The determination of the effect of stress conditions on lipid productivity and triacylglycerol (TAG) content in some microalgae species at the gene expression level and the investigation of biodiesel quality associated with the fatty acid profile

Microalgae can accumulate substantial amounts of triacylglycerol, the primary feedstock for biodiesel production, under different stress conditions. This study aimed to determine the high triacylglycerol-producing isolates among 30 microalgae isolates obtained from different environments and the stress conditions, which induce triacylglycerol accumulation and provide the ideal fatty acid profile. It was also purposed to determine the effect of stress conditions on the expression of some triacylglycerol biosynthesis genes. In this study, exposure to alkaline pH (pH 10) using carbonate-bicarbonate buffer, nitrogen starvation, and both were applied to induce triacylglycerol accumulation in microalgae. Our results illustrated that the combination of nitrogen starvation and alkaline pH led to significant increases in lipid productivity, triacylglycerol accumulation, and starch content of Auxenochlorella protothecoides KP7 and Chlorella vulgaris KP2 strains compared with nitrogen starvation. The highest triacylglycerol content (16.4%) and triacylglycerol/total lipid ratio (62.0%) were determined in A. protothecoides KP7 under combined stress. In this strain, under nitrogen starvation or combined stress, expression levels of phosphoenolpyruvate carboxylase, pyruvate dehydrogenase complex-E2, glycerol-3-phosphate dehydrogenase, and malic enzyme genes showed the highest level of expression. Furthermore, under the combined stress, biodiesel from A. protothecoides KP7 oil was associated with the lowest iodine value (97.64 g I2/100 g), highest oxidative stability (7.29 h), and highest cetane number (55.26) among the examined conditions. The use of the combination of nitrogen starvation and high pH stress for cultivation appears good strategy for enhancing triacylglycerol content and biodiesel quality in industrially relevant species.