APPLYING HIGH-THROUGHPUT RRNA GENE SEQUENCING TO ASSESS MICROBIAL CONTAMINATION OF A 40-YEAR OLD EXPOSED ARCHEOLOGICAL PROFILE

In recent years there has been a surge in the recovery of ancient organic molecules from archaeological contexts. These analyses are yielding unprecedented insights into past human activities, and are increasing paleoenvironmental resolution. However, contamination of archaeological sediments by microbes can alter ancient biomolecular data. Furthermore, the extent to which microbes can penetrate ancient archaeological sediments once these are exposed by excavation is unknown. We tested this question at Crvena Stijena, a rock shelter in the Dinaric alps (700 m.a.s.l.) in Montenegro which contains archaeological deposits spanning 80,000+ years. Excavations in the early 1960s exposed these profiles, which have been cleaned several times to permit sampling for archaeological, geological, and biomolecular analyses. The growth of algae on the exposed profiles after cleaning has prompted the question, are microbes penetrating into the profile? To test this question we sampled sediments at 1 cm intervals at depths up to 10 cm from five different horizons in the profile. Results from multivariate analysis of 16S rRNA gene libraries show that (1) samples from sediment surfaces have distinct microbial communities from samples collected >1 cm deep, (2) microbial biomass from the deep samples is very low, and (3) microbial taxa from the deep samples is distinct from that in blanks and controls. Together, this evidence strongly indicates that microbial contamination is limited to archaeological profile surfaces. This means it is unlikely that ancient biomolecules in these sediments have been damaged by recent in situ microbial invasions, and cleaning of the profiles before sampling may not need to exceed 2 cm. in depth. This is heartening news for future archaeological analyses at this and other Paleolithic sites, where an immense amount of data can be gathered through limited vertical sampling along archaeological profiles, a strategy which conserves archaeological deposits while helping us tackle key questions about the past.