Effects of finfish aquaculture on biogeochemistry and bacterial communities associated with sulfur cycles in highly sulfidic sediments

ABSTRACT: A combination of biogeochemical analyses and molecular microbiological analyses were conducted to assess the environmental impact of finfish aquaculture and to elucidate the major microbial assemblages responsible for the production and removal of reduced sulfur compounds in fish-farm sediments. The average concentrations of H2S (123 μM) and NH4+ (1310 μM) and the dissimilatory sulfite reductase (dsr) gene copy number (1.9 × 109 copies cm-3) in the sediments at the farm site were 15-, 1.5- and 2-fold higher, respectively, than those measured at the less-impacted reference site. Accordingly, the sulfate reduction rate (SRR) at the farm site (118 mmol m-2 d-1) was 19-fold higher than that measured at the reference site (6.2 mmol m-2 d-1). Analyses of dsrA and 16S rRNA gene sequences revealed that the Syntrophobacteraceae and Desulfobulbaceae groups are the major sulfate-reducing bacteria around the fish-farm sediment. Interestingly, despite the high SRR (12.2-19.6 mmol m-2 d-1), the H2S concentration was low (<8 μM) in the top 0-2 cm of the fish-farm sediments. In this sulfide-mismatched zone, sulfur-oxidizing bacteria associated with Gamma- and Epsilonproteobacteria were abundant. Especially at the 1-2 cm depth, bacteria related to Sulfurovum in the Epsilonproteobacteria showed the highest relative abundance, comprising 62% of the 16S rDNA sequences. The results strongly suggest that Sulfurovum-like bacteria play a significant ecological and biogeochemical role in oxidation and reduction of reduced sulfur compounds from the organic-rich, highly sulfidic fish-farm sediments.