Does organic sulfur make a significant and overlooked contribution to sediment sulfate reduction in low-sulfate environments?

硫是生命物质中永远存在的成分。随着水生生物的生长、死亡和腐烂，硫在生物和环境之间交换，在活细胞中的有机形式和周围水中的无机形式之间循环。硫地球化学中的一个重要过程是硫酸盐还原。这一过程是由微生物在缺氧的环境中进行的，它将硫酸盐（一种氧化的、常见的无机硫形式）转化为硫化氢，硫化氢具有很强的反应性，通常对其他生物有毒。这一过程具有多重环境意义：它调节磷等重要营养物质和汞等污染物的通量；这是一种将大量有机碳转化为二氧化碳和低分子量羧酸的途径；它还会影响野生水稻等重要商业水生植物的生命周期。在地质时间尺度上，它是硫化铁形成的原因，这些硫化铁被保存在沉积岩中，包含了可追溯到地球历史最早阶段的环境条件记录。然而，目前对硫酸盐还原的理解在很大程度上是由海洋环境的研究形成的，在海洋环境中，硫酸盐丰富，生物很容易从海水中获得。在淡水湖、河流和其他低硫酸盐环境（包括遥远地质时代的海洋和海底深处的沉积物）中，情况就不同了。在这些低硫酸盐环境中，有机硫似乎是硫酸盐还原中更重要的硫源，然而，它的循环途径及其在不同条件下的影响程度尚不清楚。来自明尼苏达大学德卢斯分校的地球化学家、有机化学家和地球微生物学家团队将通过研究硫转化和相关微生物来解决这些问题，重点研究苏必利尔湖及其最大的美国支流作为研究区域。他们将收集沉积物，分析与地球化学相关的硫物种，测量这些不同物种之间的反应速率，并进行遗传分析以确定关键的参与微生物。如果最初的假设得到证实，结果可能会改变目前在这种低硫酸盐环境中硫化学的范式，影响几个科学学科，并为更好的环境管理实践提供基础。该项目将支持两名初级研究人员，并将培训两名研究生和几名本科生。研究游船将为苏必利尔湖的几个合作项目提供免费支持。结果、模型和方法将被纳入一个创新的湖沼学课程，由国家科学基金会支持的pi正在开发。调查结果将通过一系列讲座、K-12教师教育活动（包括在YouTube上播放的R/V Blue Heron号上的苏必利尔湖教师教育游船）和德卢斯淡水水族馆的展览向公众传播。水生沉积物中微生物介导的硫酸盐还原作用使有机碳矿化，产生硫化氢，并介导其他元素（如铁、磷和汞）的地球化学循环。虽然有机物质中含有许多硫化合物，但人们对这种有机硫池在矿化过程中的命运知之甚少，更重要的是它对无机硫循环的贡献，无机硫循环是硫酸盐还原的燃料。目前硫酸盐还原的范例涉及硫从上覆水扩散到存在硫还原微生物的沉积物中。与范式相反，建模和初步结果表明，在低硫酸盐条件下，埋藏在沉积物中的有机硫可能是硫酸盐还原的主要硫源，一旦被动员，通过微生物生物转化，可能会出口到上覆水柱。有机硫的贡献可能在过去地质时期的淡水湖或海洋等环境中普遍存在。通过表征苏必利尔湖及其最大的美国支流沉积物中不同硫酸盐和有机碳水平的有机硫转化，研究人员将在一系列环境条件下解决以下问题：a .有机硫在多大程度上有助于促进硫酸盐还原的硫池？B.有机硫是否经历隐性的、微生物介导的生物地球化学转化？哪些微生物负责这些转化？他们组建了一个多学科的研究团队，结合了沉积物地球化学、有机地球化学和地球微生物学方面的专业知识，利用沉积物特征、速率测量、微生物群落的分子特征和建模来实现这些目标。该结果将量化成岩硫循环的一个重要组成部分，尽管它在地质过去的淡水湖、深层地下和低硫酸盐海洋等环境中具有潜在意义，但很少受到关注。验证提出的假设可能会导致硫和相关元素（如铁和氮）的地球化学循环的重新评价，包括硫酸盐-甲烷过渡带的隐反应；现代水生沉积物和古代沉积岩中硫同位素特征来源的重新解释硫化物的保护和管理措施影响了水体。该项目将产生新的微生物和地球化学数据，这些数据将公开提供。该奖项反映了美国国家科学基金会的法定使命，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。

Sulfur is an ever-present component of living matter. As aquatic organisms grow, die, and decay, sulfur is exchanged between the organisms and their environment, cycling between its organic forms in living cells and inorganic forms in ambient water. An important process in the geochemistry of sulfur is sulfate reduction. This process, carried out in the environment by microorganisms in the absence of oxygen, converts sulfate, the oxidized and commonly available form of inorganic sulfur, into hydrogen sulfide, which is highly reactive and generally toxic to other organisms. This process has multiple environmental significances: it regulates the fluxes of important nutrients such as phosphorus and pollutants such as mercury; it is a pathway by which significant amounts of organic carbon are converted into carbon dioxide and low molecular weight carboxylic acids; and it affects the life cycles of commercially important aquatic plants such as wild rice. Over geological time scales, it is responsible for the formation of iron sulfides, which are preserved in sedimentary rocks and contain the record of environmental conditions dating back to the earliest stages of Earth's history. Present understanding of sulfate reduction, however, has been largely shaped by studies in marine settings where sulfate is abundant and easily available to organisms from seawater. The situation is different in freshwater lakes, rivers, and other low-sulfate environments, which include the oceans of distant geologic past and sediments deep below seafloor. Organic sulfur appears to be a much more important source of sulfur for sulfate reduction in these low-sulfate environments, however the pathways by which it circulates and the magnitude of its effects in different conditions are unknown. A team of geochemists, organic chemists, and geomicrobiologists from the University of Minnesota Duluth will address these questions by studying the sulfur transformations and relevant microorganisms, focusing on Lake Superior and its largest American tributary as the study