9.2. Protein-Based Hg(II) Detectors 3467 9.3. Oligonucleotide-Based Hg(II) Detector 3467 9.4. DNAzyme-Based Hg(II) Detectors 3469 9.5. Antibody-Based Hg(II) Detector 3469 10. Mercury Detectors Based on Materials 3469 10.1. Soluble and Fluorescent Polymers 3469 10.2. Membranes, Films, and Fibers 3471 10.3. Micelles 3473 10.4. Nanoparticles 3473 11. Perspectives 3474 12. Addendum 3475 12.1. Small Molecules 3475 12.2. Biomolecules 3477 12.3. Materials 3477 13. List of Abbreviations 3477 14. Acknowledgments 3478 15. References 3478

Tools and tactics for the optical detection of mercuric ion.

Metals have been used as antimicrobial agents since antiquity, but throughout most of history their modes of action have remained unclear. Recent studies indicate that different metals cause discrete and distinct types of injuries to microbial cells as a result of oxidative stress, protein dysfunction or membrane damage. Here, we describe the chemical and toxicological principles that underlie the antimicrobial activity of metals and discuss the preferences of metal atoms for specific microbial targets. Interdisciplinary research is advancing not only our understanding of metal toxicity but also the design of metal-based compounds for use as antimicrobial agents and alternatives to antibiotics.

Antimicrobial activity of metals: mechanisms, molecular targets and applications

Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.

Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation

Mercury is a widespread pollutant with distinct toxicological profiles, and it exists in a variety of different forms (metallic, ionic, and as part of organic and inorganic salts and complexes). Solvated mercuric ion (Hg), one of the most stable inorganic forms of mercury, is a caustic and carcinogenic material with high cellular toxicity. The most common organic source of mercury, methyl mercury, can accumulate in the human body through the food chain and cause serious and permanent damage to the brain with both acute and chronic toxicity. Methyl mercury is generated by microbial biomethylation in aquatic sediments from water-soluble mercuric ion (Hg). Therefore, routine detection of Hg is central to the environmental monitoring of rivers and larger bodies of water and for evaluating the safety of aquatically derived food supplies. Several methods for the detection of Hg, based upon organic fluorophores or chromophores, semiconductor nanocrystals, cyclic voltammetry, polymeric materials, proteins, and microcantilevers, have been developed. Colorimetric methods, in particular, are extremely attractive because they can be easily read out with the naked eye, in some cases at the point of use. Although there are now several chromophoric colorimetric sensors for Hg, all of them are either limited with respect to sensitivity (current limit of detection 1 mm) and selectivity, kinetically unstable, or incompatible with aqueous environments. Recently, DNA-functionalized gold nanoparticles (DNA– Au NPs) have been used in a variety of forms for the detection of proteins, oligonucleotides, certain metal ions, and other small molecules. DNA–Au NPs have high extinction coefficients (3–5 orders of magnitude higher than those of organic dye molecules) and unique distancedependent optical properties that can be chemically programmed through the use of specific DNA interconnects, which allows one, in certain cases, to detect targets of interest through colorimetric means. Moreover, these structures, when hybridized to complementary particles, exhibit extremely sharp melting transitions, which have been used to enhance the selectivity of detection systems based upon them. By using such an approach, one can typically detect nucleic acid targets in the low nanomolar to high picomolar target concentration range in colorimetric format. The ability to use such particles to detect Hg in the nanomolar concentration range in colorimetric format would be a significant advance, especially when one considers that commercial systems for detecting Hg rely on cumbersome inductively coupled plasma approaches that are not suitable for point-of-use applications. Herein, we present a highly selective and sensitive colorimetric detection method for Hg that relies on thymidine–Hg–thymidine coordination chemistry and complementary DNA–Au NPs with deliberately designed T–T mismatches. When two complementary DNA–Au NPs are combined, they form DNA-linked aggregates that can dissociate reversibly with a concomitant purple-to-red color change. 28] For our novel colorimetric Hg assay, however, we prepared two types of Au NPs (designated as probe A and probe B, see the Supporting Information), each functionalized with different thiolated-DNA sequences (probe A: 5’HS-C10-A10-T-A103’, probe B: 5’HS-C10-T10-T-T103’), which are complementary except for a single thymidine–thymidine mismatch (shown in bold; Scheme 1). Importantly, these particles also form stable aggregates and exhibit the characteristic sharp melting transitions (full width at half maximum< 1 8C) associated with aggregates formed from perfectly complementary particles, but with a lower melting temperature Tm. [17, 18] Since it is known that Hg will coordinate selectively to the bases that make up a T–T mismatch, we hypothesized that Hg would

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200700269

Colorimetric Detection of Mercuric Ion (Hg2+) in Aqueous Media using DNA-Functionalized Gold Nanoparticles

The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO2 and the increase in O2. Consequently, the competition between O2 and CO2 for the active sites of RUBISCO became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO2 concentrations (CO2 concentrating mechanisms, CCMs) in the proximity of RUBISCO. A number of CCM variants are now found among the different groups of algae. Modulating the CCMs may be crucial in the energetic and nutritional budgets of a cell, and a multitude of environmental factors can exert regulatory effects on the expression of the CCM components. We discuss the diversity of CCMs, their evolutionary origins, and the role of the environment in CCM modulation.

