Culture-independent characterization of DOC-transforming bacteriplankton in coastal seawater
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Bacterially-mediated transformation of aromatic monomers and organic osmolytes, two important components of the dissolved organic (DOC) pool in coastal seawater, is significant in the biogeochemical cycling of essential elements including carbon, sulfur and nitrogen. A study of bacterioplankton responding to the addition of 20 ¼M dimethylsulfoniopropionate (DMSP), a sulfur-containing organic osmolyte, suggested that a subset of the bacterial community could degrade DMSP. Cells developing high nucleic acid content in the presence of DMSP included members of ±-Proteobacteria (mainly in the Roseobacter clade), ²-Proteobacteria and ³-Proteobacteria, and a lower number of Actinobacteria and Bacteroidetes. The relative importance of DMSP-active taxa varied seasonally. Another study tested whether aromatic monomers and organic osmolytes are transformed by specialist bacterial taxa. The taxonomy of coastal bacterioplankton responding to the addition of 100 nM organic osmolyte [DMSP or glycine betaine (GlyB)] or aromatic monomer [para-hydroxybenzoic acid (pHBA) or vanillic acid (VanA)] was determined using incorporation of bromodeoxyuridine (BrdU) to track actively growing cells. 16S rDNA clones of active bacterioplankton indicated that both types of DOC were transformed by bacterial assemblages composed of the same major taxa, including Actinobacteria, Bacteroidetes, ±-Proteobacteria (mainly members of the Roseobacter clade), ²-Proteobacteria, and ³-Proteobacteria (mainly members of Altermonadaceae, Chromatiaceae, Oceanospirillaceae and Pseudomonadaceae). The relative abundance of each taxon differed, however. Members of the OM60/241 and OM185, SAR11, SAR86 and SAR116 bacterioplankton groups in the active cell group indicated the ability to transport and metabolize these compounds by these ubiquitous but poorly described environmental clusters. In a final study, the phylogenetic diversity and functional capability of bacterioplankton stimulated by the addition of DMSP or VanA were explored by metagenomic analysis. Metagenomic sequences revealed a similar taxonomic structure for the bacterioplankton enriched with both DOC types, and were generally consistent with the PCR-based 16S rDNA analysis of the same template. Likewise, similar distribution patterns of functional gene categories were found for the two enriched bacterioplankton communities. These results indicate that DMSP and VanA were transformed by metabolic generalists capable of degrading both compounds. These studies provide insights into the response of coastal bacterial communities to changing environmental conditions in an ecological time frame.