Synergistic Response of Phycosphere Bacteria with Microalga Phaeodactylum tricornutum under the 2,6-Di-tert-butyl-p-cresol Stress

The emerging contaminant 2, 6-di-tert-butyl-p-cresol (BHT) is a kind of synthetic phenolic antioxidant and can pose negative effects on the microalgal cells. Enriching specific bacteria from the phycosphere environment was an important adaptive strategy for algae cells when they are under the adverse stress. However, the mechanism of phycosphere bacteria coordinating with microalgae in response to BHT stress remains poorly understood. Herein, the effect of BHT on the microalgae Phaeodactylum tricornutum was comprehensively analyzed. Results showed that the growth and photosynthetic pigment biosynthesis of P. tricornutum was inhibited with the increase of BHT concentration. BHT also led to an increase in the content of malondialdehyde, therefore microalgae responded to the oxidative stress by enhancing activities of antioxidant enzymes, including superoxide dismutase, catalase and peroxidase, to eliminate excess reactive oxygen species in the cells. Furthermore, transcriptome analysis revealed that up-regulation of genes related to photosynthesis, TCA cycle, oxidative phosphorylation and indole-3-acetic acid (IAA) in response to BHT stress. In addition, high-throughput Illumina MiSeq sequencing results demonstrated a significant increase in the relative abundance of bacteria affiliated with Halomonas, Marivita and Oceanicaulis . Microbiological assays demonstrated that Halomonas can thrive by using BHT as the sole energy source and exhibit a chemotactic response to IAA. Therefore, we conclude that the content increase of IAA secreted by microalgae in the phycosphere environment promoted the enrichment of BHT-tolerant bacterium Halomonas , thereby it is helpful for environmental pressures adaptability of P. tricornutum . Overall, this study provided a comprehensive understanding of the physiological and biochemical effects of BHT on microalgae, and we highlight the potential functional significance of IAA in establishing an interaction between microalgae and algae-associated bacteria in adverse environments.