Glycerol changes the growth and lipid profile of the marine microalga Isochrysis galbana via the regulation of photosynthetic and respiratory metabolic pathways

Isochrysis galbana is widely used in the aquaculture industry and is a potential industrial biodiesel producer. The growth of I. galbana has been enhanced in mixotrophic conditions with glycerol supplementation. Nevertheless, the effect mechanism of glycerol on I. galbana remains unclear. This study aimed to reveal how glycerol supplementation (0.1% by volume fraction) altered the growth, photosynthesis, respiration, fatty acid profile and transcriptome of I. galbana. Glycerol was shown to favor the growth of I. galbana under light. The increase in oxygen uptake rate indicates that glycerol is involved in the respiratory carbon metabolism of I. galbana. Furthermore, the added glycerol downregulated the photosynthetic activity of the alga, with a decrease in light-harvesting pigments, i.e., chlorophyll c and carotenoids, and a decrease in photosynthetic electron transfer activity as indicators. In addition, the fatty acid profile of I. galbana was affected by the supplementation of glycerol with a pronounced increase in oleic acid to 36.47%. Meanwhile, the ratio of Sigma(saturated fatty acids + monounsaturated fatty acids)/Sigma(polyunsaturated fatty acids) in glycerol-treated samples (3.43) was significantly higher than that in control samples (2.13), indicating a better composition for biodiesel. Transcriptomic analysis further revealed the molecular mechanisms involved in the impacts of glycerol on I. galbana. DEGs enriched to photosynthesis, light harvesting, chlorophyll binding, and chloroplast thylakoid membrane were mainly downregulated, and the DEGs enriched in translation, nitrate transport, stearoyl-CoA 9-desaturase activity were mainly upregulated, consistent with the results of photosynthesis, biomass and the fatty acid profile. This study demonstrated the essential role of 0.1% glycerol in accelerating the growth and altering the fatty acid profiles of I. galbana, as well as the preliminary molecular mechanism underlying these physiological changes.