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

点击翻译按钮获取中文摘要

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.