By catalyzing biological transformations as cofactors of key enzymes, trace metals like iron and zinc play a critical role in the global cycles of major nutrients such as carbon, nitrogen and phosphorus. In some cases metals also inhibit these transformations. The principal long-term research theme of our group is the elucidation, at both the molecular and the global level, of the linkages between the cycles of trace metals and those of C, N and P.  Human activities, such as the burning of fossil fuel and the production of fertilizers, have profoundly modified these cycles.  One of our research goals is to understand how trace metals modulate the responses of ecosystems to these global changes. 

A large part of our work deals with the oceans, focusing on the grand question of what physical and chemical factors control the growth and activity of phytoplankton in the sea. Marine phytoplankton are responsible for about half of global primary production and, by exporting organic matter to the deep sea, they maintain a low concentration of CO2 in surface waters and in the atmosphere. As detailed below, the elements of most interest to us in this part of our work are cationic metals from the first and second row of the periodic table –manganese, iron, cobalt, copper, nickel, zinc and cadmium-- in addition to carbon itself.  A newer research activity in our group concerns the role of metals in the cycling of nitrogen in soils; there the focus is on metals found principally as oxoanions: molybdenum, vanadium and tungsten. The biogeochemical cycling of mercury, one of the most toxic elements in aquatic systems, is the subject of one of our long-standing research activities.  Our main present interest is the formation of methylmercury, an organometallic compound that accumulates in aquatic food-chains.  

We approach our work with a mix of laboratory and field experiments using a variety of chemical, microbiological, biochemical and genetic tools, as appropriate.  Our work is also informed by theoretical considerations from a number of disciplines ranging from bioinorganic chemistry to geology and ecology.

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Links to projects

Iron Uptake by Diatoms
Iron Storage in Cyanobacteria
Inorganic Carbon Acquisition by Diatoms
The Response of Marine Phytoplankton to Increasing [CO2]
The Biological Role of Cadmium
Use of Organic Phosphorus Sources by Coccolithophores
Mercury Methylation
The Role of Metals in Nitrogen Cycling in Soils
Past Research Themes and Achievements


Iron Uptake by Diatoms.

Iron is known to limit the growth of phytoplankton, especially diatoms, in large regions of the oceans. Because of their large size and silica ballast, diatoms are particularly important in the export of organic matter to the deep ocean.  Previous laboratory data from our group and others have shown that diatoms take up the free iron (i.e., the iron that is not bound to strong chelating agents) from solution.  But, according to field data, the free iron in surface seawater is too low to support the growth of these dominant primary producers. Both new and old laboratory data show that diatoms can obtain their iron by reducing Fe(III) in some chelates, including iron bound to siderophores. We have established that Fe(III) reduction at the diatom surface is an essential step in the uptake of iron in all cases and developed a model for uptake kinetics that reconciles all available data (Shaked et al. 2005). This model provides a chemical framework to quantify the bioavailability of Fe in seawater.  In the course of this study, we have also shown that diatoms produce an abundant quantity of superoxide (O2-), by extracellular reduction of oxygen (Kustka et al. 2005).  Superoxide is an extremely reactive radical able to reduce and oxidize many solutes, including Fe, in seawater. 

To obtain a more mechanistic understanding of the iron uptake system of diatoms, we have studied the effect of Fe availability on the expression of genes coding for the proteins that are thought to be responsible for the reduction of Fe(III) and the transport of Fe in the two diatoms whose full genomes have been sequenced, Thalassiosira pseudonana and Phaedactylum tricornutum (Kustka et al. in press).  As expected, the transcription of putative ferric reductases is upregulated under iron stress.  But the steps downstream from the initial Fe(III) reduction appear to be different in the two model organisms. Recently, we have demonstrated that diatoms and other marine phytoplankters are able to acquire Fe from iron storage proteins such as ferritins and Dps proteins (Castruita et al. in review).  These proteins may thus play an important role in the biological cycling of iron in surface seawater (see below). 


Iron Storage in Cyanobacteria. 

