Volatile compounds in cave ecosystems and their roles in subterranean carbon cycling

The composition of cave atmospheres is unknown beyond ventilation studies on carbon dioxide (CO₂) and methane (CH₄). The identities, concentrations, and roles of volatile organic compounds (VOCs) and their link to CO₂ and CH₄ – although crucial for understanding subterranean carbon cycling – remains unexplored. Caves also take part in gas exchange with terrestrial ecosystems and the atmosphere, acting as sources and sinks of reactive gases. However, the magnitude of this gas exchange, and the potential effects on local and global carbon budgets, has not yet been characterized.

We analyzed VOCs, CO₂, CH₄, and oxygen in situ from the air of two caves (in Loulé and Torres Novas) in the main karst massifs of Portugal, along with cave microbiomes in sediment samples. Additionally, we isolated bacterial and fungal species from the sediment samples and investigated their volatile fingerprints in vitro. We used headspace vials and sorbent tubes for the in situ and in vitro volatile sampling combined with analysis via mass spectrometric and optical spectroscopy methods. Cave microbial compositions were analyzed with metabarcoding of 16S rRNA (bacteria) and ITS (fungi) genes.

We will present novel data connecting cave atmospheric compositions to cave microbial carbon cycling, including analysis of the origin of volatiles through stable isotope analysis (¹³C). We also present data for seasonal and spatial variation in the cave atmospheric compositions. Our results suggest that cave atmospheres are dynamic rather than stable, affected by outside conditions, and therefore, potentially compromised by climate change. Conversely, we confirm that caves can act as sources and sinks for some reactive gases, suggesting that they can also impact the surrounding environment.