Microbiological and Biogeochemical Studies of Microbial Mn(IV) and Fe(III) Reduction in Arctic Sediments (Svalbard)

Abstract

This PhD thesis focuses on the importance of microbial Mn(IV) and Fe(III) reduction for the degradation of organic material in Arctic sediments. Important regulatory mechanisms of microbial Mn and Fe reduction in Arctic sediments were identified by comparing different sediments of a fjord and the northeast coast of Svalbard. By the isolation and characterization of Mn- and Fe-reducing bacteria their potential in situ activities and their adaptation to environmental settings were investigated. The four investigated sites, a fjord of the west coast of Svalbard (Smeerenburgfjorden) and three sites at the northeast coast, were characterized by low bottom water temperatures between 2 and 3°C. Total oxygen uptake and anaerobic carbon oxidation rates of Smeerenburgfjorden sediment were as high as in comparable permanently cold and temperate sediments indicating that the microbial community is adapted to the permanently low temperature. In comparison the oxygen uptake and anaerobic carbon oxidation rates of the three sediments at the northeast coast of Svalbard were lower. At the northeast coast of Svalbard the carbon supply to the sediments is very limited due to long periods of, 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 of the northeast coast of Svalbard is probably limited by the organic carbon contents of the sediments. Microbial Fe reduction accounted for 43% of the anaerobic carbon oxidation at 0-5 cm depth of Smeerenburgfjorden sediment. The zone of microbial Fe reduction overlapped in the surface sediment of the uppermost 2 cm with the zone of sulfate reduction. High rates of organic carbon mineralization lead to a rapid depletion of Fe(III). Thus, the bacterial Fe-reducing community seemed to be mainly limited by Fe(III). Below 2 cm depth sulfate was sole electron acceptor. Due to low concentrations of solid Mn, Mn was suggested to be mainly reduced chemically by Fe(II) and sulfide. High concentrations of particulate Fe(III) and unusually high concentrations of solid Mn were measured at the three stations at the northeast coast of Svalbard. Consequently, microbial Mn and Fe reduction were the most important anaerobic respiratory processes in the sediments of the northeast coast contributing to 69 to at least 90% of anaerobic carbon oxidation. The relative contribution of Mn and Fe reduction at all four sites to carbon oxidation seemed to be regulated by the concentrations of particulate Mn and Fe(III), the carbon oxidation rate, and bioturbation. Thus, low organic carbon contents and high concentrations of particulate Mn and Fe at the northeast coast of Svalbard compared to Smeerenburgfjorden seemed to favor microbial Mn and Fe reduction. Investigations on the role of volatile fatty acids (VFA) as substrates for Fe- and sulfate-reducing bacteria showed that VFAs (acetate, propionate, and isobutyrate) were mainly consumed by sulfate-reducing bacteria as they accumulated when sulfate reduction was inhibited. The turnover of acetate and lactate accounted only for a maximum of 43% of the sulfate reduction and 6% of the Fe reduction in the sediment. Thus, other electron donors have to be utilized by the two bacterial populations. Psychrophilic and psychrotolerant Fe-reducing bacteria were enriched and isolated from two fjord stations at the west coast of Svalbard. The strains were related to species of the genera Shewanella, Desulfuromusa, Desulfuromonas, and Desulfovibrio within the Gamma- and Delta-Proteobacteria. The description as new species, Desulfuromusa ferrireducens sp. nov., Desulfuromonas svalbardensis sp. nov., Desulfovibrio ferrireducens sp. nov. and Desulfovibrio frigidus sp. nov., extends the group of Fe-reducing for psychrophilic and psychrotolerant species in pure culture. 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 of Arctic sediments. As electron donors all strains utilized important fermentation products found in marine sediments. The isolates reduced Fe, however, strains related to Desulfovibrio gained no energy for growth from this process. Accordingly, a contribution of sulfate-reducing bacteria to benthic Fe reduction might be possible. In addition to Fe the bacteria reduced other electron acceptors such as oxygen, manganese, elemental sulfur and sulfate. Additionally, a moderately thermophilic, spore-forming, sulfate-reducing bacterium was enriched and isolated at 28°C from a fjord sediment of the west coast of Svalbard. The isolated strain is related to species of the Gram-positive genus Desulfotomaculum and is proposed as new species, Desulfotomaculum arcticum sp. nov. Due to its inability to grow below 26°C the bacterium must have been present inside the sediment as an inactive spore.