The synergistic effects of gamma-aminobutyric acid and salinity during the enhancement of microalgal lipid production in photobioreactors

In the present work, combined application of gamma-aminobutyric acid (GABA) and salinity was used to stimulate biolipid production in the microalga Ankistrodesmus sp. EHY in a vertical tubular photobioreactor. Under 2.5 g L-1 NaCl combined with 50 mu M GABA, the strain showed 1.36-and 1.27-fold increases in lipid content (59.42%) and lipid productivity (235.13 mg L-1 d(-1)), respectively, compared with the control. Wide-ranging analyses of the energy yield, biodiesel quality, cellular ultrastructure, signalling molecules, and lipo-genesis genes were conducted to elucidate the role of GABA plus salinity on algal lipid accumulation. By using GABA and salinity treatments, the triacylglycerol (TAG) content and energy yield were increased, whereas the carbohydrate yield was decreased. The biodiesel quality of the algal biomass-derived lipids met the approved standards. The combinational strategy also increased the levels of endogenous reactive oxidative species (ROS), Ca2+, an autophagy-related protein (ATG8), glutathione (GSH) and proline, which might be associated with the increase in lipid synthesis and salt stress tolerance. Moreover, transcriptomic and metabolomic data indicated that the expression levels of TAG assembly, antioxidative system and autophagy-related genes, and the metabolite levels of the TCA cycle, GABA shunt and several stress hormones were consistent with the enhancements in lipid accumulation and stress tolerance. In the presence of GABA and salinity, exogenous proline (50 mu M) and salicylic acid (SA; 12.5 mg L-1) further increased the lipid productivity to 280.48 mg L-1 d(-1) and 263.58 mg L-1 d(-1), respectively. Overall, the results demonstrate the significance, mechanism and applicability of using exogenous GABA combined with salinity to effectively stimulate bioenergy production by microalgae.