Endophyte Bacillus tequilensis improves the growth of microalgae Haematococcus lacustris by regulating host cell metabolism

[Display omitted] •Endosymbiont Bacillus tequilensis was identified in Haematococcus lacustris cells.•Optimal interaction enhanced growth and astaxanthin content in H. lacustris.•The fatty acid content of H. lacustris improved 1.99-fold under optimized conditions.•Interaction upregulated amino acid and nucleotide metabolism in H. lacustris.•Bacillus tequilensis was observed to delay pigment degradation in H. lacustris. This study identified an endosymbiotic bacterium, Bacillus tequilensis, residing within the cells of the microalga Haematococcus lacustris through 16S rRNA analysis. To confirm the optimal interactive conditions between H. lacustris and B. tequilensis, the effects of different ratios of cells using H. lacustris of different growth stages were examined. Under optimized conditions, the cell density, dry weight, chlorophyll content, and astaxanthin content of H. lacustris increased significantly, and the fatty acid content improved 1.99-fold. Microscopy demonstrated the presence of bacteria within the H. lacustris cells. The interaction upregulated amino acid and nucleotide metabolism in H. lacustris. Interestingly, muramic and phenylacetic acids were found exclusively in H. lacustris cells in the presence of B. tequilensis. Furthermore, B. tequilensis delayed pigment degradation in H. lacustris. This study reveals the impact of the endosymbiont B. tequilensis on the metabolism of H. lacustris and offers new perspectives on the symbiotic relationship between them.