Cave microorganisms: hidden players in global greenhouse gas cycling and climate regulation

Caves are unique among ecosystem types because of their physical structures and biological functions. Embedded in rocks, the geological setting defines the boundaries of caves and dictates how energy and matter move through them. General features of caves, compared to surface ecosystems, include absence of light, relatively stable temperature and humidity, and oligotrophic conditions. Despite these conditions, caves are highly diverse ecosystems whose environmental properties are shaped by geological, hydrological, and climatic factors. Cave microbiomes metabolize atmospheric trace gases, such as methane, nitrous oxide, and carbon dioxide, contributing to greenhouse gas (GHG) cycling dynamics. In some cases, these microbes also form biominerals, such as calcium carbonate, highlighting critical gaps in our understanding of subterranean biogeochemical processes. Some of these gaps include the limited genomic data and geographic bias in the literature. Herein, we review the current state of knowledge surrounding the potential of cave microorganisms, including those capable of biomineralizing calcium carbonate, as agents for sustainable GHG sequestration and climate change mitigation, with emerging strategies for developing novel sustainable biotechnological solutions. By revealing the hidden microbial activity beneath the Earth's surface, this review proposes integrating subterranean ecosystems into global climate models, reframing caves as metabolically and functionally active contributors to the planet's climate system rather than isolated geological features.