William B. Whitman
Department of Microbiology
University of Georgia, Athens

Abstract:
Dimethylsulfoniopropionate (DMSP) is a common osmolyte of marine phytoplankton and ubiquitous in marine surface waters. DMSP is the precursor for the climatically active gas dimethylsulfide (DMS), which is the primary natural source of sulfur to the atmosphere.  DMS aids in cloud formation and is an anti-greenhouse gas.  Most of the DMSP-S released each year is processed by marine bacteria through one of two competing pathways. DMS is produced by the cleavage pathway, but only accounts for about 25% of the DMSP-S. Most of the DMSP-S is metabolized through the demethylation pathway, which leads to DMSP mineralization and sulfur assmiilation.  DMSP likely first became abundant in ocean environments about 250 mya in conjunction with the increased abundance of dinoflagellates and coccolithophores, and its metabolism by marine bacteria probably evolved after that time. In the marine alphaproteobacterium Ruegeria pomeroyi, the enzymes of the DMSP demethylation pathway appear to have been recruited from a variety of sources, especially short chain fatty acid degradation.  While some enzymes have undergone major changes in their catalyic properties, other have only undergone small changes in their substrate specificities. Similarly, R. pomeroyi possesses four unrelated DMSP ligases, the first enzyme of the cleavage pathway. The reason for this apparent redundancy is not understood.