Elisabeth Münster Happel:
Microorganisms play a crutial role in remineralizing dissolved organic matter (DOM) in both oceanic and coastal environments. Due to their vast diversity and metabolic capability, they drive the biogeochemical cycles that are vital to every living cell. Nitrogen (N) is a key constituent of cells and usually limits production in aquatic environments. Diverse microbial assemblages mediate the different steps of the N cycle. There are large gaps in our knowledge on how these key processes are affected by anthropogenic pressure, especially in coastal zones. The Baltic Sea is a large coastal system which is heavily affected by anthropogenic activities. N cycling is particularly important here as it is thought to remove a substantial part of N coming from land before entering the open ocean. Projected changes in precipitation is believed to increase river discharge and thus river DOM to the Baltic Sea coastal zones. The works of this thesis address the effects of riverine DOM on microbial composition and function. Over-all community gene composition and key functional genes involved with N cycling were investigated through microcosm experiments and environmental sampling covering the highly diverse coastal zones of the Baltic Sea. The composition of functional genes are, here, used as a proxy for potential ecosystem function. The results show that although communities from different areas of the Baltic Sea are distinct, there are functional responses in gene composition to riverine DOM. Although there was no over-all response to riverine DOM in microbial composition and the composition if N cycling genes, deducible responses were linked to specific taxa and processes. Further, expression of a key functional N cycling gene revealed possible active functional responses to different riverine DOM.
Collectively, the findings of this thesis provide indications of how riverine DOM can shape microbial communities, community functional profiles and key N cycling processes. These results illustrate the potential suitability of functional genes as predictors of environmental change and thus for future environmental monitoring.