Impacts of microenvironment on distributions and activities of bacteria in a salt marsh environment

Infaunal burrows and the rhizosphere of Spartina alterniflora are ecologically relevant microenvironments that impact the distributions of bacteria in a salt marsh ecosystem. Infaunal burrows produced by the fiddler crabs Uca pugilator and Uca pugnax are abundant features of southeastern United States salt marshes. These water-filled burrows become significantly enriched in Vibrionaceae, and specifically Vibrio parahaemolyticus (4875 CFU mL-1), relative to creek waters (193 CFU mL-1) and interstitial porewaters (128 CFU mL-1). As evidenced from dye transport studies, tidal inundation flushes and transports the bacterially-enriched infaunal burrow water within the estuary over semi-diurnal tidal cycles. Marsh elevation also contributes to the frequency of tidal flushing of these burrow waters. Rhizosphere microenvironments harbor seasonally responsive and seasonally non-responsive nitrogen fixing (diazotrophic) bacterial assemblages. Vibrionaceae are represented as members of the diazotrophically-active, seasonally non-responsive assemblage, indicating that this bacterial family is also a ubiquitous and potentially important component of the rhizosphere bacterial community. Bacterial communities associated with infaunal burrow waters were found to produce significant rates of diazotrophic activity, thus introducing a novel source of organically available nitrogen into a nitrogen-limited system. Infaunal burrow waters show evidence of elevated ammonium concentrations, which could limit diazotrophic capabilities. However, when environmental diazotroph isolates and burrow water samples were evaluated for ammonium inhibition they were found to exhibit depressed diazotrophic abilities in ranges of ammonium concentration that substantially exceeded in situ levels, indicating their ability to persist and remain active in spite of elevated ammonium levels.