Abundant and diverse bacteria involved in DMSP degradation in marine surface waters

Abstract

An expanded analysis of oceanic metagenomic data indicates that the majority of prokaryotic cells in marine surface waters have the genetic capability to demethylate dimethylsulfoniopropionate (DMSP). The 1701 homologues of the DMSP demethylase gene, dmdA, identified in the (2007) Global Ocean Sampling (GOS) metagenome, are sufficient for 58% (+/-9%) of sampled cells to participate in this critical step in the marine sulfur cycle. This remarkable frequency of DMSP-demethylating cells is in accordance with biogeochemical data indicating that marine phytoplankton direct up to 10% of fixed carbon to DMSP synthesis, and that most of this DMSP is subsequently degraded by bacteria via demethylation. The GOS metagenomic data also revealed a new cluster of dmdA sequences (designated Clade E) that implicates marine gammaproteobacteria in DMSP demethylation, along with previously recognized alphaproteobacterial groups Roseobacter and SAR11. Analyses of G+C content and gene order indicate that lateral gene transfer is likely responsible for the wide distribution of dmdA among diverse taxa, contributing to the homogenization of biogeochemical roles among heterotrophic marine bacterioplankton. Candidate genes for the competing bacterial degradation process that converts DMSP to the climate-active gas dimethylsulfide (DMS) (dddD and dddL) occur infrequently in the (2007) GOS metagenome, suggesting either that the key DMS-producing bacterial genes are yet to be identified or that DMS formation by free-living bacterioplankton is insignificant relative to their demethylation activity.