In spite of their pivotal importance for the persistence of coastal salt marshes in the face of sea-level rise, relatively little is known of the environmental factors structuring sediment microbial communities. Microbial community composition has been linked to varying rates of organic matter decomposition in salt marshes, and thus plays a key role in the maintenance of surface elevation. Although various factors, such as elevation, salinity, and plant community composition, have been individually linked to marsh microbes, such factors have rarely been integrated in an overall assessment of their combined effects. Here, we report on the integration of six environmental variables, including latitude, elevation, salinity, pH, redox, and plant biomass to explain variation in microbial community composition among soil samples from six salt marshes lying along the Atlantic Coast of North America. All samples were split into bulk and rhizosphere fractions prior to characterizing microbial communities using terminal restriction fragment length polymorphisms (t-RFLPs) generated from 16S rDNA. A principal components analysis (PCA) on 198 t-RFLP markers from 127 soil samples was used to summarize variation among microbial communities. Multiple regressions were run separately by soil fraction to determine if PCA scores varied significantly with any of the measured environmental variables.
Results/Conclusions
Roughly 30 principal components (PCs) were found to be informative for each soil fraction, with no single PC explaining > 11% of the variation among samples, indicating that relatively narrow components of the soil microbial community were responding independently to changing environmental conditions. Microbial community composition in both bulk and rhizosphere soil varied significantly with salinity and plant biomass, with the addition of elevation and latitude for the rhizosphere samples. Interestingly, while variation in bulk soil community composition tended to be associated with just 1-3 markers (i.e., putative microbial taxa) per PC, up to 26 markers per PC were associated with changing rhizosphere community composition. Different PCs highlighted different components of the microbial community in both bulk and rhizosphere soil, with virtually no overlap in the markers significantly loading on the different PCs. Collectively, these results reflect the tremendous importance of plants in structuring sediment microbial communities in salt marshes, and raise the question of whether plants will be decoupled from the microbial communities associated with their roots as environmental conditions are altered by climate change, with possible consequences for continued ecosystem functioning.