Biogeochemical Cycling Of Metals Research Articles

Metal binding properties of the EPS produced by Halomonas sp. TG39 and its potential in enhancing trace element bioavailability to eukaryotic phytoplankton

An emergent property of exopolysaccharides (EPS) produced by marine bacteria is their net negative charge, predominantly conferred by their high uronic acids content. Here, we investigated the EPS produced by an algal-associated marine bacterium, Halomonas sp. strain TG39, for its capacity to sequester trace metals and mediate their bioavailability to eukaryotic phytoplankton. Metal analysis of the purified EPS revealed that it contained high levels of K, Ca, Mg and several essential trace metals, including Zn, Cu, Fe and the metalloid Si. Desorption experiments with marine sediment showed that the EPS possessed a specific binding capacity for Ca, Si, Fe, Mn, Mg and Al. Depending on the ionic conditions, Fe was the third or fourth most highly-adsorbed metal out of 27 elements analyzed. Experiments employing Fe-limited synthetic ocean seawater showed that growth of the marine diatom Thalassiosira weissflogii (axenic strain) was enhanced when incubated in the presence of either purified EPS or EPS that had been pre-exposed to marine sediment, compared to non-EPS amended controls. This growth enhancement was attributed to the EPS binding and increasing the bioavailability of key trace metal elements, such as Fe(III). Since the bacterium used in this study was originally isolated from a marine micro-alga, this work highlights the possibility that bacterial associates of eukaryotic algae could be influencing the bioavailability of Fe(III) to phytoplankton via their production of polyanionic EPS. More widely, this work reinforces the potential importance of marine bacterial EPS in trace metal biogeochemical cycling.

Fields and Forests in Flames: Lead and Mercury Emissions from Wildfire Pyrogenic Activity

In the article “Fields and Forests in Flames,” Weinhold (2011) addressed the toxic health effects associated with fire smoke. Although he acknowledged the limited data on the toxicity of wildfires, several important studies on environmental emissions from fire events and their consequences were omitted. Weinhold (2011) listed multiple compounds from wildfires, back burning, and incinerated buildings, but listed only four elements: potassium, chlorine, sulfur, and silicon. Significant omissions were the toxic elements lead and mercury. Lead has been identified as one of the most environmentally pervasive and damaging metals to human health (Patterson 1965). Several studies have detailed the remobilization of metals from fire events (e.g., Finley et al. 2009; Nriagu 1989; Odigie and Flegal 2011; Young and Jan 1977). These studies showed that significant levels of toxic and nontoxic metals are emitted into the environment during fires. Young and Jan (1977) found that smoke from a 1975 Californian wildfire emitted various metals, including cadmium, chromium, copper, iron, lead, manganese, nickel, silver, and zinc, up to 100 km from the fire. Contamination of local marine waters with lead, iron, and manganese from the wildfire exceeded the polluting effects of the local municipal wastewater, the main source of metals. Nriagu (1989) and Finley et al. (2009) estimated that the amount of lead (plus other trace metals) and mercury, respectively, from fires were comparable to emissions from anthropogenic sources such as industrial processes and city pollution. Nriagu (1989) estimated that global emissions of lead from wildfires ranged from 60,000 to 3,800,000 kg/year, with an average of 1,900,000 kg/year. Global mercury emissions from wildfires are also significant, estimated at 890,000 ± 490 kg/year for gaseous elemental mercury and 170 ± 100 kg/year for particulate-bound mercury (Finley et al. 2009). Until recently it was not known whether lead released by wildfires is from natural and or industrial sources. Odigie and Flegal (2011) measured the isotopic lead composition of ash from the 2009 Jesusita Fire in Southern California. Their work showed clearly that the ash from the wildfire contained industrial lead primarily from leaded gasoline used in Southern California during the 1960s through the 1980s. Environmental media, such as air, dust, sediment, soil, and water, have well-defined and strict environmental and human health guidelines because of their damaging effect on natural and anthropogenic systems. Even low levels of atmospheric lead emissions are known to cause adverse human health effects, including irreversible neurological damage. For example, the U.S. Environmental Protection Agency (EPA) recently reduced the lead-in-air guideline by an order magnitude—from 1.5 µg/m3 to 0.15 µg/m3—after reviewing > 6.000 human health–lead-related studies (U.S. EPA 2008). Although pyrogenic activity affects environmental quality, its effects remain ill-defined, despite evidence of harmful human health effects from exposure to toxicants, even at very low levels (Lanphear et al. 2005). The risk from fires is likely to increase as the frequency of climatically driven fire events rises in response to predicted global warming (Intergovernmental Panel on Climate Change 2007). The predicted environmental changes present a significant research opportunity for those interested in monitoring the biogeochemical cycling of metals and their potential risk of harm to human and environmental health systems.

