Members of a group of marine bacteria that is numerically important in
coastal seawater and sediments were characterized with respect to their
ability to transform organic and inorganic sulfur compounds.  Fifteen
strains representing the Roseobacter group (a phylogenetic cluster of
marine bacteria in the alpha-subclass of Proteobacteria) were
isolated from seawater, primarily from the southeastern U.S.  Although over
half the isolates were obtained without any selection for sulfur metabolism,
all were able to degrade the sulfur containing osmolyte dimethylsulfoniopropionate 
(DMSP), with production of dimethylsulfide (DMS).  Five isolates also 
degraded DMSP with production of methanethiol, indicating the unusual 
occurrence of both cleavage and demethylation pathways for DMSP in the same 
organism.  Five isolates were able to reduce DMSO to DMS, and several also 
degraded DMS and methanethiol.  Sulfite oxygenase activity and methanesulfonic 
acid oxygenase activity were also present for some of the isolates.  The 
ability to incorporate reduced sulfur in DMSP and methanethiol into cellular 
material was studied in one of the isolates.  A group specific 16S rRNA probe 
indicated that uncultured bacteria in the Roseobacter group increase 
in relative abundance in seawater enriched with DMSP or DMS.  Because this 
group typically accounts for >10% of the 16S rDNA pool in coastal seawater 
and sediments of the southern U.S., clues about its potential biogeochemical 
role are of particular interest.  Studies of culturable representatives suggest 
that the group could mediate a number of steps in the cycling of both organic 
and inorganic forms of sulfur in marine environments.