Spatial and temporal variations in salt marsh microorganisms of the Wadden Sea

Salt marshes exist at the interface of the marine and the terrestrial system. Shore height differences and associated variations in inundation frequency result in altered abiotic conditions, plant communities, and resource input into the belowground system. These factors result in three unique zones, the upper salt marsh (USM), the lower salt marsh (LSM), and the pioneer zone (PZ). Marine detritus, such as micro- and macroalgae, is typically flushed into the PZ daily, with storm surges moving both salt marsh detritus and marine detritus into higher salt marsh zones. Microbial assemblages are essential for the decomposition of organic matter and have been shown to sensitively respond to changes in abiotic conditions such as oxygen supply and salinity. However, temporal and spatial dynamics of microbial communities of Wadden Sea salt marshes received little attention. We investigated the dynamics of soil microbial communities across horizontal (USM, LSM, and PZ), vertical (0-5 and 5-10-cm sediment depth), and temporal (spring, summer, and autumn) scales in the Wadden Sea salt marsh of the European North Atlantic coast using phospholipid fatty acid (PLFA) analysis. Our results show strong spatial dynamics both among salt marsh zones and between sediment depths, but temporal dynamics to be only minor. Despite varying in space and time, PLFA markers indicated that bacteria generally were the dominant microbial group across salt marsh zones and seasons, however, their dominance was most pronounced in the USM, whereas fungal biomass peaked in the LSM and algal biomass in the PZ. Only algal markers and the stress marker monounsaturated to saturated fatty acid ratio responded to seasonality. Overall, therefore, the results indicate remarkable temporal stability of salt marsh microbial communities despite strong variability in abiotic factors.