MICROBIAL COMMUNITIES INHABITING AGING SECONDARY MINERAL DEPOSITS IN A FORMER SULFIDIC STREAM PASSAGE FROM A MODERATELY THERMAL CAVE

Life plays a crucial role in hypogene caves that arise from sulfuric acid speleogenesis (SAS), where chemoautotrophic microbes harness energy from hydrogen sulfide (H₂S) oxidation. Beyond simply proliferating where H₂S-rich groundwaters emerge, acidophilic microbial communities drive precipitation of secondary minerals such as gypsum produced by sulfuric acid generating metabolisms. SAS caves are appealing natural laboratories for exploring microbial-mineral interactions, especially as acidic sulfur mineral deposits are a target for astrobiology on the Martian surface. Elucidating more information on which, and how, different lifeforms exploit these geochemical niches may advance our understanding of biosignature repositories in analogue environments.

Using high-throughput 16S rRNA gene and metagenomic sequencing, we explored microbial colonization of gypsum and S° in Grotta Nuova di Rio Garrafo, near Acquasanta Terme in central Italy: a hypogene cave fed by moderately thermal (~40 °C) sulfidic waters. Local hydrology had changed ~3 years prior, which redirected sulfidic streamflow away from a previously H₂S-rich passage. This natural experiment is a unique opportunity to study how this energy deficit changed secondary mineral-associated communities.

In contrast to sulfur-oxidizing Acidithiobacillus spp. that dominate “snottite” biofilms in cool sulfidic caves, we found that secondary minerals were colonized by high levels of Thermoplasmatales-group Archaea, >90% of rRNA gene sequences in some samples. Rather than mineralogy, proximity to the remnant sulfidic stream passage was the driver of sample diversity, where species richness was greater with distance from the relict area. The more distal samples had few known sulfur-oxidizers, while communities from the former stream passage still had chemolithotrophic taxa. We also describe initial metagenomic reconstruction from a S° deposit from a still-active sulfidic zone, including two archaeal taxa with a potential symbiotic relationship that are capable of incomplete sulfide oxidation and may contribute directly to the extensive S° deposits. We will discuss implications of short-term overprinting and how this may impact extremophile assemblages and molecular biosignature presence.