Pathways of carbon oxidation in an Arctic fjord sediment (Svalbard) and isolation of psychrophilic and psychrotolerant Fe(III)-reducing bacteria

Abstract

The main mineralization pathways were determined in permanently cold fjord sediment on the west coast of Svalbard. In whole core incubations, the total oxygen uptake rate was 4.2 ± 0.4 mmol m -2 d -1 and the sulfate reduction rate 2.6 ± 0.6 mmol m -2 d -1 at 0 to 20 cm depth. Sulfate reduction was the most important anaerobic mineralization process, accounting for 57% of anaerobic organic carbon oxidation in anoxic bag incubations of the top 5 cm of the sediment. The remaining 43% oxidation was attributed to microbial Fe(III) reduction. Both processes occurred con- currently in the uppermost 2 cm, and the Fe-reducing community appeared to be limited mainly by the availability of Fe(III). Below 2 cm, sulfate reduction was the dominant electron-accepting process. Calculations for the uppermost 10 cm of the sediment yielded the following contribution of the differ- ent respiratory pathways to total carbon oxidation: aerobic respiration 53%, sulfate reduction 34%, Fe(III) reduction 13%. In situ, the importance of Fe(III) reduction may vary through competition for substrate with oxygen- and nitrate-reducing bacteria in the surface sediment. Fe(III)-reducing bacte- ria belonging to the genera Desulfuromonas, Desulfuromusa, Shewanella and Desulfovibrio were isolated from enrichment cultures of 2 fjord sediments from Svalbard. Strains related to Desulfovib- rio reduced Fe(III) without energy generation for growth. All isolates were psychrophilic or psychro- tolerant and grew at -2°C, the freezing point of sea water, indicating adaptation to permanently cold temperatures. Besides Fe(III), the strains reduced other electron acceptors such as oxygen, man- ganese, elemental sulfur and sulfate.