Prophage induction shifts community composition and functional capacity in a <i>Sargassum</i> -derived multispecies biofilm

Abstract Background Pelagic Sargassum has undergone significant range expansion and dramatic blooms in the Atlantic over the past 15 years. This algae’s microbiome provides symbiotic functions that are believed to contribute to its ecological success. Recent research shows that Sargassum -associated bacteria are enriched in integrated prophages compared to the surrounding seawater and that these prophages are inducible by chemical and ultraviolet treatment. Results Here, we investigated a Sargassum -derived in vitro multispecies biofilm encompassing the dominant heterotrophic microbial members associated with Sargassum to probe the impacts of prophage induction on the composition of Sargassum microbiomes. Induction was quantified by coverage-based virus-to-host ratios in chemically induced treatments with Mitomycin C and non-induced controls, and the community composition and metabolic profiles were analyzed after a period of recovery post-induction. Chemical induction led to a significant increase in abundance and virus-to-host ratio of viral genomes linked to Vibrio metagenome-assembled genomes. This was accompanied by altered biofilm community composition, with a reduction in Vibrio bacterial abundance that opened niche space for other biofilm members in the genera Pseudoalteromonas , Alteromonas , and Cobetia . The induced Vibrio -associated phages encoded genes involved in quorum sensing, biofilm formation, virulence, and host metabolism. Induction led to a relative loss of 17 metabolic modules, including functions related to energy metabolism and nitrogen utilization. Conclusion Due to the high frequency of lysogeny in the Sargassum microbiome and the susceptibility of prophages to chemical and ultraviolet light induction, these results suggest that prophage integration and induction are mechanisms that significantly contribute to structuring the Sargassum microbiome and its functional profiles, potentially aiding in microbiome flexibility in changing environmental contexts.