The relationship between microbial composition and ecosystem processes in a New England tidal marsh

Background/Question/Methods   We examined the microbial composition-function relationship in a New England tidal marsh ecosystem. We used a transplant experiment designed to compare the composition (TRFLP) and activities of salt marsh and freshwater marsh sediment communities. We transplanted sediment cores (6 cm dia by 6 cm deep) contained in dialysis bags (MWCO of 8 kD). To investigate the effects of both composition and environment on ecosystem processes, we performed a short-term (1 week) experiment to limit composition shifts of transplanted communities. To examine feedbacks between the environment and composition, we also performed a longer term transplant experiment (2 months). At the end of each experiment, we compared general bacteria and sulfate reducing bacteria community composition among cores. We also performed functional assays to assess ecosystem processes. We measured soil respiration, net methane production, aminopeptidase, sulfatase, and phosphatase activities and gross nitrification rates.

Results/Conclusions   In the short-term experiment, as expected, community composition did not shift significantly when cores were transplanted. Additionally, six of the nine functional assays exhibited a significant effect of core origin, and thus community composition. In this short-term experiment, in addition to environmental effects, composition effects on measurable ecosystem processes were evident. In the long-term experiment we expected transplanted cores to exhibit composition shifts coupled with functional shifts, such that transplanted cores would resemble the site in which the cores incubated rather than the site of origin. After two months of being transplanted, a significant but limited shift in the general bacteria composition was evident. The sulfate reducing community was even more resistant and did not shift significantly. Even though community composition did not shift dramatically, a significant effect of the incubating site was evident for five of the nine functional assays, while core origin was significant for only two of the assays. Initial microbial response to new environmental conditions did not occur as large composition shifts, even though functional responses were evident. Composition may be more resistant to shifts compared to function when communities are exposed to new environments, potentially decoupling known composition – function relationships.