Characterization of ancient DNA preservation in copper-patinated human bone and tooth samples from Latvia

Bronze and copper-containing alloys are known to possess antibacterial properties, which, besides mineralization, might contribute to the observation of better preservation of bone, teeth, and other organic materials in close proximity to bronze artifacts in archaeological contexts. Often, the selection of preserved hard tissue material from burials for ancient DNA (aDNA) analysis is limited due to the sampling strategies used at the time of excavations or poor preservation in general. This study assessed the suitability of samples with copper patination for in-depth human and bacterial aDNA analysis. To characterize the difference between patinated and unpatinated samples, a qualitative assessment of human DNA preservation and metagenomic analysis using Kraken2 and MEGAN Alignment Tool was performed. For patinated samples, a very low proportion of endogenous DNA preservation was observed. Only one—the least patinated—out of five analyzed samples could be authenticated to be containing human aDNA with confidence. A patinated sample of specific bacterial genera was affiliated with soil and aquatic, mostly marine environmental, microorganisms. In conclusion, due to mineralization processes that occur during the patination of hard tissue, endogenous DNA preservation is seen to be greatly affected, thus making heavily patinated samples a compromised source of endogenous aDNA. Future in-depth research is needed, including the assessment of chemical composition of patinated tissues.