Enrichment of vitamin B12-producing Porphyrobacter in the phycosphere microbiome promotes microalgal stress adaptation to antibiotic exposure.

BACKGROUND: Planktonic microalgae deploy multifaceted responsive and adaptive strategies against anthropogenic pollutants; however, current understanding of antibiotic resistance mechanisms remains predominantly focused on intrinsic physiological adaptations. While microalgae maintain intimate relationships with the phycosphere microbiome, the ecological roles of these associated microbes in mediating host adaptation to polluted environments are inadequately characterized. RESULTS: We identified a phycosphere microbiome-involved antibiotic resistance mechanism in Dictyosphaerium sp., a pollution-tolerant Chlorophyta microalgae exhibiting remarkable enrofloxacin (ENR) tolerance. Microalgal growth displayed initial inhibition followed by significant promotion under 5 mg/L ENR exposure. This resilience was associated with the restructuring of phycosphere microbiome, characterized by Porphyrobacter enrichment and functional enhancement of algal fitness-promoting pathways, including upregulation of cobalamin biosynthesis genes (log2FC = 7.76) and a 33.3-fold increase in extracellular B12 accumulation. Consequently, we isolated the ENR-selected microbial taxa to elucidate their roles in microalgal stress adaptation. Co-culturing axenic Dictyosphaerium sp. with Porphyrobacter enhanced microalgal growth by 36.5% after 8-day ENR exposure, whereas non-dominant bacteria exhibited negligible effects. Based on the transcriptomic and metabolomic analyses of the algal system when Porphyrobacter was dominant, we subsequently compared the growth of axenic microalgae with and without B vitamin (B1, B6, B7, B12) supplementation. Experimental validation demonstrated the pivotal role of B12-producing Porphyrobacter in enhancing microalgal ENR adaptation through (i) stimulating extracellular polymeric substances production and subsequently enhancing ENR removal via EPS-mediated adsorption and (ii) alleviating intracellular oxidative stress via elevating superoxide dismutase and peroxidase activities and reducing malondialdehyde levels. Additionally, this B12-producing bacteria/B12-mediated adaptability exhibited cross-species conservation, improving ENR resistance in Chlorella vulgaris and Scenedesmus quadricauda, with analogous protection observed under ciprofloxacin and norfloxacin exposures. CONCLUSION: Collectively, our findings establish stress-induced enrichment of B12-producing Porphyrobacter within the phycosphere microbiome as a pivotal mechanism underlying microalgal antibiotic adaptation. This insight facilitates the rational development of microalgae-microbiome systems for enhanced wastewater treatment and sustainable bioproduction, with applications in aquatic feed supplementation, biofuel production, and biofertilizer development. Video Abstract.