Roles of siderophore in manganese-oxide reduction by Shewanella oneidensisMR-1

Dissimilatory metal-reducing bacteria (DMRB), such as Shewanella oneidensis MR-1, are of great interest for their importance in the biogeochemical cycling of metals and utility in biotechnological processes, such as bioremediation and microbial fuel cells. To identify genes necessary for metal reduction, this study constructed a random transposon-insertion mutant library of MR-1 and screened it for isolating mutants that were deficient in metal reduction. Examination of approximately 5000 mutants on lactate minimal-medium plates containing MnO(2) resulted in the isolation of one mutant, strain N22-7, that showed a decreased MnO(2)-reduction activity. Determination of a transposon-insertion site in N22-7 followed by deletion and complementation experiments revealed that the disruption of SO3030, a siderophore biosynthesis gene, was responsible for the decreased MnO(2)-reduction activity. In ΔSO3030 cells, iron and cytochrome contents were decreased to approximately 50% of those in the wild-type cells, when they were incubated under MnO(2)-reduction conditions. In addition, the transcription of genes encoding outer-membrane cytochromes necessary for metal reduction was repressed in ΔSO3030 under MnO(2)-reduction conditions, while their transcription was upregulated after supplementation of culture media with ferrous iron. These results suggest that siderophore is important for S. oneidensis MR-1 to respire MnO(2), because iron availability influences the expression of cytochromes necessary for metal reduction.

Metal Interactions at the Biochar-Water Interface: Energetics and Structure-Sorption Relationships Elucidated by Flow Adsorption Microcalorimetry

Plant-derived biochars exhibit large physicochemical heterogeneity due to variations in biomass chemistry and combustion conditions. However, the influence of biochar heterogeneity on biochar-metal interaction mechanisms has not been systematically described. We used flow adsorption microcalorimetry to study structure-sorption relationships between twelve plant-derived biochars and two metals (K(+) and Cd(2+)) of different Lewis acidity. Irrespective of the biochar structure, sorption of K(+) (a hard Lewis acid) occurred predominantly on deprotonated functional groups via ion exchange with molar heats of adsorption (ΔH(ads)) of -4 kJ mol(-1) to -8 kJ mol(-1). By comparison, although ion exchange could not be completely ruled out, our data pointed to Cd(2+) (a soft Lewis acid) sorption occurring predominantly via two distinct cation-π bonding mechanisms, each with ΔH(ads) of +17 kJ mol(-1). The first, evident in low charge-low carbonized biochars, suggested Cd(2+)-π bonding to soft ligands such as -C ═ O; while the second, evident in low charge-highly carbonized biochars, pointed to Cd(2+)-π bonding with electron-rich domains on aromatic structures. Quantitative contributions of these mechanisms to Cd(2+) sorption can exceed 3 times that expected for ion exchange and therefore could have significant implications for the biogeochemical cycling of metals in fire-impacted or biochar-amended systems.

Temperature and irradiance influences on cadmium and zinc uptake and toxicity in a freshwater cyanobacterium, Microcystis aeruginosa

Temperature and light irradiance are important factors affecting the occurrence of cyanobacterial blooms. In this study, we examined the influences of different temperatures (15, 24, and 30 °C) and irradiances (18, 32, and 55 μmol photons m −2 s −1) on the uptake and toxicity of cadmium (Cd) and zinc (Zn) in a freshwater cyanobacterium Microcystis aeruginosa. The subcellular distribution of Cd and Zn was analyzed. Enhanced growth rates were observed for the cyanobacterial cells incubated at higher temperature or irradiance conditions with lower metal concentrations. With increasing ambient Cd or Zn concentrations, both cellular growth rate and photosynthesis were significantly inhibited at elevated irradiance conditions. The observed increase in Cd and Zn toxicity might be attributed to the enhanced metal uptake and accumulation in Microcystis. Based on the intracellular Cd concentration, the 50% inhibition concentration (IC 50) values were higher at the higher temperature or irradiance treatment. The subcellular distribution demonstrated that Cd in the metal rich granule (MRG) faction increased with elevated [Cd 2+] concentration, suggesting that MRG may partially detoxify the Cd toxicity in the cyanobacterial cells. This study implied that temperature and irradiance may influence the biogeochemical cycling of metals during cyanobacterial blooming in eutrophic freshwater ecosystems.

