Nitrogen supplemented by symbiotic Rhizobium stimulates fatty-acid oxidation in Chlorella variabilis

Abstract

Large-scale cultivation of microalgae benefits from co-cultivation with model growth-promoting microorganisms to increase algal biomass and lipids production. Nevertheless, the effect of natural symbiotic bacteria on microalgae cultivation and lipid production is still unclear. Here, the effects of nitrogen-fixing, symbiotic Rhizobium on nitrogen content and algal growth, lipid accumulation, and gene expression were investigated in the green microalga Chlorella variabilis. The results demonstrated that compared to axenic C. variabilis cultures, the bacterial-derived nitrogen supply and algal growth were enhanced by 32.8% and 27% in cocultures, respectively. Transcriptome analyses revealed that C. variabilis growing with R. radiobacter TH729 upregulated genes involved in fatty acid oxidation and downregulated genes related to nitrogen metabolism. Our findings indicated that bacterial fixed nitrogen reduces lipid accumulation in microalgae, which is not beneficial for biodiesel production. Further studies on the effects of other bacteria on microalgae are needed to assess most optimal practices for biodiesel production.