CAREER: Do microbes form caves? Sulfide oxidation and limestone corrosion in sulfuric acid caves

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Some of the world’s largest and most spectacular limestone caves, including Carlsbad Cavern and Lechuguilla Cave in New Mexico, were created by sulfuric acid. These caves form where groundwaters carrying dissolved hydrogen sulfide are exposed to oxygen, usually at or near the cave water table. Hydrogen sulfide is unstable in the presence of oxygen, and it rapidly reacts to form sulfuric acid and other sulfur compounds that dissolve limestone and precipitate new minerals. And because of the abundant chemical energy from hydrogen sulfide, chemosynthetic microorganisms thrive at the water table and speed up sulfide oxidation, producing what might be the fastest known rates of cave enlargement. This project will use an interdisciplinary combination of microbiology, geochemistry, and modeling to determine where, how, and how fast limestone dissolves in these caves, and how specifically microorganisms contribute to bedrock corrosion. Results from this research will show us how microorganisms form caves, but also how they contribute to similar carbonate weathering processes that occur across other more widespread but less accessible subsurface environments. Carbonate mineral weathering by strong acids is an important source of carbon dioxide to the atmosphere, but sulfuric acid weathering is not well understood in the terrestrial subsurface, especially in areas with limestone and other carbonate bedrock. Sulfuric acid caves are windows through which we can directly access and study these extensive but otherwise hidden processes of subterranean carbonate weathering, gas flux, and mineral formation. And, caves are exceptional platforms for science communication. This project will take advantage of the excitement of caves to create new educational opportunities through course-based research, K-12 teacher education, and science communication activities. In actively-forming sulfuric acid caves, substantial limestone corrosion and void development can occur above the water table, where springs and streams degas hydrogen sulfide to the cave atmosphere. However, the overall impact of this degassing-driven processes is not well understood, and we don’t know the contribution of vadose corrosion for sulfuric acid cave formation, or how microorganisms affect karst development above the water table. This project will therefore combine geochemical and molecular analyses with speleogenetic modeling to address three questions: (1) Do microorganisms speed up sulfide oxidation and limestone dissolution above the water table, and can these rates explain observed cave morphologies? (2) How do microorganisms affect subaerial carbonate dissolution, both during the active phase of sulfuric acid corrosion but also throughout the long lifetime of the caves? (3) What is the impact of gas emissions from sulfuric acid karst for climate and carbon cycling? The team will address these questions by combining direct measurements of limestone dissolution and biological sulfide oxidation kinetics with metatranscriptomics and other community analyses to directly link microbial activity with cave formation both close to and far from the sulfidic aquifer. They will use air flow and speleogenetic modeling to relate these measurements to cave morphologies and gas flux. And they will explore how these processes change in ancient sulfuric acid caves that no longer have a sulfide source. Parts of Questions 2 and 3 will be addressed through course-based research that emphasizes biodiscovery, Earth systems, and science communication. This CAREER award is co-funded by the Geobiology and Low-Temperature Geochemistry Program and the Education and Human Resources Program in the NSF Division of Earth Sciences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.