Bacterial virulence against an oceanic bloom-forming phytoplankter is mediated by algal DMSP.

is a bloom-forming microalga that affects the global sulfur cycle by producing large amounts of dimethylsulfoniopropionate (DMSP) and its volatile metabolic product dimethyl sulfide. Top-down regulation of blooms has been attributed to viruses and grazers; however, the possible involvement of algicidal bacteria in bloom demise has remained elusive. We demonstrate that a strain, D7, that we isolated from a North Atlantic bloom, exhibited algicidal effects against upon coculturing. Both the alga and the bacterium were found to co-occur during a natural bloom, therefore establishing this host-pathogen system as an attractive, ecologically relevant model for studying algal-bacterial interactions in the oceans. During interaction, D7 consumed and metabolized algal DMSP to produce high amounts of methanethiol, an alternative product of DMSP catabolism. We revealed a unique strain-specific response, in which strains that exuded higher amounts of DMSP were more susceptible to D7 infection. Intriguingly, exogenous application of DMSP enhanced bacterial virulence and induced susceptibility in an algal strain typically resistant to the bacterial pathogen. This enhanced virulence was highly specific to DMSP compared to addition of propionate and glycerol which had no effect on bacterial virulence. We propose a novel function for DMSP, in addition to its central role in mutualistic interactions among marine organisms, as a mediator of bacterial virulence that may regulate blooms.