Dimethyl sulfide mediates microbial predator-prey interactions between zooplankton and algae in the ocean.

Phytoplankton are key components of the oceanic carbon and sulfur cycles. During bloom events, some species can emit large amounts of the organosulfur volatile dimethyl sulfide (DMS) into the ocean and consequently the atmosphere, where it can modulate aerosol formation and affect climate. In aquatic environments, DMS plays an important role as a chemical signal mediating diverse trophic interactions. Yet, its role in microbial predator-prey interactions remains elusive with contradicting evidence for its role in either algal chemical defence or in the chemo-attraction of grazers to prey cells. Here we investigated the signalling role of DMS during zooplankton-algae interactions by genetic and biochemical manipulation of the algal DMS-generating enzyme dimethylsulfoniopropionate lyase (DL) in the bloom-forming alga Emiliania huxleyi. We inhibited DL activity in E. huxleyi cells in vivo using the selective DL-inhibitor 2-bromo-3-(dimethylsulfonio)-propionate and overexpressed the DL-encoding gene in the model diatom Thalassiosira pseudonana. We showed that algal DL activity did not serve as an anti-grazing chemical defence but paradoxically enhanced predation by the grazer Oxyrrhis marina and other microzooplankton and mesozooplankton, including ciliates and copepods. Consumption of algal prey with induced DL activity also promoted O. marina growth. Overall, our results demonstrate that DMS-mediated grazing may be ecologically important and prevalent during prey-predator dynamics in aquatic ecosystems. The role of algal DMS revealed here, acting as an eat-me signal for grazers, raises fundamental questions regarding the retention of its biosynthetic enzyme through the evolution of dominant bloom-forming phytoplankton in the ocean.