Chemical speciation and partitioning of trace metals (Cd, Co, Cu, Ni, Pb) in the lower Athabasca river and its tributaries (Alberta, Canada)

Concentrations of Cd, Co, Cu, Ni and Pb were measured in particulate and dissolved phases at 11 sites located upstream and near Athabasca oil sands development. The in situ discrimination between non-labile and labile dissolved metals was done using diffusive gradients in thin-films (DGT) devices. The DGT-labile fraction of Co and Ni was 30% lower near development sites whereas Cu, Cd and Pb showed minor changes spatially. It was found that an 8-fold increase in dissolved organic matter (DOM) near development induced a rapid decrease in DGT-labile metals. Dissolved metal concentrations were used along with DOM, major ions, nutrients, pH and conductivity to calculate the distribution of dissolved metal species using the speciation model WHAM. Labile-DGT metal concentrations agreed well with WHAM-predicted concentrations. It was also found that a significant amount of metals were associated with the non-DGT labile fraction (i.e. colloidal DOM) and colloid abundance was more important than suspended particulate matter abundance in influencing metal mobility near Athabasca oil soils development. Since changes in colloidal DOM levels are likely to be the result of surface mining activities, this confirms the serious effects of oil sands activities on metal biogeochemical cycles in the lower Athabasca River.

TOUGHREACT Version 2.0: A simulator for subsurface reactive transport under non-isothermal multiphase flow conditions

TOUGHREACT is a numerical simulation program for chemically reactive non-isothermal flows of multiphase fluids in porous and fractured media, and was developed by introducing reactive chemistry into the multiphase fluid and heat flow simulator TOUGH2 V2. The first version of TOUGHREACT was released to the public through the U.S. Department of Energy's Energy Science and Technology Software Center (ESTSC) in August 2004. It is among the most frequently requested of ESTSC's codes. The code has been widely used for studies in CO 2 geological sequestration, nuclear waste isolation, geothermal energy development, environmental remediation, and increasingly for petroleum applications. Over the past several years, many new capabilities have been developed, which were incorporated into Version 2 of TOUGHREACT. Major additions and improvements in Version 2 are discussed here, and two application examples are presented: (1) long-term fate of injected CO 2 in a storage reservoir and (2) biogeochemical cycling of metals in mining-impacted lake sediments.

Metallomics: lessons for metalliferous soil remediation

The term metallomics has been established for the investigation of transcriptome, proteome, and metabolome changes induced by metals. The mechanisms allowing the organisms to cope with metals in the environment, metal resistance factors, will in turn change biogeochemical cycles of metals in soil, coupling the metal pool with the root system of plants. This makes microorganisms key players in introducing metals into food webs, as well as for bioremediation strategies. Research on physiological and metabolic responses of microorganisms on metal stress in soil is thus essential for the selection of optimized consortia applicable in bioremediation strategies such as bioaugmentation or microbially enhanced phytoextraction. The results of metallomics studies will help to develop applications including identification of biomarkers for ecotoxicological studies, bioleaching, in situ soil regeneration, and microbially assisted phytoremediation of contaminated land. This review will therefore focus on the molecular understanding of metal resistance in bacteria and fungi, as can be derived from metallomics studies.

