Adipocere formation in biofilms – a first step in soft tissue preservation

The preservation of soft tissue in the fossil record is mostly due to the replacement of organic structures by minerals (e.g. calcite, aragonite or apatite) called pseudomorphs. In rare cases soft tissues were preserved by pyrite. We assume that adipocere, as the shaping component, might be a preliminary stage in the pyritisation of soft tissues under anaerobic conditions. Using high-performance liquid chromatography coupled to ultraviolet and mass spectrometric detection (HPLC-UV/MS) and confocal Raman spectroscopy (CRS) we were able to demonstrate the transformation of the hepatopancreas (digestive gland) of the crayfish Cambarellus diminutus into adipocere within only 9 days, just inside a biofilm. Microorganisms (bacteria and fungi) which were responsible for the biofilm (Sphaerotilus and Pluteus) and maybe the adipocere formation (Clostridium) were detected by 16S r-RNA gene amplicon sequencing. Furthermore, micro-computed tomography (µ-CT) analyses revealed a precipitation of calcite and further showed that in animals with biofilm formation calcite precipitates in finer grained crystals than in individuals without biofilm formation, and that the precipitates were denser and replicated the structures of the cuticles better than the coarse precipitates.