Donato Giovannelli, Peter H. Barry, Joost M de Moor, Gerdhard L. Jessen, Matthew O Schrenk, Karen G. Lloyd

Despite being one of the largest microbial ecosystems on Earth, with >1029 microbial cells, many basic open questions remain about how life exists and thrives in the deep subsurface biosphere, inside Earth’s crust. Much of this ambiguity is due to the fact that it is exceedingly difficult and (often prohibitively expensive) to directly sample the deep subsurface, requiring elaborate drilling programs and/or access to deep mines. We propose a sampling approach which involves collection of a large suite of geological, geochemical, and biological data from numerous deeply-sourced seeps—including lower temperature sites—over large spatial scales. This enables research into interactions between the geosphere and the biosphere, expanding the classic local approach to regional or even planetary scales. Understanding the interplay between geology, geochemistry and biology on such scales is essential for building models of the subsurface ecosystem and extrapolating ecological and biogeochemical roles of subsurface microbes beyond single site interpretations. This approach has been used successfully across the Central American and South American Convergent Margins, and can be applied more broadly to other types of geological regions (i.e., actively rifting, intraplate volcanic and/or hydrothermal settings). Working across geological spatial scales, inherently encompasses broad temporal scales (e.g., millions of years of volatile cycling across a single convergent margin), providing access to a framework for interpreting evolution and ecosystem functions through deep time and space. We pose that tectonic interactions are fundamental to maintaining planetary habitability through feedbacks that stabilize the ecosphere (e.g., plate tectonics controlling the distribution of volatiles throughout Earth), and deep biosphere studies are fundamental to understanding geo-bio feedbacks on these processes on a global scale.