Volatilized ammonia supports extremophilic cave ecosystems with unusual nitrogen isotopic signatures

The sulfidic 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 (δ ¹⁵ N values less than -20‰). In this study, we tested the hypothesis that these low δ ¹⁵ N values result from the volatilization, trapping, and uptake of ammonia degassed from the circumneutral streams. We found that dissolved ammonium in the streams had δ ¹⁵ N values near +3‰, whereas NH ₃ ( g ) in the cave atmosphere above streams exhibited δ ¹⁵ N values as low as -27‰, consistent with fractionation by NH ₃ volatilization. Extremely acidic condensation droplets on cave walls efficiently trapped airborne NH ₃ , accumulating up to 4 mM NH ₄ ⁺ with δ ¹⁵ N 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.