Transformation of sulfur compounds by an abundant lineage of marine bacteria in the alpha-subclass of the class Proteobacteria

Abstract

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 the class Proteobacteria) were isolated from seawater, primarily from the southeastern United States. Although more than one-half of the isolates were obtained without any selection for sulfur metabolism, all of the isolates were able to degrade the sulfur-containing osmolyte dimethyl sulfoniopropionate (DMSP) with production of dimethyl sulfide (DMS). Five isolates also degraded DMSP with production of methanethiol, indicating that both cleavage and demethylation pathways for DMSP occurred in the same organism, which is unusual. Five isolates were able to reduce dimethyl sulfoxide to DMS, and several isolates also degraded DMS and methanethiol. Sulfite oxygenase activity and methanesulfonic acid oxygenase activity were also present in some of the isolates. The ability to incorporate the reduced sulfur in DMSP and methanethiol into cellular material was studied with one of the isolates. A group-specific 16S rRNA probe indicated that the relative abundance of uncultured bacteria in the Roseobacter group increased in seawater enriched with DMSP or DMS. Because this group typically accounts for >10% of the 16S ribosomal DNA pool in coastal seawater and sediments of the southern United States, clues about its potential biogeochemical role are of particular interest. Studies of culturable representatives suggested that the group could mediate a number of steps in the cycling of both organic and inorganic forms of sulfur in marine environments.

FIG. 1

Phylogenetic tree for bacteria belonging to the Roseobacter group and representative bacteria not belonging to this group. Sequences whose designations begin with the prefixes GAI and GAC are isolates and 16S rDNA clones, respectively, obtained from southeastern United States seawater. The shaded boxes indicate strains characterized in this study. The tree is based on positions 50 to 410 (E. coli numbering) of the 16S rRNA gene and is unrooted; Hirschia baltica is the outgroup. Bootstrap values greater than 50% are indicated. The bar indicates a Jukes-Cantor distance of 0.05.

FIG. 2

Time course for production of DMS from 20 μM DMSP for six representative isolates. The pattern exhibited by DSS-1 (○) and DSS-3 (□) was also exhibited by DSS-2 and DSS-8; the pattern exhibited by GAI-16 (◊) and GAI-37 (▿) was also exhibited by GAI-5, GAI-21, GAI-101, ISM, and S. pontiacus ChLG 10; and the pattern exhibited by GAI-109 (▵) and S. stellata E-37 (▹) was also exhibited by GAI-111. Independent tests showed that the lack of DMS accumulation in GAI-109, GAI-111, and S. stellata E-37 cultures was due to the rapid consumption of DMS that occurred simultaneously with DMS production.

FIG. 3

Pathways for degradation of organic sulfur compounds in members of the Roseobacter group. Most isolates were able to cleave DMSP to DMS, whereas five isolates also demethylated DMSP and produced methanethiol. Methanethiol was assimilated into cell material or was further degraded to inorganic sulfur compounds that could be enzymatically oxidized.

FIG. 4

Contribution of bacteria in the Roseobacter group to enrichment community 16S rDNA, based on results of hybridizations with probe MALF-1 (A) and cell numbers in enrichment cultures as determined by acridine orange direct counting (B). Enrichment cultures were prepared in duplicate. The error bars indicate ±1 standard deviation.