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

世界上一些最大、最壮观的石灰岩洞穴，包括新墨西哥州的卡尔斯巴德洞穴和龙舌兰洞，都是由硫酸形成的。这些洞穴形成于携带溶解的硫化氢的地下水暴露于氧气的地方，通常在洞穴地下水位处或附近。硫化氢在氧气存在下不稳定，它迅速反应形成硫酸和其他硫化合物，溶解石灰石并沉淀新的矿物。由于硫化氢具有丰富的化学能，化学合成微生物在地下水中繁衍生息，加速了硫化物的氧化，产生了可能是已知最快的洞穴扩大速度。该项目将使用微生物学，地球化学和建模的跨学科组合，以确定石灰石在这些洞穴中溶解的位置，方式和速度，以及微生物如何具体促进基岩腐蚀。这项研究的结果将向我们展示微生物是如何形成洞穴的，以及它们是如何促进在其他更广泛但更难进入的地下环境中发生的类似碳酸盐风化过程的。碳酸盐矿物被强酸风化是大气中二氧化碳的重要来源，但硫酸风化在陆地地下，特别是在石灰岩和其他碳酸盐基岩地区还没有得到很好的了解。硫酸洞穴是一扇窗户，通过它我们可以直接进入和研究这些广泛但隐藏的地下碳酸盐风化、气体流动和矿物形成过程。洞穴是科学交流的绝佳平台。该项目将利用洞穴的兴奋，通过基于课程的研究，K-12教师教育和科学交流活动创造新的教育机会。在活跃形成的硫酸洞穴中，大量的石灰石腐蚀和空隙发育可能发生在地下水位以上，那里的泉水和溪流将硫化氢脱气到洞穴大气中。然而，这种脱气驱动过程的整体影响还没有得到很好的理解，我们不知道渗流腐蚀对硫酸洞穴形成的贡献，或者微生物如何影响地下水位以上的岩溶发育。因此，本项目将联合收割机地球化学和分子分析与洞穴形成模型相结合，以解决三个问题：（1）微生物是否加速了地下水位以上的硫化物氧化和石灰石溶解，这些速率能否解释观察到的洞穴形态？(2)微生物是如何影响陆上碳酸盐溶解的，无论是在硫酸腐蚀的活跃阶段，还是在洞穴的整个漫长寿命期间？(3)硫酸岩溶的气体排放对气候和碳循环有什么影响？该团队将通过将石灰石溶解和生物硫化物氧化动力学的直接测量与元转录组学和其他社区分析相结合来解决这些问题，以将微生物活动与靠近和远离硫化物含水层的洞穴形成直接联系起来。他们将使用气流和洞穴生成模型将这些测量与洞穴形态和气体通量联系起来。他们将探索这些过程如何在不再有硫化物来源的古代硫酸洞穴中变化。问题2和3的部分内容将通过基于课程的研究来解决，重点是生物发现，地球系统和科学传播。 该职业奖由地球生物学和低温地球化学计划以及NSF地球科学部的教育和人力资源计划共同资助。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.