Soil microbial ecology of terrestrial-aquatic interfaces from a freshwater and estuarine coastal system

Coastal soils are dynamic systems where unique microbial niches are shaped by the intensity and duration of flooding combined with differences in soil and vegetation types between the purely terrestrial and purely aquatic boundaries of the terrestrial-aquatic interface (TAI). This study aims to understand the soil microbial ecology along the TAIs of a freshwater and estuarine region and how it relates to organic matter. We analyzed soil microbial (16S rRNA gene) and organic matter (Fourier-transform ion cyclotron resonance mass spectrometry, FTICR-MS) composition from upland (forested), transition (stressed forest), and wetland positions at three sites in each of the Lake Erie (freshwater, Erie) and Chesapeake Bay (estuarine, Chesapeake) regions. We found that region, transect position, and site had significant influence on soil bacterial community structure. To understand commonalities and distinctions across transects within a region, we identified core microbes conserved across all sites and transect positions, and indicator taxa that were unique to a given transect position (a TAI ecosystem state). We also found several indicator taxa shared between upland-transition and transition-wetland zones, with no indicator taxa shared between upland-wetland zones suggesting that presence of specific microbes was less predictive of the transitional ecosystem state. Contrary to expectations, given their dynamic hydrologies, transitions represented a midpoint in microbial richness and diversity metrics. Using a weighted gene co-expression network analysis, we correlated the feature-level beta-nearest-taxon indices of bacterial taxa and organic compounds across all samples within a transect position. We found that microbial indicator taxa such as nitrogen fixers, and sulfate reducers, and organic matter features such as lignin-like, and aromatic compounds showed positive relationships in their contributions to community assembly processes. This study highlights microbial indicators of ecosystem states along the TAIs of distinct coastal regions with potential coupling of microbial and organic matter responses.