Direct Measurement and Modeling of the Importance of Bacterial Adsorption of Cd in Natural Samples

细菌存在于广泛的地质环境中，在地表沃茨、土壤甚至更深的地下含水层中发现了高浓度的细菌。地表水和地下水中的金属阳离子吸附在细菌上，这种吸附可以影响元素的全球循环、生物矿化、土壤和地下水系统中重金属污染物的移动性、矿物溶解以及地下水生物修复技术的有效性。在过去的20年里，已经开发了模型来描述细菌表面的金属吸附，计算表明，细菌金属吸附可以控制地质系统中的金属形态。然而，还没有研究直接量化细菌金属吸附在复杂的自然环境中的重要性。因此，虽然细菌表现出吸附金属的高亲和力，但只有间接证据表明细菌吸附影响真实的系统中的金属分布。研究人员将使用一种创新的方法，包括共聚焦激光扫描显微镜，直接测量与河流和湿地水系统复杂样品中细菌细胞相关的吸附金属的比例。这项研究将是第一个直接和定量地确定细菌吸附在影响自然样品中金属分布方面的重要性，因此也将是第一个测试当前热力学模型解释这种分布的能力的研究。资助的研究最终将导致更准确地预测金属在一系列含细菌的地质环境中的命运和运输，并可用于优化受污染地下水和地表水系统的修复策略。在大量的研究中已经测量了金属对细菌的吸附，并且结果已经被用于确定金属-细菌络合物的热力学稳定常数。 有两个基本假设证明了这一大型研究机构的合理性：1）细菌对金属的吸附可以影响含菌地质系统中金属的分布，2）从简单的单一金属，单一细菌物种实验中确定的结合常数可以准确预测金属在这些复杂系统中的分布。这些假设都没有经过严格的自然系统测试，主要是由于测量复杂样品中的金属形态的困难。这项拟议中的研究将首次对自然、非人工系统的这两个假设进行定量测试，从而提高我们对细菌如何影响地质系统中物质运输的理解。研究人员将表征一系列天然样品，量化细菌，有机物和矿物含量;他们将向每个样品中添加含水金属，并将使用一种新的共聚焦激光扫描显微镜方法，结合最近开发的金属特异性荧光探针，以确定这些复杂系统中细菌上结合的金属浓度。独立地，研究人员将使用先前确定的热力学稳定常数来预测每个实验系统中金属的分布，以便预测和观察到的金属分布之间的比较将使他们能够确定建模方法的准确性。这些实验将是第一个定量确定细菌吸附多组分天然样品中金属的重要性的实验，因此具有变革性，因为它们将提供对地质系统中金属分布控制的新理解，以及量化这些系统中金属分布的方法。

Bacteria are present in a wide range of geologic environments, with high concentrations found in surface waters, soils, and even deeper subsurface aquifers. Aqueous metal cations in surface and groundwaters adsorb onto bacteria, and this adsorption can affect the global cycling of elements, biomineralization, heavy metal contaminant mobility in soils and groundwater systems, mineral dissolution, and the effectiveness of groundwater bioremediation techniques. Over the past 20 years, models have been developed to describe metal adsorption onto bacterial surfaces, and calculations suggest that bacterial metal adsorption can control metal speciation in geologic systems. However, there have been no studies that directly quantify the importance of bacterial metal adsorption in complex natural settings. Therefore, although bacteria exhibit a high affinity for adsorbing metals, there is only circumstantial evidence that bacterial adsorption affects metal distributions in real systems. The investigators will use an innovative approach involving confocal laser scanning microscopy to directly measure the proportion of adsorbed metal that is associated with bacterial cells in complex samples from river and wetland water systems. The research will be the first to directly and quantitatively determine the importance of bacterial adsorption in affecting metal distributions in natural samples, and hence will also be the first to test the ability of current thermodynamic models to account for that distribution. The funded research will ultimately lead to more accurate predictions of the fate and transport of metals in a range of bacteria-bearing geologic settings, and can be used to optimize remediation strategies for contaminated groundwater and surface water systems. Metal adsorption onto bacteria has been measured in scores of studies, and the results have been used to determine thermodynamic stability constants for metal-bacteria complexes. There are two underlying assumptions that justify this large body of research: 1) that bacterial adsorption of metals can affect the distribution of metals in bacteria-bearing geologic systems, and 2) that the binding constants determined from simple single metal, single bacterial species experiments can accurately predict metal distributions in those complex systems. Neither of these assumptions has been rigorously tested for natural systems, primarily due to the difficulty of the measuring metal speciation in complex samples. The proposed research will, for the first time, quantitatively test both of these assumptions for natural, non-artificial, systems, and hence will improve our understanding of how bacteria affect mass transport in geologic systems. Investigators will characterize a range of natural samples, quantifying the bacterial, organic matter, and mineralogical contents; they will add aqueous metal to each sample, and they will use a novel confocal laser scanning microscopy approach, in conjunction with recently-developed metal-specific fluorescent probes, to determine the concentration of metal bound onto the bacteria in these complex systems. Independently, investigators will use previously determined thermodynamic stability constants to predict the distribution of metal in each experimental system, so that the comparison between the predicted and observed metal distributions will enable them to determine the accuracy of the modeling approach. The experiments will be the first to quantitatively determine the importance of bacterial adsorption of metals in multi-component natural samples, and hence are transformative in that they will provide both a new understanding of the controls on metal distributions in geologic systems as well as a means for quantifying metal distributions in those systems.