Sulfur disproportionating microbial communities in a dynamic, microoxic-sulfidic karst system.

Area of Science:

• Geomicrobiology
• Biogeochemical Sulfur Cycling
• Karst System Ecology

Background:

• Sulfidic karst systems rely on sulfide oxidation, but elemental sulfur (S⁰) fate remains unclear.
• The Frasassi Cave system exhibits Proterozoic-like conditions due to aquifer mixing, supporting sulfur-based chemolithoautotrophy.
• Understanding S⁰ cycling is crucial for comprehending these unique ecosystems.

Purpose of the Study:

• To investigate the geochemistry and microbiology of sediments in the Frasassi Cave system.
• To elucidate the role of elemental sulfur (S⁰) in biogeochemical sulfur cycling.
• To assess the contribution of microbial processes to S⁰ fate and ecosystem function.

Main Methods:

• Analysis of sediment geochemistry and microbial community composition (16S rRNA amplicon sequencing).
• Identification of key microbial players, including uncultivated chemolithoautotrophs (e.g., Desulfocapsa, Sulfurovum).
• Gibbs energy calculations (∆G_r) and microsensor profiling to assess S⁰ disproportionation and chemical gradients.

Main Results:

• Sediments harbor diverse sulfur-cycling chemolithoautotrophs and anaerobic heterotrophs.
• Desulfocapsa and Sulfurovum, capable of S⁰ disproportionation, are abundant (12%-26% of sequences).
• S⁰ disproportionation is thermodynamically favorable and occurs in situ, contributing to acidity below Beggiatoa mats.

Conclusions:

• Microbial S⁰ disproportionation is a significant sink for elemental sulfur in this oxygen-limited karst environment.
• This process plays a role in the biogeochemical sulfur cycle within the Frasassi Cave system.
• S⁰ disproportionation may contribute to the weathering of carbonate rocks and sediments in sulfur-rich karst settings.