NITROGEN SOURCES AND CYCLING IN THE FRASASSI CAVES, ITALY: IMPLICATIONS FOR NITROGEN CYCLING IN THE SHALLOW SUBSURFACE

Sulfidic caves form from the dissolution of limestone by sulfuric acid. Dissolved hydrogen sulfide (H₂S) is carried into caves by groundwater, and a portion of this H₂S degasses into the cave atmosphere, where it is oxidized to form sulfuric acid. At the deeper levels of the cave, these is very minimal surface organic input, and nearly all organic production in this environment is the result of in-situ chemolithoautotrophic primary production. For example the H₂S supports diverse microbial communities of sulfur oxidizers such as those that form extremely acidic (pH <1) ‘snottite’ biofilms. The dominance of these chemolithoautotrophs is evident from the isotopic composition of organic δ¹³C and δ¹⁵N signatures, which are much lighter than surface sources.

From a nitrogen perspective, these extremely acidic snottites are able to effectively ‘trap’ ammonia gas that has volatilized from the cave stream as bioavailable ammonium for nitrogen assimilation. This volatilized ammonia gas is extremely isotopically light (δ¹⁵N as low as -29‰), leaving a distinct isotopic signature in these acidic biofilms and other acidic cave surfaces. With distance from the cave stream, there is less ammonia available and, with the lower H₂S concentrations, fewer sulfur oxidizers. In these areas of the cave, there are no snottites, and microbial communities are usually only mildly acidic (pH 5-6). These mildly acidic microbial communities have heavier nitrogen isotope signatures (δ¹⁵N -12‰ - +4‰), which suggests different nitrogen sources. These potential nitrogen sources in the Frasassi Caves include nitrate and nitrite from localized drip water pools, atmospheric nitrogen, and ammonia from other sources such as bat guano near cave entrances. In this presentation, we will be describe isotopic signatures of biovermiculations and different cave various nitrogen sources. We will report results from incubations with ¹⁵N₂ to evaluate the presence of nitrogen fixing microorganisms, and metagenomic insights into potential nitrogen metabolisms from 15 samples that were selected to represent the range of organic δ¹⁵N values above the water table.