area. They will collect sediments, analyze them for geochemically relevant sulfur species, measure reaction rates between these different species, and perform genetic analyses to identify key involved microbes. If the initial hypotheses are confirmed, the results are likely to transform the current paradigm of sulfur chemistry in such low-sulfate environments, influencing several scientific disciplines and providing a foundation for better environmental management practices. The project will support two beginning investigators and will train two graduate and several undergraduate students. Research cruises will provide no-cost support for several collaborative efforts on Lake Superior. Results, models, and methods will be incorporated into an innovative Limnology curriculum being developed by the PIs with NSF support. Findings will be communicated to public through a series of talks, K-12 teacher education events including a teacher education cruise on Lake Superior aboard the R/V Blue Heron broadcasted on YouTube, and exhibitions at Duluth Freshwater Aquarium. Microbially mediated sulfate reduction in aquatic sediments mineralizes organic carbon, generates hydrogen sulfide, and mediates the geochemical cycles of other elements, such as iron, phosphorus, and mercury. While organic matter contains a number of sulfur compounds, little is known of the fate of this organic sulfur pool during mineralization and more importantly its contribution to the inorganic sulfur cycle that fuels sulfate reduction. The current paradigm of sulfate reduction involves diffusion of sulfur from overlying water into the sediments where sulfur-reducing microorganisms are present. Contrary to the paradigm, modeling and preliminary results demonstrate that under low-sulfate conditions organic sulfur buried in sediment may be the dominant source of sulfur for sulfate reduction, and once mobilized, via microbial biotransformation, may be exported to the overlying water column. Contributions from organic sulfur may be pervasive in environments such as oligotrophic freshwater lakes or the oceans of the geologic past. By characterizing the organic sulfur transformations in sediments across a range of sulfate and organic carbon levels in Lake Superior and its largest American tributary, investigators will address the following questions under a range of environmental conditions: A. To what extent does organic sulfur contribute to the pool of sulfur that fuels sulfate reduction? B. Does organic sulfur undergo cryptic, microbially-mediated biogeochemical transformations, and what microorganisms are responsible for these transformations? They have assembled a multidisciplinary research team that combines expertise in sediment geochemistry, organic geochemistry, and geomicrobiology to address these objectives using sediment characterizations, rate measurements, molecular characterizations of microbial communities, and modeling. The results will quantify an important part of the diagenetic sulfur cycle that has received little attention despite its potential significance in environments such as freshwater lakes, deep subsurface, and the low sulfate oceans of the geological past. Verifying the proposed hypotheses may lead to reevaluation of the geochemical cycles of sulfur and associated elements such as iron and nitrogen, including cryptic reactions in the sulfate-methane transition zone; reinterpretation of the origins of the isotopic signatures of sulfur preserved in both modern aquatic sediments and ancient sedimentary rocks; and conservation and management practices in sulfide affected water bodies. The project will generate novel microbial and geochemical data that will be publicly available.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.