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.