Characterisation of ancient DNA preservation in human bone and teeth samples from Latvia with copper patination

Bronze and other copper containing alloys are known to possess antibacterial properties, which might partly explain the observation of better preservation of bone, teeth, and other organic materials in close proximity to bronze artifacts in archaeological contexts. The selection of bone and teeth samples for ancient DNA (aDNA) analysis often can be limited due to excavation techniques used in the past, whereby only selected bones were removed from the burial for curation, and poor preservation of osteological material in general. The aim of this study was to compare the metagenomic profiles and endogenous DNA preservation between osteological samples with and without copper patination obtained from different burials in Lejasbiteni cemetery (7th-10th centuries CE). Sequencing data obtained from patinated samples showed much lower human genome aligning read percentages than non-patinated samples. Most of the analysed samples did not have enough reads for aDNA authentication or did not display the characteristic aDNA damage patterns at the ends of sequences. Metagenomic analyses indicated that one of the samples (an intensively patinated and highly porous hand phalanx) also had an increased proportion of sequencing reads aligning to Sphingomonas – a genus of Gram-negative, chemoheterotrophic, strictly aerobic bacteria with biodegradative capabilities including organometallic compounds. Overall, despite the apparently good preservation of bones and teeth located in close proximity to metals and alloys with antibacterial properties, sequencing data suggests poor endogenous human DNA preservation and a possibility of changes in metagenomic profiles in favour of more metal-resistant bacteria. For determination of specific causes of lower endogenous DNA percentage in patinated samples, further research is required.