Ancient environmental microbiomes and the cryosphere.

In this review, we delineate the unique set of characteristics associated with cryosphere environments (namely, ice and permafrost) which present both challenges and opportunities for studying ancient environmental microbiomes (AEMs). In a field currently reliant on several assumptions, we discuss the theoretical and empirical feasibility of recovering microbial nucleic acids (NAs) from ice and permafrost with varying degrees of antiquity. We also summarize contamination control best practices and highlight considerations for the latest approaches, including shotgun metagenomics, and downstream bioinformatic authentication approaches. We review the adoption of existing software and provide an overview of more recently published programs, with reference to their suitability for AEM studies. Finally, we summarize outstanding challenges and likely future directions for AEM research.