| Summary: | Nitrogen (N) is often a limiting nutrient for primary productivity in the ocean and plays a critical role in the global carbon cycle. Fixation of N2 into reactive N (Nr) by photosynthetic prokaryotes in surface waters is a primary source of N to the marine nitrogen cycle. Removal of reactive nitrogen from the marine environment is accomplished primarily through two microbial processes, denitrification and anammox, that produce gaseous N as terminal metabolic byproducts. Denitrification is the respiratory reduction of nitrate (NO3-) to gaseous end products (N2, N2O), is mediated by a phylogenetically diverse group of microbes, and may be heterotrophic or autotrophic. Anammox involves the anaerobic oxidation of ammonium (NH4+) with nitrite (NO2-) and is an autotrophic process. Only one genus, Scalindua, within the Planctomycetes has been identified as an anammox bacterium in the marine environment. Historically, all gaseous N production in the oceans was attributed to heterotrophic denitrification, but it is now clear that a more diverse group of microbes mediate Nr removal. An estimated 50 to 70% of the global removal of marine Nr occurs in sediments, and Nr removal from continental shelves comprises approximately half of the total sediment contribution. It follows that Arctic continental shelves, which contribute 18% of the global continental shelf area, may contribute significantly to global Nr removal. High export production in the subarctic Bering Sea and in the arctic Chukchi and Barents Sea fuels locally high rates of N2 production that largely overlap with those of temperate shelf sediments. It appears that the permanently cold (< 4 °C) conditions do not limit Nr removal rates from sediments in these regions of the Arctic. This lack of inherent temperature limitation has been observed for other microbial respiratory processes in permanently cold sediments and has been attributed to the activity of psychrophilic ("cold-loving") sediment microbial communities. The main objective of this dissertation was ...
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