SULFUR CYCLING AND NUTRIENT SPIRALING IN KARST

Microorganisms are a fundamental aspect of sulfur redox chemistry, and biogeochemical transformations within the sulfur cycle occur as a consequence of specialized metabolic processes. At Lower Kane Cave, Wyoming, we have documented redox cycling of sulfur in thick, aphotic microbial mats containing sulfur-oxidizing (SB) and sulfate-reducing (SRB) bacteria, as well as iron-reducing bacteria, aerobic and anaerobic heterotrophs, and rare methanogens. These mats form a complex microbial ecosystem supported by primary production of SB, but sustained by SRB and heterotrophs.

Sulfidic springs discharge anaerobic water into the cave but contribute minimal allochthonous, reduced carbon to the cave ecosystem; total dissolved organic carbon is < 80 mmol/L. At the spring orifices, SB and SRB microbial communities are initially separated, but progressively coalesce downstream to produce a thick stratified mat where oxidizers, in contact with aerobic advective stream water, cover an anaerobic consortium within a diffusion-dominated mat interior. Bulk mat d³⁴S ratios are up to 2‰ lower than the spring allochthonous sulfide source of -22‰, suggesting that SB incorporate biogenic sulfide produced by SRB. The aerobic SB maintain an invariant d¹³C value downstream, averaging -36‰, indicating that the SB are predominately autotrophic. Higher d¹³C values for anaerobic populations reflect characteristically smaller ¹³C discrimination by chemolithoautotrophic SRB, as well as recycling of carbon by chemoorganotrophs and heterotrophs. Downstream, the anaerobic populations in direct contact with SB have lower d¹³C values, progressively approaching those of the SB populations.  

Mat redox stratification creates an environment for nutrient cycling of both autotrophically fixed carbon by chemoorganotrophic and heterotrophic populations, and also sulfide produced by SRB which is utilized by SB. As this process is occurring in an advective system, nutrients are cycled between multiple components of the ecosystem through redox boundaries while being advected downstream in each subsequent cycle. This process of nutrient spiraling is previously undescribed from chemosynthetic or subterranean ecosystems.