Abstract
Summary This study describes the microbial community structure of three sandy sediment stations that differed with respect to median grain size and permeability in the German Bight of the Southern North Sea. The microbial community was investigated using lipid biomarker analyses and fluorescence in situ hybridization. For further characterization we determined the stable carbon isotope composition of the biomarkers. Biomarkers identified belong to different bacterial groups such as members of the Cytophaga‐Flavobacterium cluster and sulfate‐reducing bacteria (SRB). To support these findings, investigations using different fluorescent in situ hybridization probes were performed, specifically targeting Cytophaga‐Flavobacterium , γ‐Proteobacteria and different members of the SRB. Depth profiles of bacterial fatty acid relative abundances revealed elevated subsurface peaks for the fine sediment, whereas at the other sandy sediment stations the concentrations were less variable with depth. Although oxygen penetrates deeper into the coarser and more permeable sediments, the SRB biomarkers are similarly abundant, indicating suboxic to anoxic niches in these environments. We detected SRB in all sediment types as well as in the surface and at greater depth, which suggests that SRB play a more important role in oxygenated marine sediments than previously thought.