Vegetated coastal habitats such as seagrass meadows (e.g. Zostera marina) are considered critical ecosystems for carbon burial in the marine environment. Zostera is one of a few marine angiosperms with an extensive root system that facilitates long-term carbon sequestration within rhizosphere-associated anoxic sediments. Compared to their terrestrial counterparts, relatively little is understood about the impact of microbial activities within the rhizosphere-processes that ultimately influence net carbon storage. Here we propose to develop an experimental system for the detailed study of the anaerobic microbial community and their interactions in seagrass beds. We will leverage Caltech's Kerckhoff Marine Laboratory facilities, local university collaborations, and our expertise in marine microbial ecology to study two key processes within the anoxic seagrass rhizosphere: root-associated nitrogen-fixation and methanogenesis.