Metagenomic analysis of a glacial ice core record from the contiguous United States

Glacial ice preserves time-sequenced records of preserved microbes, offering access to historic pre-anthropic metagenomes. As proof-of-concept, we three tested methods to recover ancient DNA from a ~250-year ice core from Wyoming's Upper Fremont Glacier for metagenomic sequencing. Direct amplification of filter-concentrated melt water (eDNA) was a simple and effective method for metagenomics. We observed higher microbial diversity, enriched nitrogen-cycling genes, and higher abundance of Nitrosopira, Rhodanobacter and Polaromonas sequences in 1790 CE samples compared to 1900-1961 CE samples. The documented microbial variation may be attributable to changes in climate and land use over the last two centuries.