Microbial iron reduction in the formation of iron ore caves

传统上，人们认为洞穴是由地下水溶解岩石形成的；然而，硫酸洞穴中不寻常的化学成分表明，微生物也可能在洞穴形成中发挥重要作用。通过研究这种生物地球化学，研究人员发现，洞穴可能是通过微生物氧化硫化氢气体形成的，硫化氢气体占全球洞穴形成的25%。他们假设了一种新的微生物驱动洞穴形成的潜在机制，基于富铁岩石（被称为带状铁地层；BIF）中铁（III）矿物的微生物呼吸作用。BIF洞穴形成过程的识别具有重要意义，因为它极大地扩展了洞穴形成的环境，为许多珍稀濒危动物提供了重要的地下栖息地。这些BIF洞穴的位置与具有全球经济意义的铁矿石之间也存在很强的相关性，铁矿石为钢铁生产提供了原始材料。这种相关性表明，洞穴形成过程可能与这些重要铁矿床的形成有关。通过更好地了解形成洞穴的微生物过程，有可能有选择地确定导致矿石形成的过程。这可能允许更精确地有目标地识别和开采铁矿石矿床，限制勘探这种矿石经常产生的环境影响。研究人员的初步研究表明，在BIF洞穴中存在活性铁（III）还原微生物群落，孔隙流体中存在大量溶解的铁（II），以及铁（III）相还原性溶解的结构证据。总之，这些观察结果表明，微生物铁（III）的还原可能是导致物质分离的原因，而地下水的流动可能是铁（II）的去除造成洞穴空隙的原因。因此，研究人员将检验铁（III）还原微生物的活动是铁矿洞形成的原因这一假设。为了验证这一假设，他们将使用一种整合生物地球化学、环境微生物学、实验室微观世界、铁（III）生物还原动力学研究和现场规模经验数据的方法。铁（III）还原速率和铁（II）输运模型将用于将这些经验数据与从微观到区域的一系列尺度上潜在的洞穴形成过程相结合。这些数据将使他们能够限制铁洞形成的机制和速率，并确定微生物在铁洞洞穴形成中所起的作用。

Traditionally, it was thought that caves formed only through the dissolution of rock by groundwater; however, the unusual chemistry of sulfuric acid caves suggested that microorganisms could also play an important role in cave formation. By studying this biogeochemistry, researchers discovered that caves could form through microbial oxidation of hydrogen sulfide gas, which accounted for as much as 25% of cave formation worldwide. They have hypothesized a new potential mechanism for microbially-driven cave formation, based on microbial respiration of iron(III) minerals in iron-rich rocks (known as banded iron formations; BIF). The identification of cave forming processes in BIF is significant as it dramatically expands the environments in which caves can form, which provide critical subterranean habitats for many rare and endangered animal species. There is also a strong correlation between the location of these BIF caves and the presence of iron ores of global economic significance, providing the source material for the production of steel. This correlation suggests that the cave forming processes may be linked to the creation of these important iron ore deposits. By gaining a better understanding of the microbial processes that form caves, it may be possible to selectively identify the processes that lead to ore formation. This may allow for more precisely targeted identification and mining of iron ore deposits, limiting the environmental impact that prospecting for such ores often generates. Investigators' preliminary research has demonstrated the presence of active Fe(III) reducing microbial communities within BIF caves, abundant dissolved Fe(II) in pore fluids, and textural evidence of reductive dissolution of Fe(III) phases. Together, these observations suggest that microbial Fe(III) reduction may be responsible for driving mass separation, while groundwater flow may be responsible for Fe(II) removal to create the cave voids. Investigators will therefore test the hypothesis that the activities of Fe(III) reducing microorganisms are responsible for the formation of iron ore caves. To test this hypothesis they will use an approach that integrates biogeochemistry, environmental microbiology, laboratory microcosms, kinetic studies of Fe(III) bioreduction, and field-scale empirical data. Models of Fe(III) reduction rates and Fe(II) transport will be used to integrate this empirical data with potential cave forming processes across a range of scales, from microscopic to regional. Together these data should allow them to constrain the mechanisms and rates of iron cave formation and determine the role that microbes play in iron cave speleogenesis.