Metals, minerals and microbes: geomicrobiology and bioremediation

Microbes play key geoactive roles in the biosphere, particularly in the areas of element biotransformations and biogeochemical cycling, metal and mineral transformations, decomposition, bioweathering, and soil and sediment formation. All kinds of microbes, including prokaryotes and eukaryotes and their symbiotic associations with each other and 'higher organisms', can contribute actively to geological phenomena, and central to many such geomicrobial processes are transformations of metals and minerals. Microbes have a variety of properties that can effect changes in metal speciation, toxicity and mobility, as well as mineral formation or mineral dissolution or deterioration. Such mechanisms are important components of natural biogeochemical cycles for metals as well as associated elements in biomass, soil, rocks and minerals, e.g. sulfur and phosphorus, and metalloids, actinides and metal radionuclides. Apart from being important in natural biosphere processes, metal and mineral transformations can have beneficial or detrimental consequences in a human context. Bioremediation is the application of biological systems to the clean-up of organic and inorganic pollution, with bacteria and fungi being the most important organisms for reclamation, immobilization or detoxification of metallic and radionuclide pollutants. Some biominerals or metallic elements deposited by microbes have catalytic and other properties in nanoparticle, crystalline or colloidal forms, and these are relevant to the development of novel biomaterials for technological and antimicrobial purposes. On the negative side, metal and mineral transformations by microbes may result in spoilage and destruction of natural and synthetic materials, rock and mineral-based building materials (e.g. concrete), acid mine drainage and associated metal pollution, biocorrosion of metals, alloys and related substances, and adverse effects on radionuclide speciation, mobility and containment, all with immense social and economic consequences. The ubiquity and importance of microbes in biosphere processes make geomicrobiology one of the most important concepts within microbiology, and one requiring an interdisciplinary approach to define environmental and applied significance and underpin exploitation in biotechnology.

Effects of bacteria on metal bioavailability, speciation, and mobility in different metal mine soils: a column study

Purpose Successful phytoremediation depends mainly on the bioavailability of heavy metals in the soil. Recently, soil microbes possess several mechanisms that are able to change metal bioavailability in the soil, which provides a new strategy for investigating biogeochemical cycling of metals in contaminated soils. Three metal mines soils with elevated concentrations of Cd, Pb, and Zn from China were applied in this column study to (1) evaluate the effects of metal tolerant bacterial inoculation (Burkholderia cepacia, accession number: AB051408) on metal release, (2) monitor the migration of metals in the rhizospheric horizon (0–20 cm), and (3) investigate metal speciation and sequential fractions in soil.

The role of 4-hydroxyphenylpyruvate dioxygenase in enhancement of solid-phase electron transfer by Shewanella oneidensis MR-1

We hypothesized that Shewanella oneidensis MR-1, a model dissimilatory metal-reducing bacterium, could utilize environmentally relevant concentrations of tyrosine to produce pyomelanin for enhanced Fe(III) oxide reduction. Because homogentisate is an intermediate of the tyrosine degradation pathway, and a precursor of a redox-cycling metabolite, pyomelanin, we evaluated the process of homogentisate production by S. oneidensis MR-1, in order to identify the key steps involved in pyomelanin production. We determined that two enzymes involved in this pathway, 4-hydroxyphenylpyruvate dioxygenase and homogentisate 1,2-dioxygenase are responsible for homogentisate production and oxidation, respectively. We used genetic analysis and physiological characterization of MR-1 strains either deficient in or displaying substantially increased pyomelanin production. The relative significance imparted by pyomelanin on solid-phase electron transfer was also addressed using electrochemical techniques, which allowed us to extend the genetic and physiological findings to biogeochemical cycling of metals. Based on our findings, environmental production of pyomelanin from available organic precursors could contribute to the survival of S. oneidensis MR-1 when dissolved oxygen concentrations become low, by providing an increased capacity for solid-phase metal reduction. This study demonstrates the role of organic precursors and their concentrations in pyomelanin production, solid phase metal reduction and biogeochemical cycling of iron.

The effects of dissolved organic carbon, acidity and seasonality on metal geochemistry within a forested catchment on the Precambrian Shield, central Ontario, Canada

