Defense Date

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Integrative Life Sciences

First Advisor

Rima Franklin

Abstract

Microbial communities play an essential role in carrying out the biogeochemical cycles that sustain life on Earth, yet we know very little about their ecology. One question of particular interest is how environmental conditions shape microbial community structure (i.e., the types of organisms found in the community and their relative abundance), and whether such changes in structure are related to biogeochemical function. It is the aim of this dissertation to address this question via the examination of carbon (C) and nitrogen (N) cycling in wetland ecosystems, which due to their diverse hydrology have a profound influence on biogeochemical cycles. With respect to N cycling, the community structure of denitrification- and dissimilatory nitrate reduction to ammonium (DNRA)-capable organisms was evaluated in response to changes in resource availability, specifically organic matter (OM) and nitrate (NO3-), using an in situ field manipulation. Interactive regulation of microbial community composition was exhibited in both groups, likely due to variation in C substrate preferences and NO3- utilization efficiency. Subsequent experimentation considering only denitrification revealed that resource regulation of activity rates was mediated through changes in denitrifier community composition. The resource regulation of wetland C cycling also was evaluated using an in situ OM manipulation. OM characteristics (e.g., degree of decomposition) affected microbial extracellular enzyme activity (EEA) and changed the community structure of bacteria, archaea, and methanogens. These changes were linked with carbon dioxide and methane production via a conceptual model diagramming the importance of microbial community structure and EEA in greenhouse gas production. The investigation of C cycling in wetlands was extended to consider an important global change threat: saltwater intrusion into freshwater tidal wetlands. Bacterial community structure and EEA were examined along a natural salinity gradient. Salinity was strongly associated with bacterial community structure and positively correlated with EEA. These results suggested that salinity-induced increases in decomposition were responsible for reduced soil OM content in more saline wetlands. This work demonstrates that microbial communities in wetlands are structured by environmental conditions including resource availability and salinity. Further, the research provides evidence that environmental regulation of important biogeochemical processes in wetlands (e.g., methanogensis, denitrification, etc.) is mediated through changes in microbial community structure.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

January 2014

Included in

Life Sciences Commons

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