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

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Nearly 50 years ago, inorganic carbon was shown to be fixed in microalgae as the C3 compound phosphoglyceric acid. The enzyme responsible for C3 carbon fixation, ribulose-1,5-bisphosphate carboxylase (Rubisco), however, requires inorganic carbon in the form of CO2 (ref. 2), and Rubisco enzymes from diatoms have half-saturation constants for CO2 of 30-60 microM (ref. 3). As a result, diatoms growing in seawater that contains about 10 microM CO2 may be CO2 limited. Kinetic and growth studies have shown that diatoms can avoid CO2 limitation, but the biochemistry of the underlying mechanisms remains unknown. Here we present evidence that C4 photosynthesis supports carbon assimilation in the marine diatom Thalassiosira weissflogii, thus providing a biochemical explanation for CO2-insensitive photosynthesis in marine diatoms. If C4 photosynthesis is common among marine diatoms, it may account for a significant portion of carbon fixation and export in the ocean, and would explain the greater enrichment of 13C in diatoms compared with other classes of phytoplankton. Unicellular C4 carbon assimilation may have predated the appearance of multicellular C4 plants.

Biological nitrogen fixation constitutes the main input of fixed nitrogen to Earth's ecosystems, and its isotope effect is a key parameter in isotope-based interpretations of the N cycle. The nitrogen isotopic composition (δ 15 N) of newly fixed N is currently believed to be ∼-1‰, based on measurements of organic matter from diazotrophs using molybdenum (Mo)-nitrogenases. We show that the vanadium (V)-and iron (Fe)-only "alternative" nitrogenases produce fixed N with significantly lower δ 15 N (-6 to -7‰). An important contribution of alternative nitrogenases to N 2 fixation provides a simple explanation for the anomalously low δ 15 N (<-2‰) in sediments from the Cretaceous Oceanic Anoxic Events and the Archean Eon. A significant role for the alternative nitrogenases over Mo-nitrogenase is also consistent with evidence of Mo scarcity during these geologic periods, suggesting an additional dimension to the coupling between the global cycles of trace elements and nitrogen. Mo is ∼+2‰ (3-6)], which is thought to be the most abundant in nature. As a result, the δ 15 N of newly fixed N is ∼-1‰ (i.e., ∼+2‰ lower than the δ 15 N of dissolved N 2 substrate, +0.7‰, Fig. 1A).In addition to Mo-nitrogenase (the most common form of the enzyme), diazotrophs can possess two other nitrogenase isozymes (7). These so-called "alternative" nitrogenases differ chiefly from Mo-nitrogenases in that V or Fe replaces Mo in the active site. They also contain an additional protein subunit and exhibit slower kinetics compared with the Mo-nitrogenase (8). Such differences could result in distinct isotope effects, a possibility supported by Rowell et al. (9), who reported small but significant variations in biomass δ 15 N from growth of wild-type diazotrophs possessing all three isozymes in media containing Fe and amendments of Mo, V, or neither metal. However, the use of multiple nitrogenase isozymes in the wild type (10) precludes the direct association between biomass δ 15 N and the isotope effect of a particular isozyme. Here we (i) measured directly the isotope effects for Mo-, V-, and Fe-only nitrogenases using diazotroph mutant strains that could express only a single nitrogenase isozyme, (ii) determined the impact of metal limitation on alternative nitrogenase use and N isotope fractionation in wild-type bacteria, and (iii) provide several examples of how these results on N isotope fractionation may change our understanding of the N cycle in the past. Results and DiscussionIsotope Fractionation During Nitrogen Fixation by Mo-, V-, and Fe-only Nitrogenases. We measured the in vivo isotope effect associated with each type of nitrogenase in two phylogenetically and metabolically distinct diazotrophic bacteria, Rhodopseudomonas palustris and Azotobacter vinelandii. R. palustris is an alpha-proteobacterium. It fixes N 2 anaerobically and was grown under anaerobic and photoheterotrophic conditions. A. vinelandii is a gamma-proteobacterium. It fixes N 2 aerobically and was grown under aerobic chemoheterotrophic conditions. All three nitroge...

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 potentialsurface 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.

Biological nitrogen fixation, the main source of new nitrogen to the Earth's ecosystems, is catalysed by the enzyme nitrogenase. There are three nitrogenase isoenzymes: the Mo-nitrogenase, the V-nitrogenase and the Fe-only nitrogenase. All three types require iron, and two of them also require Mo or V. Metal bioavailability has been shown to limit nitrogen fixation in natural and managed ecosystems. Here, we report the results of a study on the metal (Mo, V, Fe) requirements of Azotobacter vinelandii, a common model soil diazotroph. In the growth medium of A. vinelandii, metals are bound to strong complexing agents (metallophores) excreted by the bacterium. The uptake rates of the metallophore complexes are regulated to meet the bacterial metal requirement for diazotrophy. Under metal-replete conditions Mo, but not V or Fe, is stored intracellularly. Under conditions of metal limitation, intracellular metals are used with remarkable efficiency, with essentially all the cellular Mo and V allocated to the nitrogenase enzymes. While the Mo-nitrogenase, which is the most efficient, is used preferentially, all three nitrogenases contribute to N₂ fixation in the same culture under metal limitation. We conclude that A. vinelandii is well adapted to fix nitrogen in metal-limited soil environments.

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Biological di-nitrogen fixation (N2) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N2 fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N2 conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature. Here we show in tropical forests of lowland Panama that the limiting element on asymbiotic N2 fixation shifts along a broad landscape gradient in soil P, where Mo limits fixation in P-rich soils while Mo and P co-limit in P-poor soils. In no circumstance did P alone limit fixation. We provide and experimentally test a mechanism that explains how Mo and P can interact to constrain asymbiotic N2 fixation. Fixation is uniformly favored in surface organic soil horizons - a niche characterized by exceedingly low levels of available Mo relative to P. We show that soil organic matter acts to reduce molybdate over phosphate bioavailability, which, in turn, promotes Mo limitation in sites where P is sufficient. Our findings show that asymbiotic N2 fixation is constrained by the relative availability and dynamics of Mo and P in soils. This conceptual framework can explain shifts in limitation status across broad landscape gradients in soil fertility and implies that fixation depends on Mo and P in ways that are more complex than previously thought.