Seasonal composition and activity of sulfate-reducing prokaryotic communities in seagrass bed sediments
Smith, April C.
Kostka, Joel E.
Yates, Diane F.
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Sulfate-reducing prokaryotes (SRP) play a key role in the carbon and nutrient cycles of coastal marine, vegetated ecosystems; however, the interactions of SRP communities with aquatic plants remain little studied. The abundance, activity, and community composition of SRP were studied in relation to sediment geochemical gradients and plant growth state in a Thalassia testudinum seagrass bed and in adjacent unvegetated areas. Geochemical analyses indicated significantly higher concentrations of microbial respiration products in vegetated sediments during summer than during winter. Depth-integrated sulfate reduction rates were 3 to 5 times higher in vegetated (52.5 mmol m-2 d-1 in summer and 20.4 to 26.5 mmol m-2 d-1 in winter) compared to unvegetated sediments (10.7 mmol m-2 d-1 in summer and 3.6 to 7.6 mmol m-2 d-1 in winter), and depth-integrated activities further showed a strong correlation with seagrass biomass. Most probable number (MPN) counts of SRP were 10 times higher in vegetated compared to unvegetated sites in the summer during the period of maximum growth for seagrasses, whereas no difference was observed for counts between sites during the winter. The community composition of SRP was determined using restriction fragment length polymorphism (RFLP) screening and amino acid sequence comparisons inferred from partial dissimilatory bisulfite reductase (dsrA and B) genes that were PCR-amplified and cloned from DNA extracted from sediment samples. The majority of unique DSR sequences were not affiliated with any known SRP group, and clustered at levels indicative of new SRP. Some DSR sequences grouped on the basis of originating from vegetated or unvegetated sediments, although the relationship did not appear to be strong. The diversity of SRP in seagrass bed sediments, as indicated by dsr analysis, was high and did not appear to covary with the other environmental parameters tested. Our results indicate that seagrass growth state enhances the abundance and activity of SRP, while SRP community composition remains relatively stable across the environmental parameters tested.