Volatilized Ammonia Supports Subterranean Ecosystems with Unusual Nitrogen Isotopic Signatures

The Frasassi cave system hosts a robust subterranean ecosystem based on microbial lithoautotrophy. Curiously, acidic biofilms forming above degassing sulfidic cave streams, and the invertebrates that feed on them, are extremely depleted in nitrogen-15 (δ¹⁵N values less than -20‰). In this study, we tested the hypothesis that these low δ¹⁵N values result from the volatilization, trapping, and uptake of ammonia degassed from circumneutral streams. We found that dissolved ammonium in the streams had δ¹⁵N values near +3‰, whereas NH_3(g) in the cave atmosphere above streams exhibited δ¹⁵N values as low as -27‰, consistent with fractionation by NH₃ volatilization. Extremely acidic condensation droplets on cave walls efficiently trapped airborne NH₃, accumulating up to 4 mM NH₄⁺ with δ¹⁵N values as low as -29‰, thereby confirming volatilized and trapped ammonia as the primary N source to cave wall biofilms and the extensive subsurface ecosystem they support. Airborne ammonia trapping represents a novel mechanism for biological N acquisition and provides abundant N for growth in an extreme subsurface environment that otherwise receives very limited nutrient input.