Abstract
Despite the central role of wetland microbial communities in nutrient cycling and carbon sequestration, little is known of how sediment bacteria differ by salinity, particularly between fresh, intermediate, brackish, and saltmarshes. In this study, the sediment bacterial community and extracellular enzyme activity of 22 wetlands along the Gulf and Atlantic coasts of the southeastern U.S. were surveyed. This included nine tidally connected pairs of wetlands, differing in salinity regime but separated by < 20 km. Disentangling sediment bacterial dynamics in response to salinity variation at multiple temporal (0-300 days), concentration (0 ppt – 39 ppt), and geospatial scales (0 km – 2,000 km) is necessary to better predict the impacts of saltwater intrusion on the wetland microbiome. Five sediment cores were taken from each site and separated into surface and root-zone fractions. Bacterial DNA was extracted, the V4 region of the 16S rRNA gene amplified, and amplicons sequenced using Illumina MiSeq. Activities of the extracellular enzymes β-glucosidase, NAGase, peroxidase, phenol oxidase, and acid phosphatase were assayed to infer mineralization rates of cellulose, lignin, chitin, and organic phosphates. Wetland bacterial communities differed significantly by salinity (p< 0.001) and each pairwise comparison of wetland salinity class was significant (p=0.006). Ordinations show a stepwise differentiation of bacterial composition from freshwater to saltwater. Ordination distances between bacterial communities at different salinity levels revealed a strong but non-linear response, with low levels of salinity increase (0 ppt – 5 ppt) as well as salinity increases across the saltmarsh domain (18 ppt – 30 ppt) having much stronger effects than across brackish salinity marshes (5 ppt – 18 ppt). Enzyme activity also varied non-linearly, but generally decreased with salinity. Thus, even small changes in salinity may have large impacts on coastal microbiomes.
