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

Abstract

AbstractEmiliania huxleyiis a bloom forming microalga that impacts the global sulfur cycle by producing large amounts of dimethylsulfoniopropionate (DMSP) and its volatile metabolic product dimethyl sulfide (DMS). Top-down regulation ofE. huxleyiblooms is attributed to viruses and grazers, however, the possible involvement of algicidal bacteria in bloom demise is still elusive. We isolated from a North AtlanticE. huxleyibloom aRoseobacterstrain,SulfitobacterD7, which exhibited algicidal effects againstE. huxleyiupon co-culturing. Both the alga and the bacterium were found to co-occur during a naturalE. huxleyibloom, therefore establishing this host-pathogen system as an attractive, ecologically relevant model for studying alga-bacterium interaction in the oceans. During interaction,SulfitobacterD7 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 whichE. huxleyistrains that exuded higher amounts of DMSP were more susceptible toSulfitobacterD7 infection. Intriguingly, exogenous application of DMSP enhanced bacterial virulence and induced susceptibility in a resistant algal strain to the bacterial pathogen. This DMSP-dependent pathogenicity was highly specific as compared to supplementation of propionate and glycerol. We propose a novel function for DMSP, in addition to its central role in mutualistic interactions, as a mediator of bacterial virulence that may regulateE. huxleyiblooms.