Collaborative Research: Humics and Iron Redox Reactions in Bangladesh Aquifer

合作研究：孟加拉国含水层腐殖质和铁氧化还原反应。据观察，在缺氧条件下，许多沉积含水层的地下水中砷（As）浓度升高，对世界一些地区的人类健康构成重大威胁。含砷铁（III）氧化物的微生物还原被用来解释砷的富集，尽管铁（III）氧化物还原的机制还没有完全解决。微生物Fe(III)-氧化物还原的一个潜在重要因素是称为腐殖质物质的复杂有机分子的可用性和丰度，它可以在微生物和Fe(III)-氧化物之间传递电子。本研究评估了腐殖质电子穿梭在孟加拉国含水层中各种化学、空间和季节梯度的feii还原和随后的As释放中的作用。季节性补给的化学变化被假设为微生物群落的结构。反过来，假设微生物通过腐殖质的电子穿梭调节局部铁还原速率，最终影响砷向含水层的释放。该研究旨在确定在浅层含水层补给过程中腐殖质的电子穿梭是否增强了Fe（III）氧化物的还原，以及电子穿梭是否在流动路径下游普遍缺乏有机物的沉积物中受到限制。这项研究在孟加拉国的阿拉哈扎尔进行，那里的浅层含水层显示出一系列砷浓度和再补给速率。除了分析水和沉积物的化学性质（pH、Eh、DO、硝酸盐、Fe、Mn、硫酸盐）和氧化还原性质（FeII/FeIII、AsIII/AsV）外，本研究还采用荧光光谱法评估野外腐殖质的氧化还原状态，并采用分子系统发育方法进行微生物群落表征。沉积物电子穿梭能力也将利用在野外开始的孵育来测量。该研究提供了关键数据（例如，电子穿梭化合物浓度和微生物群落变化的代用物），以增强对季节性洪水、农业和人类排泄物同时影响碳动态的系统中Fe（III）还原机制的理解。由于铁还原的许多重要的自然生物地球化学后果，我们的研究将对其他三角洲环境产生影响，远远超出对As迁移率的解释。

Collaborative Research: Humics and Iron Redox Reactions in Bangladesh AquiferY. Zheng, D. Nemergut and D. McKnightElevated concentrations of arsenic (As) in groundwater in many sedimentary aquifers have been observed to occur under anoxic conditions, posing significant threats to human health in some regions of the world. Microbial reduction of As-bearing Fe(III)-oxides has been invoked to explain the enrichment of As, although the mechanisms of Fe(III)-oxide reduction are not fully resolved. One potentially important factor in microbial Fe(III)-oxide reduction is the availability and abundance of complex organic molecules called humic substances which can shuttle electrons between microbes and Fe(III)-oxides. This study evaluates the role of electron-shuttling by humic substances in FeIII-reduction and subsequent As release in Bangladesh aquifers over a variety of chemical, spatial and seasonal gradients. The chemistry of the seasonally variable recharge is hypothesized to structure microbial communities. The microbes, in turn, are hypothesized to regulate local rates of Fe reduction through electron shuttling by humics, ultimately influencing As release to the aquifer. The study seeks to ascertain if Fe(III) oxide reduction is enhanced by electron shuttling of humic substances during recharge to the shallow aquifer, and if electron shuttling is limited in the generally organic-poor sediment further down the flow path . The study takes place in Araihazar, Bangladesh, where the shallow aquifer has shown a range of As concentrations and recharges rates. In addition to assaying the chemical (pH, Eh, DO, nitrate, Fe, Mn, sulphate) and redox properties (FeII/FeIII, AsIII/AsV) of water and sediment, the study employs fluorescence spectroscopy to evaluate redox state of humics in the field and molecular phylogenetic methods for microbial community characterization. Sediment electron shuttling capacities will also be measured using incubation initiated in the field. This study provides critical data (e.g., proxies for concentrations of electron shuttling compounds and microbial community shifts) to enhance the understanding of the mechanisms of Fe(III) reduction in a system where seasonal flooding, agriculture, and human waste are simultaneously influencing carbon dynamics. Because of the many important natural biogeochemical consequences of Fe-reduction, our study will have implications for other deltaic environments far beyond the interpretation of As mobility.