Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania

Metals in soils may positively or negatively affect plants as well as soil micro-organisms and mesofauna, depending on their abundance and bioavailability. Atmospheric deposition and biological uplift commonly result in metal enrichment in surface soils, but the relative importance of these processes is not always resolved. Here, we used an integrated approach to study the cycling of phosphorus and a suite of metals from the soil to the canopy (and back) in a temperate watershed. The behavior of elements in these surface soils fell into three categories. First, Al, Fe, V, Co, and Cr showed little to no enrichment in the top soil layers, and their concentrations were determined primarily by soil production fluxes with little influence of either atmospheric inputs or biological activity. Second, P, Cu, Zn and Cd were moderately enriched in surface soils due to a combination of atmospheric deposition and biological uplift. Among the metals we studied, Cu, Zn and Cd concentrations in surface soils were the most sensitive to changes in atmospheric deposition fluxes. Finally, Mo and Mn showed strong enrichment in the top soil layer that could not be explained strictly by either current atmospheric deposition or biological recycling processes, but may reflect both their unique chemistry and remnants of past anthropogenic fluxes. Mn has a long residence time in the soil partly due to intense biological uplift that retains Mn in the top soil layer. Mo, in spite of the high solubility of molybdate, remains in the soil because of strong binding to natural organic matter. This study demonstrates the need to consider simultaneously the vegetation and the soils to understand elemental distribution within soil profiles as well as cycling within watersheds

Uptake of molybdenum and vanadium by a nitrogen-fixing soil bacterium using siderophores.

Nitrogen fixation, the reaction that transforms atmospheric nitrogen into bioavailable ammonia and is responsible for the supply of nitrogen to Earth's ecosystems, is mediated by the enzyme nitrogenase. This reaction requires molybdenum (Mo) or vanadium (V) in addition to iron (Fe) (refs 1,2). Therefore, the availability of these trace metals may control the Earth's nitrogen cycle 3,4 . Many bacteria release strong iron-binding compounds (siderophores) for iron acquisition In metal-replete diazotrophic cultures, the gram-negative soil bacterium Azotobacter vinelandii expresses the Mo nitrogenase, which is most efficient, preferentially to the V nitrogenase or the Fe-only nitrogenase 1 and its growth can be limited by Fe, Mo or V Under our culture conditions, A. vinelandii produces various types of siderophore. The monocatechol 2,3-dihydroxybenzoic acid (DHBA) and the tris(catechol) protochelin are produced in higher concentrations than the bis(catechol) azotochelin Whereas the metal affinity of DHBA is relatively poor 13 , protochelin and azotochelin are strong complexing agents for Fe(III), molybdate and vanadate. For example, azotochelin (LH 5 ) reacts with molybdate to form a 1:1 complex with Mo(VI) (LH (ref. 14). As revealed by mass spectrometry, the reaction of molybdate with protochelin also yields a 1:1 complex (Mo-protochelin), with a structure probably similar to that of Mo-azotochelin 15 To determine whether protochelin and azotochelin actually complex Mo and V in the culture medium, we used a high-performance liquid chromatograhy (HPLC) separation coupled to inductively coupled plasma mass spectrometry (ICP-MS) analysis of collected fractions to quantify the catechols and catechol-metal (Fe, Mo, V) −6 M), we accounted for 80% of the Mo originally present in the medium in the form of the Mo-protochelin comple

Effect of iron limitation on the isotopic composition of cellular and released fixed nitrogen in Azotobacter vinelandii

Most biological nitrogen transformations have characteristic kinetic isotope effects used to track these processes in modern and past environments. The isotopic fractionation associated with nitrogen fixation, the only biological source of fixed nitrogen (N), provides a particularly important constraint for studies of nitrogen cycling. Nitrogen fixation using the ‘canonical’ Mo-nitrogenase produces biomass with a δ^(15)N value of ca. −1‰ (vs. atmospheric N_2). If the ‘alternative’ V- and Fe-only nitrogenases are used, biomass δ^(15)N can be between −6‰ and −7‰. These biomass values are assumed to be relatively invariant and to reflect the cellular level expressed isotope effect of nitrogen fixation. However, field and laboratory studies report wide ranges of diazotrophic biomass δ^(15)N (from −3.6‰ to +0.5‰ for Mo-based nitrogen fixation). This variation could be partly explained by the release of dissolved organic N (DON) that is isotopically distinct from biomass. The model nitrogen fixer Azotobacter vinelandii secretes siderophores, small molecules that aid in Fe uptake and can comprise >30% of fixed nitrogen. To test whether siderophores (and other released N) can decouple biomass δ^(15)N from the isotope effect of nitrogen fixation we measured the isotopic composition of biomass and released N in Fe-limited A. vinelandii cultures fixing nitrogen with Mo- and V-nitrogenases. We report that biomass δ^(15)N was elevated under Fe limitation with a maximum value of +1.2‰ for Mo-based nitrogen fixation. Regardless of the nitrogenase isozyme used, released nitrogen δ^(15)N was also 2–3‰ lower than biomass δ^(15)N. Siderophore nitrogen was found to have a slightly higher δ^(15)N than the rest of the DON pool but was still produced in large enough concentrations to account for increases in biomass δ15N. The low δ^(15)N of siderophores (relative to biomass) is consistent with what is known from compound specific isotope studies of the amino acids used in siderophore biosynthesis, and indicates that other amino-acid derived siderophores should also have a low δ^(15)N. The implications for studies of nitrogen fixation are discussed

Effect of iron limitation on the isotopic composition of cellular and released fixed nitrogen in Azotobacter vinelandii

