Compositional Differences in Bacterial Communities in Fresh and Saltwater Wetlands of the Gulf Coast

Wetlands are important reservoirs for biodiversity and ecosystem services such as nutrient cycling. However, the anthropogenic stressors of sea-level rise and eutrophication threaten these habitats. In this study, I examined the bacterial communities at six locations along the US Gulf Coast between Louisiana and Florida, focusing on how these communities were affected by salinity, depth, and site location. At each of six locations, five 30cm-deep soil cores were taken from a tidal fresh marsh and either a tidal brackish marsh or tidal saltmarsh. Bacterial DNA was extracted from both the surface layer and the root layer of each soil core. Illumina MiSeq was used to sequence the V4 region of the 16S rRNA gene. NMDS ordination and analysis of similarity were calculated from a Bray-Curtis dissimilarity matrix to determine sample differences and their environmental drivers. Wetland type had a significant effect on sediment microbial composition with fresh marsh, saltmarsh, and brackish marsh all differing. Ranking wetlands by salinity in 5 ppt increments revealed that the only non-significant comparisons were between the three lowest salinity groups and two moderate salinity groups. This pattern was further refined by NMDS ordination, which showed distinct clustering of communities by salinity and, generally, tighter grouping of samples in higher salinity wetlands and wider distribution in lower salinity wetlands. Salinity, depth in sediment, and site all had significant effects on sediment bacterial community composition. This study represents one of the largest surveys of the wetland microbiome both spatially and in number of samples. The results reflect previous data from these sites that showed that site was the most influential factor in determining enzyme activity, followed by salinity. It is reasonable to predict that in these and similar sites, sea-level rise will cause shifts in the sediment microbiome and its activities