The bacteria associated with oceanic algal blooms are acknowledged to play important roles in carbon, nitrogen, and sulfur cycling, yet little information is available on their identities or phylogenetic affiliations. Three culture-independent methods were used to characterize bacteria from a dimethylsulfoniopropionate (DMSP)-producing algal bloom in the North Atlantic. Group-specific 16S rRNA-targeted oligonucleotides, 16S ribosomal DNA (rDNA) clone libraries, and terminal restriction fragment length polymorphism analysis all indicated that the marine Roseobacter lineage was numerically important in the heterotrophic bacterial community, averaging >20% of the 16S rDNA sampled. Two other groups of heterotrophic bacteria, the SAR86 and SAR11 clades, were also shown by the three 16S rRNA-based methods to be abundant in the bloom community. In surface waters, the Roseobacter, SAR86, and SAR11 lineages together accounted for over 50% of the bacterial rDNA and showed little spatial variability in abundance despite variations in the dominant algal species. Depth profiles indicated that Roseobacter phylotype abundance decreased with depth and was positively correlated with chlorophyll a, DMSP, and total organic sulfur (dimethyl sulfide plus DMSP plus dimethyl sulfoxide) concentrations. Based on these data and previous physiological studies of cultured Roseobacter strains, we hypothesize that this lineage plays a role in cycling organic sulfur compounds produced within the bloom. Three other abundant bacterial phylotypes (representing a cyanobacterium and two members of the ␣ Proteobacteria) were primarily associated with chlorophyll-rich surface waters of the bloom (0 to 50 m), while two others (representing Cytophagales and ␦ Proteobacteria) were primarily found in deeper waters (200 to 500 m).The bacterial communities associated with oceanic algal blooms play critical roles in carbon and nitrogen cycling through their influence on the formation and fate of dissolved organic matter (4, 7), nutrient availability (24), sinking flux (45), and many other processes. In blooms dominated by algal species that produce dimethylsulfoniopropionate (DSMP), bloom-associated bacteria also play an important role in organic-sulfur cycling. Degradation of DMSP by marine bacteria is one of the primary routes for the formation of dimethyl sulfide (DMS), a volatile sulfur compound that influences global climate through effects on backscatter and cloud formation (6). Recent studies have suggested that marine bacteria may control DMS formation through the expression of a competing pathway that routes the sulfur in DMSP through methanethiol (MeSH) rather than to DMS (21, 27, 46). New evidence is pointing to one particular lineage of marine bacteria as a key participant in DMSP biogeochemistry in the ocean. Both culture-independent (i.e., 16S rRNA-based) and culture-dependent studies indicate that members of the ␣ Proteobacteria belonging to the Roseobacter lineage are abundant in coastal and open-ocean environments (15,17,18), where they ar...

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The composition of bacterial communities growing at the expense of high-molecular weight (HMW; >1000 Da) and low-molecular weight (LMW; <1000 Da) fractions of dissolved organic carbon from a southeastern US estuary was determined by sequencing and terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene amplicons in 2 enrichment studies. 16S rDNA sequence analysis indicated that the bacterial communities growing on the LMW fraction were dominated by γ-and ε-Proteobacteria related to Pseudomonas fluorescens and Arcobacter nitrofigilis (accounting for 90% of the clones) while the communities using the HMW fraction were dominated by α-, β-, and γ-Proteobacteria and Cytophaga-Flexibacter-Bacteroides related to Rhizobium-Agrobacterium, Janthinobacterium lividum, Pseudomonas fluorescens, Marinobacterium georgiense, Pseudoalteromonas, and Sphingobacterium comitans (accounting for 98% of the clones). Methylotrophic bacteria were present in the inoculum community but not found in either LMW or HMW enrichments. T-RFLP analysis of the enrichment communities showed measurable changes in community composition during the enrichment period, and companion respiration assays confirmed utilization of sufficient HMW and LMW carbon to support several bacterial generations. Although the composition of the estuarine inoculum used for the 2 enrichment studies (conducted in April 1997 and May 1999) was quite similar, the communities developing on the HMW and LMW fractions differed between experiments, potentially reflecting temporal variations in the chemical composition of the dissolved organic carbon. KEY WORDS: Dissolved organic carbon (DOC) · Ultrafiltration · Terminal restriction fragment length polymorphism (T-RFLP) analysis · Bacterial community composition · 16S rDNAResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 25: 127-139, 2001 molecular weight (LMW) components of DOC (DOC less than 1000 Da) can contain highly labile sugars and amino acids (Coffin 1989, Keil & Kirchman 1991 as well as the refractory remains of larger biomolecules that have already been extensively processed microbially (Amon & Benner 1996). In specific investigations of DOC turnover rates of molecular weight fractions, Ford & Lock (1985) and Meyer et al. (1987) found the LMW fraction supported more bacterial growth in several river systems, while Amon & Benner (1994, 1996 found the HMW fraction to be the most bioavailable in open ocean settings.To this point, studies of DOC cycling in aquatic ecosystems have focused primarily on the rates at which bacteria utilize DOC, with little emphasis on the identity of the bacteria responsible for uptake. This 'microbial black box' approach has been dictated by the difficulties involved in culturing environmental bacteria, which prevent detailed study of the identity and capabilities of individual bacteria. Techniques borrowed from molecular biology, however, have revolutionized our understanding of bacteria in the environment by...

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