Effects of Antibiotic and Temperature Treatments on Phycosphere Bacterial Communities of Kelp Gametophytes.

Microbiome dysbiosis represents a critical bottleneck in sustainable kelp aquaculture, yet the distinct regulatory mechanisms of antibiotics and temperature remain poorly characterized. This study systematically elucidates the divergent effects of penicillin-streptomycin exposure and temperature gradients on Saccharina japonica gametophytes and their epiphytic microbiota. Antibiotic treatment significantly reduced Proteobacteria from 93.02% to 49.70% and increased Bacteroidetes from 6.98% to 49.20%, replacing beneficial Sulfitobacter and Colwellia with opportunistic Kordia and Klebsiella, indicating severe dysbiosis driven by the creation of unoccupied ecological niches. In contrast, temperature variation did not alter dominant phyla: Proteobacteria remained predominant, but Vibrio peaked at 15°C (44.11%) alongside the maximal gametophyte specific growth rate of 5.21% per day, concurrently elevating pathogenic loads of Vibrio and Pseudomonas. These findings reveal that antibiotics disrupt core community architecture, whereas temperature modulates metabolic rates within stable taxonomic frameworks. Consequently, we propose a stratified preservation strategy: short-term seedling cultivation at 15°C requires stringent antimicrobial measures to mitigate disease risk, while lower temperatures are preferable for long-term germplasm storage to minimize contamination potential. This framework provides essential insights for precision microbial management, balancing growth promotion with disease control in kelp aquaculture systems.