High- and low-affinity binding sites for Cd on the bacterial cell walls of Bacillus subtilis and Shewanella oneidensis     

Bhoopesh Mishra  Maxim Boyanov  Shelly D. Kelly  Jeremy B. Fein 《Geochimica et cosmochimica acta》2010,74(15):4219-9603

Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3-9 that requires the presence of four distinct sites, with pK_a values of 3.3 ± 0.2, 4.8 ± 0.2, 6.7 ± 0.4, and 9.4 ± 0.5, and site concentrations of 8.9(±2.6) × 10⁻⁵, 1.3(±0.2) × 10⁻⁴, 5.9(±3.3) × 10⁻⁵, and 1.1(±0.6) × 10⁻⁴ moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1-200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.  相似文献   

Effect of temperature and mass transport on transition metal isotope fractionation during electroplating     

Jay R. Black  Seth John  Abby Kavner 《Geochimica et cosmochimica acta》2010,74(18):5187-715

Transition metal stable isotope signatures can be useful for tracing both natural and anthropogenic signals in the environment, but only if the mechanisms responsible for fractionation are understood. To investigate isotope fractionations due to electrochemistry (or redox processes), we examine the stable isotope behavior of iron and zinc during the reduction reaction  + 2e⁻ = M_metal as a function of electrochemical driving force, temperature, and time. In all cases light isotopes are preferentially electroplated, following a mass-dependent law. Generally, the extent of fractionation is larger for higher temperatures and lower driving forces, and is roughly insensitive to amount of charge delivered. The maximum fractionations are δ^56/54Fe = −4.0‰ and δ^66/64Zn = −5.5‰, larger than observed fractionations in the natural environment and larger than those predicted due to changes in speciation. All the observed fractionation trends are interpreted in terms of three distinct processes that occur during an electrochemical reaction: mass transport to the electrode, chemical speciation changes adjacent to the electrode, and electron transfer at the electrode. We show that a large isotope effect adjacent the electrode surface arises from the charge-transfer kinetics, but this effect is attenuated in cases where diffusion of ions to the electrode surface becomes the rate-limiting step. Thus while a general increase in fractionation is observed with increasing temperature, this appears to be a result of thermally enhanced mass transport to the reacting interface rather than an isotope effect associated with the charge-transfer kinetics. This study demonstrates that laboratory experiments can successfully distinguish isotopic signatures arising from mass transport, chemical speciation, and electron transfer. Understanding how these processes fractionate metal isotopes under laboratory conditions is the first step towards discovering what role these processes play in fractionating metal isotopes in natural systems.  相似文献   

The impact of over 100 years of wildfires on mercury levels and accumulation rates in two lakes in southern California,USA     

Sarah E. Rothenberg  Matthew E. Kirby  Broxton W. Bird  Margie B. DeRose  Chu-Ching Lin  Xinbin Feng  Richard F. Ambrose  Jennifer A. Jay 《Environmental Earth Sciences》2010,60(5):993-1005

In southern California, USA, wildfires may be an important source of mercury (Hg) to local watersheds. Hg levels and Hg accumulation rates were investigated in dated sediment cores from two southern California lakes, Big Bear Lake and Crystal Lake, located approximately 40-km apart. Between 1895 and 2006, fires were routinely minimized or suppressed around Big Bear Lake, while fires regularly subsumed the forest surrounding Crystal Lake. Mean Hg concentrations and mean Hg accumulation rates were significantly higher in Crystal Lake sediments compared to Big Bear Lake sediments (Hg levels: Crystal Lake 220 ± 93 ng g⁻¹, Big Bear Lake 92 ± 26 ng g⁻¹; Hg accumulation: Crystal Lake 790 ± 1,200 μg m⁻² year⁻¹, Big Bear 240 ± 54 μg m⁻² year⁻¹). In Crystal Lake, the ratio between post-1965 and pre-1865 Hg concentrations was 1.1, and several spikes in Hg levels occurred between 1910 and 1985. Given the remote location of the lake, the proximity of fires, and the lack of point sources within the region, these results suggested wildfires (rather than industrial sources) were a continuous source of Hg to Crystal Lake over the last 150 years.  相似文献   

Spatiotemporal Dynamics of a Páramo Ecosystem in the Northern Ecuadorian Andes 1988-2007     

Oliver WIGMORE  Jay GAO 《山地科学学报》2014,11(3):708-716

Páramo is a term used to describe tropical alpine vegetation between the continuous timberline and the snow line in the Northern Andes. Páramo environments provide important species habitat and ecosystem services. Changes in spatial extent of the páramo ecosystem at Pambamarca in the Central Cordillera of the northern Ecuadorian Andes were analysed using multi-temporal Landsat TM/ETM+ satellite data. The region suffered a loss of 1826.6 ha or 20% of the total area at a rate of 100 ha/annum during 1988-2007 period. It is found that permanent páramo cover decreased from 8350 ha in 1988 to 5864 ha in 2007 at a fairly constant rate(R2=0.94). This loss is attributed to expansion of commercial agriculture and floriculture in the valleys coupled with increased population pressure. Land at higher elevations has been cleared for small scale agriculture. Loss of the páramo ecosystem will exert a number of negative impacts on ecosystem services and livelihoods of the local population at Pambamarca.  相似文献