CO2 CONCENTRATING MECHANISMS IN ALGAE: Mechanisms, Environmental Modulation, and Evolution

Because it is very toxic and accumulates in organisms, particularly in fish, mercury is an important pollutant and one of the most studied. Nonetheless we still have an incomplete understanding of the factors that control the bioconcentration of mercury. Elemental mercury is efficiently transported as a gas around the globe, and even remote areas show evidence of mercury pollution originating from industrial sources such as power plants. Besides elemental mercury, the major forms of mercury in water are ionic mercury (which is bound to chloride, sulfide, or organic acids) and organic mercury, particularly methylmercury. Methylmercury rather than inorganic mercury is bioconcentrated because it is better retained by organisms at various levels in the food chain. The key factor determining the concentration of mercury in the biota is the methylmercury concentration in water, which is controlled by the relative efficiency of the methylation and demethylation processes. Anoxic waters and sediments are an important source of methylmercury, apparently as the result of the methylating activity of sulfatereducing bacteria. In surface waters, methylmercury may originate from anoxic

/pdf/the-chemical-cycle-and-bioaccumulation-of-mercury-53npodz0gl.pdf

The chemical cycle and bioaccumulation of mercury

Biological nitrogen fixation limits plant growth and carbon exchange at local to global scales. Long-term nutrient manipulation experiments in forests and short-term manipulation experiments in microcosms suggest that the micronutrient molybdenum, a component of the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by asymbiotic bacteria in tropical soils in Panama. Nitrogen fixation, the biological conversion of di-nitrogen to plant-available ammonium, is the primary natural input of nitrogen to ecosystems1, and influences plant growth and carbon exchange at local to global scales2,3,4,5,6. The role of this process in tropical forests is of particular concern, as these ecosystems harbour abundant nitrogen-fixing organisms1,4 and represent one third of terrestrial primary production4,7,8. Here we show that the micronutrient molybdenum, a cofactor in the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by free-living heterotrophic bacteria in soils of lowland Panamanian forests. We measured the fixation response to long-term nutrient manipulations in intact forests, and to short-term manipulations in soil microcosms. Nitrogen fixation increased sharply in treatments of molybdenum alone, in micronutrient treatments that included molybdenum by design and in treatments with commercial phosphorus fertilizer, in which molybdenum was a ‘hidden’ contaminant. Fixation did not respond to additions of phosphorus that were not contaminated by molybdenum. Our findings show that molybdenum alone can limit asymbiotic nitrogen fixation in tropical forests and raise new questions about the role of molybdenum and phosphorus in the tropical nitrogen cycle. We suggest that molybdenum limitation may be common in highly weathered acidic soils, and may constrain the ability of some forests to acquire new nitrogen in response to CO2 fertilization9.

Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils

A consistent thermodynamic model is developed for metal sorption on expanding 2:1 layer clays such as montmorillonite. The particle of clay, including lamellae and interlayers, is represented as a porous solid bearing a permanent negative charge (resulting from isomorphic substitution) with an infinite plane interface (i.e., edges) with the solution. Cation exchange occurs inside the clay particle as the result of the negative potential of the clay. Surface complexation reactions take place at the interface whose surface charge and potential are pH dependent. The potential in the bulk of the clay and near the interface, as well as the surface potential–surface charge density relation, are calculated taking into account the effect of the permanent negative charge. The results are discussed and compared with the classic Gouy–Chapman theory. A subroutine (Clayeql) with the new potential–charge relationships is implemented in the thermodynamic equilibrium program Mineql +3.0 and is used to fit an extensive published experimental data set on adsorption of transition metals on montmorillonite. The model is shown not only to fit satisfactorily all the data, but also to explain specific features of adsorption on clays compared to oxides. In particular, the increase in the surface concentration of protons with decreasing ionic strength is successfully reproduced and the weaker dependence of metal sorption on pH compared to oxides is correctly fitted.

http://www3.geosc.psu.edu/~kzk10/Kraepiel_jcis_99.pdf

A Model for Metal Adsorption on Montmorillonite

Geochemistry of trace metals in the Gironde estuary

The micronutrient molybdenum is necessary for nitrogen fixation, but is very rare and highly soluble in soils. X-ray spectroscopy analysis of forest soil samples indicates that most of the molybdenum in the litter layer binds to organic matter. The micronutrient molybdenum is a necessary component of the nitrogen-fixing enzyme nitrogenase1,2. Molybdenum is very rare in soils, and is usually present in a highly soluble form, making it susceptible to leaching3,4. However, it is generally thought that molybdenum attaches to mineral surfaces in acidic soils; this would prevent its escape into the groundwater, but would also impede uptake by microbes3. Here we use X-ray spectroscopy to examine the chemical speciation of molybdenum in soil samples from forests in Arizona and New Jersey. We show that in the leaf litter layer, most of the molybdenum forms strong complexes with plant-derived tannins and tannin-like compounds; molybdenum binds to these organic ligands across a wide pH range. In deeper soils, molybdenum binds to both iron oxides and natural organic matter. We suggest that the molybdenum bound to organic matter can be captured by small complexing agents that are released by nitrogen-fixing bacteria; the molybdenum can then be incorporated into nitrogenase. We conclude that the binding of molybdenum to natural organic matter helps prevent leaching of molybdenum, and is thus a critical step in securing new nitrogen in terrestrial ecosystems.

Anne M. L. Kraepiel

Papers

The chemical cycle and bioaccumulation of mercury

Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils

A Model for Metal Adsorption on Montmorillonite

Geochemistry of trace metals in the Gironde estuary

Storage and bioavailability of molybdenum in soils increased by organic matter complexation