METAGENOME-ASSEMBLED GENOMES FROM CAVE WALL DEPOSITS PROVIDE NEW INSIGHTS INTO MICROBIAL SULFIDE OXIDATION IN SULFURIC ACID CAVES

Sulfuric acid caves are rare karst systems in which carbonate dissolution is driven to a large extent by the oxidation of hydrogen sulfide (H₂S) to sulfuric acid. In these caves, substantial corrosion occurs above the water table where H₂S gas volatilizes from cave streams and is oxidized to sulfuric acid on subaerial cave surfaces. H₂S oxidation is catalyzed by diverse chemolithotrophic microorganisms that colonize cave walls and produce distinctive biofilms and mineralogical features. In order to learn more about the sulfide-oxidizing microorganisms that are important for speleogenesis above the water table, we used Phase Genomics’ ProxiMeta Hi-C technology to generate metagenomes of anastomosing wall sediments known as “biovermiculations” from two locations in the Frasassi cave system, Italy. With the ProxiMeta Hi-C process, DNA from intact microbial cells is cross-linked in situ and sequenced following a proximity ligation procedure that is used to guide assembly and binning. We applied this technique to biovermiculations collected from sites 2 m and 18 m above an actively degassing stream, which are expected to experience very different levels of H₂S gas flux. Based on genes encoding SoxB and sulfide quinone reductase (SQR), populations capable of oxidizing reduced inorganic sulfur compounds are slightly more abundant and diverse closer to the stream, but only slightly. Metagenome-assembled genomes recovered from the Hi-C dataset indicate that some of the most abundant inorganic sulfur-oxidizing populations include members of the families Nitrosomonadales and Xanthomonadaceae at the higher H₂S flux site, and other unclassified Betaproteobacteria and Alphaproteobacteria at the site further from the H₂S source. We will discuss the diversity and metabolic potential of these and other populations based on metagenomic analysis, and the implications for microbial sulfide oxidation in sulfuric acid caves.