Examining the role of biotic iron reduction as a life-sustaining process at the potential temperature limit of the deep subseafloor biosphere (IODP Expedition 370) (RESPIRE)

本项目将为国际海洋发展计划第370次考察队的研究问题提供帮助：Muroto深海生物圈的t极限。在南开海槽（日本）的C0023钻探点，温度在1.2 km深度增加到~120°C，从而达到微生物可能耐受的最大值。然而，对营养贫乏的深层沉积物进行灭菌的更合理的T估计是80-90°C。远征370的目的是评估微生物群落如何随深度变化，哪些因素控制变化以及微生物生命在哪里停止。这是科学计划的一部分，旨在研究深层可用的能量基质，并识别区分生物和非生物领域和/或其过渡的独特地球化学和微生物特征。获得了高分辨率和高精度的孔隙水数据，可以识别反应前沿、潜在的微生物活动和热液蚀变。大部分取心层段不含甲烷和硫酸盐。因此，微生物活动依赖于硫酸盐以外的电子受体。最近的研究表明，经典的氧化还原序列需要由甲烷沉积物中的铁和锰还原来补充，而自然系统中的生物地球化学过程与矿物学的联系比与严格的垂直反应序列的联系更强。铁（III）还原是微生物呼吸最古老的形式之一，铁还原体可以在高温度和高压条件下生长，这表明铁（III）还原体是接近深生物圈温度极限的潜在候选生物。我们在C0023站点的甲烷沉积物中发现了铁和锰的还原带。通过应用顺序萃取，我们旨在评估哪些Fe和Mn相可作为电子受体，以及原生矿物被成岩改变的程度。特别令人感兴趣的是火山灰层，因为这些已经被确定为微生物生命的热点。灰层在C0023沉积物中无处不在，通常富含铁和锰。微生物铁还原使孔隙水中的54Fe富集，从而使自生铁矿物（如脉铁矿、磁铁矿）富集，而与硫化物的非生物反应导致溶解相中更多的56Fe。我们的目的是利用溶解和活性固体铁的稳定铁同位素来区分铁还原的微生物和非生物驱动因素。固体铁的d56Fe组成将在铁碳酸盐、铁水合石+绢云母、针铁矿+赤铁矿和磁铁矿上进行测定。对磁性数据解释有影响的硫化程度将通过提取酸挥发性硫和铬可还原硫来确定。本项目旨在评估C0023站点深层沉积物中氧化铁对微生物呼吸的作用及其相关的成岩变化。

This project will contribute to the research questions of IODP Expedition 370: T-Limit of the Deep Biosphere off Muroto. Temperatures at the Drill Site C0023 in the Nankai Trough (Japan) increase to ~120°C in 1.2 km depth and thus reach the maximum that can potentially be tolerated by microbes. However, a more reasonable T estimate for sterilisation of nutrient-poor deep sediments is 80-90°C. Aim of Expedition 370 was to assess how microbial communities change with depth, by which factors the changes are controlled and where microbial life ceases. It is part of the scientific program to investigate energy substrates available at depth and to identify unique geochemical and microbial signatures that differentiate the biotic and abiotic realms and/or their transitions. High-resolution and high-precision pore water data were produced enabling an identification of reaction fronts, potential microbial activity and hydrothermal alteration. A large part of the cored interval was methanic and sulfate-free. Microbial activity hence depends on electron acceptors other than sulfate. Recent studies indicate that the classical redox sequence needs to be complemented by Fe and Mn reduction in methanic sediments and that biogeochemical processes in natural systems show a stronger link to mineralogy than to a strict vertical sequence of reactions according to calculated energy yields. Fe(III) reduction is one of the most ancient forms of microbial respiration and iron reducers can grow under high T and pressure conditions, which suggests that Fe(III) reducers are potential candidates to survive close to T limit of the deep biosphere.We identified Fe and Mn reduction zones in methanic sediments of Site C0023. By applying sequential extractions we aim at assessing which Fe and Mn phases are available as electron acceptors and how strongly primary minerals have been diagenetically altered. Of particular interest are ash layers as those have been identified earlier as hotspots for microbial life. Ash layers are ubiqous in C0023 sediments and are typically rich in Fe and Mn.Microbial Fe reduction enriches 54Fe in pore water and, thus, authigenic Fe minerals (e.g. siderite, magnetite), whereas abiotic reactions with sulfide lead to more 56Fe in the dissolved phase. We aim at using stable Fe isotopes of dissolved and reactive solid Fe to discriminate microbial and abiotic drivers of Fe reduction. The d56Fe composition of solid Fe will be measured on Fe-carbonates, ferrihydrite + lepidocrocite, goethite + hematite and magnetite. The extent of sulfidation that has implications for the interpretation of magnetic property data will be determined by extracting acid volatile sulfur and chromium reducible sulfur. It is the aim of this project to assess the role of iron oxides for microbial respiration and the related diagenetic alterations in deep sediments of Site C0023.