Mobile genetic elements used by competing coral microbial populations increase genomic plasticity.

Intraspecies diversification and niche adaptation by members of the Vibrio genus, one of the most diverse bacterial genera, is thought to be driven by horizontal gene transfer. However, the intrinsic driving force of Vibrio species diversification is much less explored. Here, by studying two dominant and competing cohabitants of the gastric cavity of corals, we found that a phenotype influencing island (named VPII) in Vibrio alginolyticus was eliminated upon coculturing with Pseudoalteromonas. The loss of VPII reduced the biofilm formation and phage resistance, but activated motility, which may allow V. alginolyticus to expand to other niches. Mechanistically, we discovered that the excision of this island is mediated by the cooperation of two unrelated mobile genetic elements harbored in Pseudoalteromonas spp., an integrative and conjugative element (ICE) and a mobilizable genomic island (MGI). More importantly, these mobile genetic elements are widespread in cohabitating Gram-negative bacteria. Altogether, we discovered a new strategy by which the mobilome is employed by competitors to increase the genomic plasticity of rivals.