The port of Hamburg represents a transition zone between upstream, shallow regions of high net primary production and downstream deep and more turbulent waters in the tidal Elbe River in northwestern Germany. Correspondingly, strong gradients of degradable organic matter (OM) on a distance of a few river kilometers had been identified. This study links microbial community composition using 16S metagenomic amplicons and extracellular polymeric substances (EPS) composition to the observed gradients of sediment OM lability. It was hypothesized that lability gradients caused by higher concentrations of biogenic, autochthonous OM upstream and greater share of already stabilized OM downstream reflect in gradients of microbial community composition, diversity and EPS characteristics.

Indeed, available OM was found to act as key driver regulating syntrophic microbial community composition and associated metabolic features, with location-specific overriding the effect of seasonal variations. Upstream sites with high available OM featuring lower bacterial but increased archaeal diversity and elevated methane and carbon dioxide fluxes, whereas lower OM lability downstream fostered a more diverse bacterial but decreased archaeal diversity. The ratio between microbial taxon richness and biomass correlated inversely with OM transformation rates. These patterns also reflected in increased EPS concentration produced in response to metabolic needs (i.e. polysaccharides and proteins), whereas structural components such as lipids, which can be more resistant under the prevailing anoxic conditions, remained more evenly distributed along the transect. Although bacterial relative abundances exceeded archaeal abundances (<1 %) by far, archaeal functional significance remained pivotal for the final release of carbon as methane and carbon dioxide under the mostly reducing conditions in the deposited sediment.