Competition between dissimilatory nitrate reduction to ammonium and denitrification in marine sediments

Abstract

Nitrogen is one of the essential elements for all living organisms, as is it a constituent of many important biomolecules. Traditionally, stated as one of the factors limiting biological productivity in the marine realm, high concentrations of fixed nitrogen are now one of the biggest challenges for marine ecosystems. With the industrialization of the chemical conversion of N2 to NH3, an enormous supply of nitrogen-based fertilizer in agriculture started. Washed out from farmland, nitrogen fertilizers enter river networks through which they arrive in coastal marine areas. Nowadays, aquatic ecosystems and especially coastal ecosystems are increasingly affected by his artificial nitrogen input often resulting in man-made eutrophication. Hence, eutrophication of an ecosystem is tightly coupled to the understanding of the nitrogen cycle and their controlling environmental factors. The aim of the thesis was to get a deeper insight into the biogeochemical nitrogen cycle in coastal marine sediments, with particular emphasis on the relative importance of dissimilatory nitrate reduction to ammonium (DNRA)in comparison to denitrification (DEN). Even though both processes reduce NO3-, only DEN, the reduction from NO3- to N2, removes fixed nitrogen from coastal sediments, thus counteracting eutrophication. In contrast,DNRA, preserves nitrogen as NH4 in a bioavailable form inside the ecosystem, possibly maintaining eutrophication. Therefore, the balance between these two processes and the environmental factors influencing this balance play a crucial role in eutrophic marine ecosystems as thereby the N-loss and N-recycling of an ecosystem is defined. Environmental conditions often regarded as controlling factors of the competition between DEN and DNRA include the Corg/NO3- ratio, availability of inorganic electron donors (e.g., sulfide and iron) or temperature. However, until now, a direct comparison of these two NO3- reducing processes, inside the zone of NO3- reduction, in relation to the environmental factors was limited by the available methods for the detection of DNRA profiles in sediments. This thesis presents the first method to measure depth-resolved near in situ activity of DNRA in intact freshwater and marine sediment cores (Chapter 2). The combined gel probe and isotope labelling technique allows the direct comparison of DNRA and DEN activity and an insight into the geochemical environmental factors inside the intact zone of NO3 reduction. In a second step, the novel gel probe method was applied to five different coastal sediments that differed in several environmental and sediment parameters (Chapter 2 and 3). The method proved to be a useful extension of the current methods used for the detection of DNRA activity profiles in intact sediment cores. However, the controlling factors for the two dissimilatory nitrate reduction processes, DEN and DNRA, could not be unravelled (Chapter 3). Despite the geochemical differences between the sediments, DEN was the dominant NO3- reduction process and DNRA was only detectable on a consistently low background level. Moreover, two bioreactors for the treatment of NO3--contaminated saline wastewater were operated to favour either DEN or DNRA, but showed both the same unexpected pattern with dominance of DEN activity and only low DNRA activity (Chapter 4). The work presented in this thesis highlights that the choice of methodology for the detection of DNRA activity in marine sediments is of vast importance, as inappropriate methods may significantly influence the partitioning between DEN and DNRA and thus lead to false conclusions (Chapter 3). In this thesis, factors commonly assumed to have an influence on the competition between DEN and DNRA for NO3 proved not to have the highest selective pressure on either process (Chapter 3 and 4). Therefore, other factors, than the one investigated, that have a higher selective priority on the competition for NO3 have to be considered. Supported by the results of this, DNRA should be regarded as a quantitatively less important NO3 removing process in marine sediments and the hypothesized shift towards DNRA under man-made eutrophic conditions is not expected.