Community structure of delta-proteobacteria associated with pathways of microbial respiration in vegetated and unvegetated intertidal mudflat

Abstract

The objectives of this study were to relate the major pathways of anaerobic microbial respiration to molecular phylogenetic evidence in the unvegetated mudflat (UMF) and vegetated mudflat (VMF) of the intertidal sediments of Ganghwa Island, South Korea. Depth integrated C mineralization rates were 5 times higher at VMF (351 mmol C m-2 d-1) in comparison to UMF (73 mmol C m-2 d-1), and sulfate reduction rates at VMF (35 mmol S m-2 d-1) were 2 times higher than at UMF (18 mmol S m-2 d-1). Dissimilatory Fe(III) reduction rates were 5 times higher at VMF (525 mmol m-2 d-1) in comparison to UMF (103 mmol m-2 d-1). Higher rates of microbial respiration at the VMF site indicated that actively growing plant roots and rhizomes excrete dissolved organic matter, fueling anaerobic bacterial metabolism. Dissimilatory sulfate- and Fe(III) reduction accounted for 49.3% and 35.6%, respectively, of anaerobic C oxidation at UMF site, but represented 19.9% and 37.3%, respectively at VMF site. The results indicated that both sulfate- and Fe(III) reduction play a significant role in C oxidation pathways in UMF site, whereas Fe(III) reduction was more significant at VMF site, and thus vegetation (i.e., Suaeda japonica) plays a significant role in regulating the rates and pathways of C mineralization. Clones related to δ-Proteobacteria accounted for 25.2% of 16S rRNA gene sequences for 539 clones retrieved in clone libraries, which was a good agreement with the results of T-RFLP analysis. Relative abundance of each phylotype of the δ-Proteobacteria was markedly different with depth and site. Phylotypes affiliated with known Fe(III) or So reducing bacteria (Desulfuromonas/Pelobacter) were abundant in clone libraries from UMF (32.4%) and VMF (48.6%) sediments at deeper depths, whereas sequences closely related to sulfate reducers (Desulfobacterium and Desulfobulbus) were retrieved from all depths of the sediments sampled. Interestingly, the majority of Desulfuromonas/Pelobacter-like clone sequences (82%; 32 out of total 39 clones) were previously uncultured, and a major fraction (56.2%; 18 among 32 clones) of the uncultured Desulfuromonas/Pelobacter group was observed in the rhizosphere of VMF site (VMF-5 cm co). Therefore, the enhanced significance of Fe(III) reduction to C oxidation at the VMF site paralleled with the molecular phylogenetic analysis of the δ-Proteobacteria. Overall, the combination of microbial community characterization and elucidation of C oxidation pathways points to several anaerobic microbial groups that are likely to be responsible for specific carbon oxidation pathways in dynamic intertidal marine sediments.