Metal pollution, in combination with other environmental stressors such as acid deposition and climate change, may disturb metal biogeochemical cycles. To investigate the influence of dissolved organic carbon, acidity and seasonality on metal geochemistry, this study has described concentrations of 19 metals as they pass through an acidified forested catchment on the Precambrian Shield in south-central Ontario, Canada. Metal, dissolved organic carbon (DOC) and sulphate (SO4 2−) concentrations fluctuate throughout the catchment compartments as the water passes through and interacts with vegetation, soils and bedrock. Relationships among metals, DOC and SO4 2− are most pronounced in compartments where DOC and SO4 2− exhibit high variability, namely in the throughfall, organic horizon soil water, and wetland-draining stream. Metal, DOC and SO4 2− concentrations varied seasonally in the streams, and temporal coherence occurred among metal, DOC and SO4 2− concentrations in the organic horizon soil water and the wetland-draining stream (PC1). In the wetland-draining stream, the highest DOC, Cr, Cu, Fe, Pb, and V concentrations occur in the summer, whereas concentrations of SO4 2− and most other metals peak in the fall after a period of drought. Despite the rural location, provincial water quality objectives for surface water were exceeded for many metals when the peak fall values occurred.

Effect of Humic Acid on Cd(II), Cu(II), and Pb(II) Uptake by Freshwater Algae: Kinetic and Cell Wall Speciation Considerations

Thepresent study examines the effect of humic acid on the uptake kinetics of Cd(II), Cu(II), and Pb(II) by the freshwater alga Chlorella kesslerii. The results demonstrated that the relative proportion of Pb in the cell wall layer is greater than that of the internalized Pb, while internalized Cd and Cu were comparable or greater than the adsorbed metal concentration. In the presence of 10 mg L(-1) humic acid (HA), Cd and Cu uptake kinetics were consistent with that predicted by measured free metal concentrations. For Pb, the uptake flux and amount of lead bound to internalization and adsorption sites were an order of magnitude higher than those found at the same free lead ion concentrations in the presence of citric acid. Chemodynamic modeling suggested that the enhancement of the Pb uptake flux in the presence of HA originates from an increasing amount of Pb bound to the internalization sites through a ternary complex formation between lead-humic acid complex and internalization sites. Cell wall speciation calculations indicated that the lead-humic acid complex is the predominant species in the cell wall layer, while for Cu(II) and Cd(II) metal bound to the internalization (Cu) and adsorption (Cd) sites significantly dominated over the M-HA complex. The findings of the work show the relevance of the cell wall layer concentration and speciation and its key role in defining the local equilibrium conditions between metal and internalizations sites. The results of the present kinetic study have important consequences for improvement of the mechanistic understanding of the role of dissolved organic matter in metal uptake in phytoplankton and biogeochemical cycling of metals in the surface waters.

Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications

BackgroundAcidithiobacillus ferrooxidans is a major participant in consortia of microorganisms used for the industrial recovery of copper (bioleaching or biomining). It is a chemolithoautrophic, γ-proteobacterium using energy from the oxidation of iron- and sulfur-containing minerals for growth. It thrives at extremely low pH (pH 1–2) and fixes both carbon and nitrogen from the atmosphere. It solubilizes copper and other metals from rocks and plays an important role in nutrient and metal biogeochemical cycling in acid environments. The lack of a well-developed system for genetic manipulation has prevented thorough exploration of its physiology. Also, confusion has been caused by prior metabolic models constructed based upon the examination of multiple, and sometimes distantly related, strains of the microorganism.ResultsThe genome of the type strain A. ferrooxidans ATCC 23270 was sequenced and annotated to identify general features and provide a framework for in silico metabolic reconstruction. Earlier models of iron and sulfur oxidation, biofilm formation, quorum sensing, inorganic ion uptake, and amino acid metabolism are confirmed and extended. Initial models are presented for central carbon metabolism, anaerobic metabolism (including sulfur reduction, hydrogen metabolism and nitrogen fixation), stress responses, DNA repair, and metal and toxic compound fluxes.ConclusionBioinformatics analysis provides a valuable platform for gene discovery and functional prediction that helps explain the activity of A. ferrooxidans in industrial bioleaching and its role as a primary producer in acidic environments. An analysis of the genome of the type strain provides a coherent view of its gene content and metabolic potential.

