ISOTOPIC SIGNATURES OF NITROGEN CYCLING IN SULFURIC ACID CAVES

Sulfidic caves form through the dissolution of limestone by sulfuric acid. Dissolved hydrogen sulfide is carried into the cave in stream and spring waters, where a portion of it volatilizes as H₂S into the cave atmosphere, supporting abundant microbial communities on cave walls. The dominance of chemolithoautotrophy in the Frasassi Cave system in central Italy is consistent with observations of organic δ¹³C and δ¹⁵N signatures that are much lower than surface sources.

The sulfidic cave stream in Frasassi also carries other reduced compounds which may degas into the cave atmosphere such as dissolved NH₄⁺. Volatilization of NH₃(g) has been proposed to explain extremely low organic δ¹⁵N values found in biofilms directly above streams. We tested this hypothesis by collecting air samples, and sampling microbial biomass throughout the cave to characterize the distribution of nitrogen content and isotopic composition. In the area of the cave where stream H₂S and NH₃(g) fluxes are highest, we documented extremely low δ¹⁵N values (down to -30‰) in gas, aerosol, and biomass samples, which contrast with the NH₄⁺ in cave stream water (+1 to +4‰). We present and discuss new data that strongly suggest that volatilized NH₃ serves as the sole nitrogen source for the microbial communities near degassing streams, which can ‘trap’ aerosolized and gaseous ammonia on their acidic surfaces (pH ~1). The low d¹⁵N values result from the large equilibrium effect between NH₄⁺ and NH₃ (~30‰) expressed during volatilization of NH₃from the cave stream. Further away from the stream, we observe other, less acidic biofilms, such as biovermiculations, with pH values ranging from 5-7. While our gas and aerosol samplers in these areas confirmed small amounts of NH₃(g) emanating from the cave stream, biovermiculations do not exhibit such low δ¹⁵N values. Our results suggest that near-neutral cave wall communities are utilizing a different nitrogen source, as they are less able to access the small amounts of ammonia transported away from the stream. We discuss these higher d¹⁵N values and alternative nitrogen sources in areas of the cave less strongly affected by degassing from the sulfidic aquifer. Taken together, these data demonstrate the importance of volatile compounds in cave environments and how they influence the biogeochemistry of these systems.