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PhD Project Experimentally simulated global warming and nitrogen enrichment effects on microbial litter decomposers in a marsh Many important processes in ecosystems are mediated by microorganisms. In particular, microbes are indispensable for most biogeochemical transformations, such as decomposition of organic matter and cycling of nutrients in both aquatic and terrestrial ecosystems. As atmospheric warming and increased nitrogen deposition are altering habitat conditions for organisms, the composition of species in communities may be changed, including in microbial communities, with possible effects on ecosystem processes. A key question is, therefore, to what extent rates of biogeochemical processes and other properties of ecosystems are affected by changes in microbial communities. Answering this question requires knowledge on the structure of microbial communities. In this light we determined bacterial community structure and microbial biomass and activity on decomposing litter on two occasions. Standard batches of litter that had been placed in coarse-mesh and fine-mesh litter bags were submerged in mesocosms just after leaf fall in November. They were retrieved in the following spring and summer and the litter analyzed for a range of parameters characterizing microbial communities. Denaturing gradient gel electrophoresis (DGGE) indicated that the apparent number of dominant microbial genotypes tended to increase from spring to summer, although the difference was only significant on litter from coarse-mesh bags. Cluster analyses of the DGGE community profiles also supported the idea that communities differed between sampling dates and between litter from coarse-mesh and fine-mesh bags. Furthermore, the branching pattern indicates that communities were generally influenced also by simulated global warming and, for litter from fine-mesh bags collected in summer, by nitrogen enrichment. Bacterial biomass was affected neither by warming nor by N addition, whereas fungal biomass in litter from the heated mesocosms was reduced. Where effects of global change factors emerged, they were, generally weak even when significant. Overall this data suggests that microbial communities associated with decomposing litter in freshwater marshes may be relatively robust to both extra nitrogen supplies and temperature increases in the range predicted by global climate models for this century. |
Denaturing gradient gel electrophoresis of a bacteria community |
A mesocosm in winter. |