Shewanella secretes flavins that mediate extracellular electron transfer

Bacteria able to transfer electrons to metals are key agents in biogeochemical metal cycling, subsurface bioremediation, and corrosion processes. More recently, these bacteria have gained attention as the transfer of electrons from the cell surface to conductive materials can be used in multiple applications. In this work, we adapted electrochemical techniques to probe intact biofilms of Shewanella oneidensis MR-1 and Shewanella sp. MR-4 grown by using a poised electrode as an electron acceptor. This approach detected redox-active molecules within biofilms, which were involved in electron transfer to the electrode. A combination of methods identified a mixture of riboflavin and riboflavin-5'-phosphate in supernatants from biofilm reactors, with riboflavin representing the dominant component during sustained incubations (>72 h). Removal of riboflavin from biofilms reduced the rate of electron transfer to electrodes by >70%, consistent with a role as a soluble redox shuttle carrying electrons from the cell surface to external acceptors. Differential pulse voltammetry and cyclic voltammetry revealed a layer of flavins adsorbed to electrodes, even after soluble components were removed, especially in older biofilms. Riboflavin adsorbed quickly to other surfaces of geochemical interest, such as Fe(III) and Mn(IV) oxy(hydr)oxides. This in situ demonstration of flavin production, and sequestration at surfaces, requires the paradigm of soluble redox shuttles in geochemistry to be adjusted to include binding and modification of surfaces. Moreover, the known ability of isoalloxazine rings to act as metal chelators, along with their electron shuttling capacity, suggests that extracellular respiration of minerals by Shewanella is more complex than originally conceived.

Ultrastructural and molecular evidence for potentially symbiotic bacteria within the byssal plaques of the deep-sea hydrothermal vent mussel Bathymodiolus azoricus

This study reports on the presence of a putatively symbiotic bacterial flora within the byssus plaque of the deep sea hydrothermal mussel Bathymodiolus azoricus, contributing to metal sequestration/deposition and testing positive to methane oxidizing symbiont-specific fluorescent probes. Combining an array of approaches including histology, electron microscopy, X-ray microanalysis, analytical chemistry, and microbiology we provide evidence for the frequently assumed, but rarely shown influence of prokaryotes on the biogeochemical cycling of metals as well as inorganic C sources (i.e., methane) at deep sea hydrothermal vents. Our results indicate that in spite of its antibacterial protective sheath, the byssus plaque gives access to a whole range of prokaryotic organisms which may be responsible for the extremely high concentration of metallic elements (Fe, Cu, Zn, Mn, Co, Mo, Cd, Pb and Hg) measured in this attachment organ. The very high levels of metals in byssus, together with its frequent renewal rate due to the dynamic nature of the habitat, suggest that intra-byssal bacteria may have a major influence on biomineralisation/deposition of metals. The presence of a methanotroph morphotype within the byssus plaque was confirmed by FISH and TEM. The implications of the biogeochemical cycling of metals and methane at hydrothermal vents are discussed.

Second Acyl Homoserine Lactone Production System in the Extreme AcidophileAcidithiobacillus ferrooxidans

The acidophilic proteobacterium Acidithiobacillus ferrooxidans is involved in the industrial biorecovery of copper. It is found in acidic environments in biofilms and is important in the biogeochemical cycling of metals and nutrients. Its genome contains a cluster of four genes, glyQ, glysS, gph, and act, that are predicted to encode the alpha and beta subunits of glycine tRNA synthetase, a phosphatase, and an acyltransferase, respectively (GenBank accession no. DQ149607). act, cloned and expressed in Escherichia coli, produces acyl homoserine lactones (AHLs) principally of chain length C14 according to gas chromatography and mass spectrometry measurements. The AHLs have biological activity as shown by in vivo studies using the reporter strain Sinorhizobium meliloti Rm41 SinI-. Reverse transcription-PCR (RT-PCR) experiments indicate that the four genes are expressed as a single transcript, demonstrating that they constitute an operon. According to semiquantitative RT-PCR results, act is expressed more highly when A. ferrooxidans is grown in medium containing iron than when it is grown in medium containing sulfur. Since AHLs are important intercellular signaling molecules used by many bacteria to monitor their population density in quorum-sensing control of gene expression, this result suggests that A. ferrooxidans has two quorum-sensing systems, one based on Act, as described herein, and the other based on a Lux-like quorum-sensing system, reported previously. The latter system was shown to be upregulated in A. ferrooxidans grown in sulfur medium, suggesting that the two quorum-sensing systems respond to different environmental signals that may be related to their abilities to colonize and use different solid sulfur- and iron-containing minerals.

