남해안 가두리 어류 양식장에서의 생지화학적 황 순환과 관련된 미생물 생태 연구

Abstract

The sediment under caged finfish farm is generally characterized by high organic loading resulting from uneaten fish food and fecal materials, which accelerates anaerobic mineralization of the organic matter such as sulfate reduction. We investigated (1) the vertical distribution of biogeochemical conditions, SRR and the abundance and diversity of SRP in the fish farm environment using dissimilatory sulfite reductase (dsr) gene, and (2) community structures of microorganisms associated with the sulfur cycles with special emphasis on the Epsilonproteobacteria in surface sediment underlying the caged finfish (rockfish and sea bream) aquaculture near Yeosu in the south coast of Korea. Hydrogen sulfide (H2S) and ammonium (NH4+) in pore-water of the farm sediment (11.8 mmol S m-2, 106 mmol N m-2) were respectively 19 and 1.5 times higher than that of the control site (0.62 mmol S m-2, 72 mmol N m-2). Accordingly, the SRR was 19 times higher at the farm site (118 mmol S m-2 d-1) than at the control site (6.2 mmol S m-2 d-1). Depth-integrated prokaryotic cell number estimated from 16S rRNA gene copy number was similar at both sites (16.9 x 1014 cells m-2, 23.3 x 1014 cells m-2, respectively), but the abundance of SRP estimated from dsr gene copy number was 1.8 times higher at the farm site (1.8 x 1014 cells m-2) compared to the control site (1.0 x 1014 cells m-2). Composition of SRP communities revealed from the dsr gene libraries appeared similar at both sites. The most predominant members of SRP were affiliated to Syntrophobacteraceae group (58% and 66% of total dsr gene at the farm and control sites, respectively) that is known to oxidize incompletely propionate, private and lactate using sulfate as an electron acceptor. The second dominant SRP group (22% and 20% of total dsr gene clones at farm and control sites, respectively) was closest to the clone TopDsr59 in Pearl River sediment (similarity, 87%), and was not affiliated to known SRP. Desulfobacter-Desulfobulbus was detected as third dominant group (8% and 9% at the farm and control sites, respectively). Analysis of 16S rRNA gene libraries revealed that major bacterial groups, including Gamma-, Delta-, and Epsilonproteobacteria, were closely affiliated with sulfur cycle. Most Epsilonproteobacteria belonged to Sulfurovum lithotrophicum, that use a variety of sulfur compounds (elemental sulfur or thiosulfate) as an electron donor and oxygen or nitrate as an electron acceptor. Relative abundance of Epsilonproteobacteria exhibited the highest by comprising 54.5% of total sequences in the 1 ― 2 cm depth of the farm site where sulfate reduction rates appeared maximum value without an accumulation of sulfide, the metabolic product of sulfate reduction. Overall, fish farm sediment was characterized by remarkably reduced geochemical condition and enhanced abundance of SRP and SRR. Predominance of incomplete organic matter oxidizing SRP (i.e., Syntrophobacteraceae and Desulfobulbus) further indicated that microbial composition of SRP at both sites were highly affected by excess loading of artificial organic matter such as fish food. And the results implied that the Epsilonproteobacteria played a significant role as a sulfur oxidizer in the highly sulfidic fish farm sediment.