Enhanced rates of microbial chemoautotrophic production in subsurface sediments are often linked to infauna burrows. Two independent methodological approaches revealed that bacterial populations that should be responsible for increased carbon dioxide dark fixation in burrow walls of the polychaete worm Nereis diversicolor, showed no enrichment effects. By using phospholipid fatty acid and hydroxy fatty acid biomarkers as biomass-specific signature compounds, it became clear that, in contrast to phototrophic eukaryotes (biomarker: 20:5w3), bacteria including sulfate reducers (biomarker: 10 Me 16:9) as well as microfauna (biomarker: 20:5w6) did not accumulate in the 1.5 mm thick superficially oxidized burrow walls. On the other hand, evidence for strong grazing was indicated by high levels of poly-b-hydroxybutyrate (poly-b-hydroxyalkanoates),a bacterial storage product, in the wall layer. This was in agreement with increased levels of partly extracellular enzyme activities such as scleroprotease and sulfatase.