Effect of Pseudokirchneriella subcapitata (Chlorophyceae) exudates on metal toxicity and colloid aggregation

The effects of Pseudokirchneriella subcapitata exudates on the acute toxicity of metals (Cd, Cu, Pb and Zn) were studied using (1 h) algal photosynthesis inhibition tests. The metal concentrations tested were 30, 60, 120, 250 and 500 μg L −1 for Cd and Zn; and 500, 1000, 2000, 3000, 4000 μg L −1 for Cu and Pb. It was established that P. subcapitata exudates markedly decreased metal toxicity. This effect was ascribed to: (i) the presence of organic ligands that bind metals and reduce the concentration of free ionic metal, and/or (ii) interaction of exudates with the cell surface with a subsequent decrease in metal uptake. The effects of the exudates on colloid aggregation were also examined using two different types of single particle counters (SPC). Exudates facilitated the colloid removal, likely via acceleration of aggregation (bridging). The results clearly demonstrate that algal exudates play an important role in the biogeochemical cycling of metals in natural surface water: (1) by reducing free metal concentrations and toxicity to living organisms and (2) by favoring colloid aggregation leading to the removal of colloid-bound metals (colloidal pumping). Such results highlight one potential application of the algae in the remediation of metal-contaminated waters. The results also suggest that current algal toxicity testing protocols, in particular long-term and static tests, may underestimate metal toxicity because of the presence of algal exudates.

Enzymology of Electron Transport: Energy Generation With Geochemical Consequences

Dissimilatory metal-reducing bacteria (DMRB) are important components of the microbial community residing in redox-stratified freshwater and marine environments. DMRB occupy a central position in the biogeochemical cycles of metals, metalloids and radionuclides, and serve as catalysts for a variety of other environmentally important processes including biomineralization, biocorrosion, bioremediation and mediators of ground water quality. DMRB are presented, however, with a unique physiological challenge: they are required to respire anaerobically on terminal electron acceptors which are either highly insoluble (e.g., Fe(III)- and Mn(IV)-oxides) and reduced to soluble end-products or highly soluble (e.g., U(VI) and Tc(VII)) and reduced to insoluble end-products. To overcome physiological problems associated with metal and radionuclide solubility, DMRB are postulated to employ a variety of novel respiratory strategies not found in other gram-negative bacteria which respire on soluble electron acceptors such as O2, NO3- and SO42-. The novel respiratory strategies include (1) direct enzymatic reduction at the outer membrane, (2) electron shuttling pathways and (3) metal solubilization by exogenous or bacterially-produced organic ligands followed by reduction of soluble organic-metal compounds. The first section of this chapter highlights the latest findings on the enzymatic mechanisms of metal and radionuclide reduction by two of the most extensively studied DMRB (Geobactermore » and Shewanella), with particular emphasis on electron transport chain enzymology. The second section emphasizes the geochemical consequences of DMRB activity, including the direct and indirect effects on metal solubility, the reductive transformation of Fe- and Mn-containing minerals, and the biogeochemical cycling of metals at redox interfaces in chemically stratified environments.« less

Cation binding in a mor layer: batch experiments and modelling

SummaryThe understanding of cation binding in the mor layer is important to correctly assess the biogeochemical cycling of metals and other cations in forested ecosystems. In a series of batch experiments, the binding of cations was examined in two mor layers from central Sweden. We examined the effect of Ca and Al on the binding of Zn, and also the binding of added Pb, Cu and Cd. Two models, WinHumicV and the Stockholm Humic Model (SHM), were tested for their ability to describe the data obtained. We found that for Zn, the pH at 50% sorption was increased from 2.8 to 4.2 after the addition of 3 mMAl. The proton titration data were well described by both WinHumicV and SHM after optimization of the concentrations of ‘active’ Al and humic substances. Applying generic parameters for cation binding produced deviations between the model simulations and the observations, particularly for the dissolved Pb and Cu concentrations, which were underestimated. A revised set of cation complexation constants was presented that improved the fit, particularly for SHM. For WinHumicV, there were still poor overall fits. The difference in model performance may be due to the greater number of adjustable parameters in the SHM, but probably also to other model‐specific differences. According to the SHM simulations, the binding of Ca, Mg and Mn was mainly non‐specific, whereas Pb, Cu and Al were bound as mono‐ or bidentate complexes. For Zn and Cd, binding occurred through both counter‐ion accumulation and monodentate complexation.