on the metabolic response of microalgal cells to salt stress. Thus, we investigated how salt stress promotes lipid accumulation inside Botryococcus sp. NJD-1 by tracking critical metabolites and proteases as well as analyzing the cellular redox balance. The results showed that the lipid content of microalgal cells followed a negative binomial correlation with intracellular reactive oxygen species (ROS), while the neutral lipid content increased linearly with salinity in range of 0-20 g/L NaCl. The optimal lipid production was achieved as 54.5 % and 110.5 mg/L d-1 at 10 g/L NaCl. Further comparing the metabolomics and proteomics data at 0 and 10 g/L NaCl, the adenosine triphosphate (ATP) and nicotinamide adenine nucleotide phosphate (NADPH) were found significantly increased to quench the Na+-induced ROS. The created NADPH and ATP accord the enhanced activity of the upper Pyruvate-Malate cycle and tricarboxylic acid cycle (TCA) cycle. Therefore, ROS detoxification is proposed to be the essential mechanism for microalgal lipid accumulation under salt stress. The cellular triacylglycerol (TAG) synthesis triggered by salinity is mainly mediated by an acetyl-CoA-dependent Kennedy pathway. These findings will deepen our understanding of the nexus of ambient salt, intracellular ROS and lipid metabolism for microalgae and advance the exploit of saline wastewater for microalgal lipid production.