In vitro decomposition of bone collagen by soil bacteria: The implications for stable isotope analysis in archaeometry

To understand biogenic collagen type I decomposition and to establish how diagenesis may bias archaeometric data, modern mammalian bone was inoculated with a selection of ubiquitous soil bacteria. The presence of exogenous microbial biomass in the inoculated specimens was then checked microscopically prior to collagen extraction. The experimentally degraded bone collagen showed altered amino acid compositions, attributable to the selective breakdown of certain amino acids by the bacteria. While both the bulk collagen extract and the single amino acids exhibited shifts to more negative δ13 C-values, enrichment was recorded for general δ13 N, and a depletion trend relative to unaltered collagen was observed for individual amino acid δ13 N. One explanation for the enrichment of the global δ15N-values is cleavage of peptide bonds, which leaves 15 N within the substrate, while the change of 13C is mostly due to the altered amino acid composition. On the other hand, possible repolymerization of cleavage products under experimental conditions may also be responsible for the depletion trend of individual amino acid δ13C- and δ15 N-values. This paper discusses the results as a basis for the development of a method for the reconstruction of the isotopic abundance of the original collagen, using the amino acid composition of the degraded product, the contribution of individual amino acids to its global δ-values and of isotope discriminations implied in the microbial decomposition