Influence of Environmental Conditions and Inundation History on Bacterial Diversity of Salt Marsh Soils in Southern Louisiana

Diversity patterns and controls on bacterial community composition were investigated from coastal salt marsh soils in southern Louisiana (USA) from 2012 – 2014. These salt marshes are part of an extensive coastal landscape that is experiencing land loss due to subsidence, sea-level rise, and anthropogenic activities, including from the impacts of the Deepwater Horizon oil spill in 2010. Prior to the oil spill, microbiology research focused predominately on biogeochemical roles and not on taxonomic representation in the soils or on understanding the significance of taxonomic diversity at the microbial level to marsh food webs or ecosystem dynamics. The purpose of this research was to characterize the taxonomic diversity of marsh soils and examine which sets of environmental parameters, including water inundation frequency and depth, vegetation, and salinity, contributed to the most variance in microbiome taxonomic diversity through time. Historical datasets and on-site measurements from the marshes were used to model marsh elevation and local flooding history, and multivariate statistical analyses were applied to determine bacterial community structure and variance. Regardless of sampling time or geographic location, bacterial communities were 80% similar at the phylum level, meaning that marshes were comprised of similar bacterial groups that likely reflected comparable ecosystem function. Subtle differences in marsh bacterial communities were coupled to geographic region, the depth of water that flooded the marsh surfaces, and salinity of that water, with most of the compositional variations being among the Alphaproteobacteria, Gammaproteobacteria, different classes of Chloroflexi, and subgroups within Cyanobacteria. Collectively, these results indicate that some bacterial groups are ubiquitous in natural salt marsh soils, and that efforts to remediate or restore coastal marshes after a disturbance need to consider the importance of key environmental drivers, like salinity, to how marsh soil bacterial communities are structured and how ecological function can be maintained.