Most biological nitrogen transformations have characteristic kinetic isotope effects used to track these processes in modern and past environments. The isotopic fractionation associated with nitrogen fixation, the only biological source of fixed nitrogen (N), provides a particularly important constraint for studies of nitrogen cycling. Nitrogen fixation using the ‘canonical’ Mo-nitrogenase produces biomass with a δ^(15)N value of ca. −1‰ (vs. atmospheric N_2). If the ‘alternative’ V- and Fe-only nitrogenases are used, biomass δ^(15)N can be between −6‰ and −7‰. These biomass values are assumed to be relatively invariant and to reflect the cellular level expressed isotope effect of nitrogen fixation. However, field and laboratory studies report wide ranges of diazotrophic biomass δ^(15)N (from −3.6‰ to +0.5‰ for Mo-based nitrogen fixation). This variation could be partly explained by the release of dissolved organic N (DON) that is isotopically distinct from biomass. The model nitrogen fixer Azotobacter vinelandii secretes siderophores, small molecules that aid in Fe uptake and can comprise >30% of fixed nitrogen. To test whether siderophores (and other released N) can decouple biomass δ^(15)N from the isotope effect of nitrogen fixation we measured the isotopic composition of biomass and released N in Fe-limited A. vinelandii cultures fixing nitrogen with Mo- and V-nitrogenases. We report that biomass δ^(15)N was elevated under Fe limitation with a maximum value of +1.2‰ for Mo-based nitrogen fixation. Regardless of the nitrogenase isozyme used, released nitrogen δ^(15)N was also 2–3‰ lower than biomass δ^(15)N. Siderophore nitrogen was found to have a slightly higher δ^(15)N than the rest of the DON pool but was still produced in large enough concentrations to account for increases in biomass δ15N. The low δ^(15)N of siderophores (relative to biomass) is consistent with what is known from compound specific isotope studies of the amino acids used in siderophore biosynthesis, and indicates that other amino-acid derived siderophores should also have a low δ^(15)N. The implications for studies of nitrogen fixation are discussed

New ABA-Hypersensitive Arabidopsis Mutants Are Affected in Loci Mediating Responses to Water Deficit and Dickeya dadantii Infection

On water deficit, abscisic acid (ABA) induces stomata closure to reduce water loss by transpiration. To identify Arabidopsis thaliana mutants which transpire less on drought, infrared thermal imaging of leaf temperature has been used to screen for suppressors of an ABA-deficient mutant (aba3-1) cold-leaf phenotype. Three novel mutants, called hot ABA-deficiency suppressor (has), have been identified with hot-leaf phenotypes in the absence of the aba3 mutation. The defective genes imparted no apparent modification to ABA production on water deficit, were inherited recessively and enhanced ABA responses indicating that the proteins encoded are negative regulators of ABA signalling. All three mutants showed ABA-hypersensitive stomata closure and inhibition of root elongation with little modification of growth and development in non-stressed conditions. The has2 mutant also exhibited increased germination inhibition by ABA, while ABA-inducible gene expression was not modified on dehydration, indicating the mutated gene affects early ABA-signalling responses that do not modify transcript levels. In contrast, weak ABA-hypersensitivity relative to mutant developmental phenotypes suggests that HAS3 regulates drought responses by both ABA-dependent and independent pathways. has1 mutant phenotypes were only apparent on stress or ABA treatments, and included reduced water loss on rapid dehydration. The HAS1 locus thus has the required characteristics for a targeted approach to improving resistance to water deficit. In contrast to has2, has1 exhibited only minor changes in susceptibility to Dickeya dadantii despite similar ABA-hypersensitivity, indicating that crosstalk between ABA responses to this pathogen and drought stress can occur through more than one point in the signalling pathway

Harmful Elements in Estuarine and Coastal Systems

Estuaries and coastal zones are dynamic transitional systems which provide many economic and ecological benefits to humans, but also are an ideal habitat for other organisms as well. These areas are becoming contaminated by various anthropogenic activities due to a quick economic growth and urbanization. This chapter explores the sources, chemical speciation, sediment accumulation and removal mechanisms of the harmful elements in estuarine and coastal seawaters. It also describes the effects of toxic elements on aquatic flora and fauna. Finally, the toxic element pollution of the Venice Lagoon, a transitional water body located in the northeastern part of Italy, is discussed as a case study, by presenting the procedures adopted to measure the extent of the pollution, the impacts on organisms and the restoration activities

Vanadium Requirements and Uptake Kinetics in the Dinitrogen-Fixing Bacterium Azotobacter vinelandii▿

Vanadium is a cofactor in the alternative V-nitrogenase that is expressed by some N2-fixing bacteria when Mo is not available. We investigated the V requirements, the kinetics of V uptake, and the production of catechol compounds across a range of concentrations of vanadium in diazotrophic cultures of the soil bacterium Azotobacter vinelandii. In strain CA11.70, a mutant that expresses only the V-nitrogenase, V concentrations in the medium between 10−8 and 10−6 M sustain maximum growth rates; they are limiting below this range and toxic above. A. vinelandii excretes in its growth medium micromolar concentrations of the catechol siderophores azotochelin and protochelin, which bind the vanadate oxoanion. The production of catechols increases when V concentrations become toxic. Short-term uptake experiments with the radioactive isotope 49V show that bacteria take up the V-catechol complexes through a regulated transport system(s), which shuts down at high V concentrations. The modulation of the excretion of catechols and of the uptake of the V-catechol complexes allows A. vinelandii to precisely manage its V homeostasis over a range of V concentrations, from limiting to toxic