The marine sediments around Svalbard and in the Northern Barents Sea experience permanently low temperatures around 0°C. At the northeast coast of Svalbard the carbon supply to the sediments is very limited due to long periods of ice coverage, restricting primary production. In contrast the west coast is influenced by relatively warm Atlantic water and a higher deposition of organic material to the sea floor. Thus, the benthic microbial community, which degrades organic matter by a series of respiratory pathways, of the northeast coast of Svalbard is probably more limited by the organic carbon contents compared to the sediments of the west coast.

To determine the rates and importance of the respiratory processes in the sediments around Svalbard four sites were investigated: a fjord of the west coast of Svalbard and three sites off the northeast coast in the open Barents Sea. Special focus was laid on the regulation and contribution of microbial Mn(VI) and Fe(III) reduction to the degradation of organic material in Arctic sediments. Isolation and characterization of Mn- and Fe-reducing bacteria allowed to investigate their potential in situ activities and their adaptation to environmental settings.

Psychrophilic and psychrotolerant Fe-reducing and sulfate-reducing bacteria were isolated from fjord sediments at the west coast of Svalbard. All isolates were able to grow at –2°C, the freezing point of sea water, showing that they are adapted to the permanently low temperatures. As electron donors all strains utilized important fermentation products found in marine sediments. The isolates reduced Fe(III), however, strains related to Desulfovibrio gained no energy for growth from this process. Thus, a contribution of sulfate-reducing bacteria to benthic Fe reduction might be possible. In addition to Fe(III) the bacteria reduced other electron acceptors such as oxygen, Mn(IV), elemental sulfur, or sulfate.

Aerobic respiration and sulfate reduction were the most important respiration processes in the fjord sediment of the west coast. Microbial Fe reduction in 0-10 cm sediment depth was the third important pathway with a contribution of 13% to total carbon oxidation. Rates of total oxygen uptake and anaerobic carbon oxidation of the fjord sediment were as high as in comparable permanently cold and temperate sediments indicating that the microbial community is adapted to the permanently low temperatures. At the three sites of the northeast coast of Svalbard the oxygen uptake and anaerobic carbon oxidation rates of were substantially lower. Here microbial Mn and Fe reduction were the most important anaerobic respiratory processes contributing 69 to at least 90% to anaerobic carbon oxidation. Thus, in regions with relatively low input of organic carbon – here due to the influence of cold currents and therefore longer periods of sea ice coverage - the suboxic processes such as microbial Mn and Fe reduction gain in importance while sulfate reduction was below the detection limit at two of the three sites. In conclusion the microbial Mn- and Fe-reducing community in the fjord sediment seemed to be limited by the low concentration of Mn(IV) and Fe(III) while in the sediments of the northeast coast it was limited by the low input of organic biomass from the water column. I could show that low Arctic temperatures are not limiting benthic microbial process but that the flux of organic material to the sea floor controls the mineralization and the contribution of the different respiratory pathways.