| Summary: | 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 concurrently 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 different 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 (111)-reducing bacteria belonging to the genera Desulfuromonas, Desulfuromusa, Shewanella and Desulfovibrio were isolated from enrichment cultures of 2 fjord sediments from Svalbard. Strains related to Desulfovibrio reduced Fe(III) without energy generation for growth. All isolates were psychrophilic or psychro-tolerant and grew at -2 degrees 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, manganese, elemental sulfur and sulfate.
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