In addition to directly limiting primary production in some oceanic regions, iron is thought to limit it indirectly in many other regions by controlling the input of fixed nitrogen. This is because the nitrogenase enzyme, which is responsible for dinitrogen (N2) fixation, requires a lot of Fe. The most important nitrogen fixer in the sea, the cyanobacterium Trichodesmium, thrives in tropical and subtropical regions where iron inputs from atmospheric dust are highly episodic. We have thus been studying the mechanism of Fe storage in Trichodesmium.  As a first step, we have identified, isolated, over-expressed and partially characterized a Dps protein (DNA-binding protein from starved cells) from this organism (Castruita et al. 2006).  This protein, the first Fe storage protein isolated from a marine microbe, is able to store vast quantities of Fe; it is also able to bind to DNA and protect it from degradation.  This second attribute may be important to protect the genetic material of the organism during periods of dormancy when nutrient concentrations are low or other environmental conditions are unfavorable.



Inorganic Carbon Acquisition by Diatoms.

A few years ago, we reported that diatoms growing under present day atmospheric conditions function as unicellular C4 plants; i.e., that they concentrate carbon by accumulating an intermediate C4 organic compound before CO2 fixation in their chloroplast (Reinfelder et al. 2000, 2004; Morel et al. 2002). This work, which implies that CO2, may be a limiting nutrient in the ocean, has been controversial. Our ongoing work shows that specific inhibitors of the two enzymes involved in the formation of the C4 intermediates and the release of CO2 from them --PEPC (phophoenol pyruvate carboxylase) and PEPCK (phophoenol pyruvate carboxykinase)-- inhibit photosynthesis in diatoms but not in green algae which operate a well-characterized C3 photosynthetic system (McGinn et al. in prep.).  Another key enzyme in inorganic carbon acquisition is carbonic anhydrase, CA, which catalyses the dehydration of bicarbonate to CO2. (CA is a metalloprotein that is the focus of one of our projects; see below.)  The inhibition of CA cripples photosynthesis in green algae because it slows down the supply of CO2 to the carboxylating enzyme RubisCO.  In contrast, CA inhibition has only a modest effect on C fixation in diatoms.  All these observations are consistent with our C4 model of inorganic carbon acquisition in diatoms and imply that the concentration of CO2 may be an important factor in  the productivity and ecology of marine phytoplankton.



The Response of Marine Phytoplankton to Increasing [CO2].

The concentration of CO2 in surface seawater is increasing along with that of the atmosphere as a result of fossil fuel combustion and other human activities. In view of the critical role of the ocean biota in the global carbon cycle, the question of the response of marine phytoplankton to increasing [CO2] is of prime importance. Aside from the indirect effect of lowered nutrient fluxes resulting from the increased stratification caused by warming, two direct effects must be considered: the effect of increasing [CO2] on photosynthesis and the effects of decreasing pH on key chemical and biological processes such as the precipitation of calcium carbonate and the availability of major and trace nutrients. Our ongoing work on inorganic carbon acquisition in diatoms implies that increasing [CO2] should increase carbon fixation rates in these organisms and that this effect may not be uniform among various families of marine phytoplankton.  Indeed, preliminary field experiments have shown that diatoms are favored under high [CO2] conditions (Tortell et al. 2002).  We are presently examining in laboratory cultures the effects of [CO2] changes on the growth of model phytoplankters under conditions of iron or nitrogen limitation.



The Biological Role of Cadmium.  

Cadmium, an element which has been thought to be only toxic to organisms, behaves exactly like a nutrient in the sea: it is depleted to very low concentration as result of biological uptake at the surface and remineralized at depth.  Because of its excellent correlation with phosphate, cadmium is used as a paleotracer for nutrients. Over the past several years, we have demonstrated that cadmium is an important micronutrient for marine phytoplankton. In particular, we have discovered that the diatom Thalassiosira weissflogii possesses a Cd-carbonic anhydrase, CdCA, which is involved in the acquisition of inorganic carbon for photosynthesis.  We have obtained the full DNA sequence for this enzyme and characterized its active center by X-ray spectroscopy (Lane et al. 2005). Recently, we have been able to over-express an active form of the enzyme (the first known Cd enzyme) and, in collaboration with Yigong Shi’s group in Molecular Biology, obtained its crystal structure (Xu et al. in prep). We are in the process of quantifying the affinity of CdCA for Cd and Zn.

We have also shown that many diatom species possess closely homologous versions of the CdCA enzyme found in T. weissflogii (Park et al. 2007).  Together with our ongoing work on the effects of CO2, Cd and Zn concentrations on the expression of CdCA, these results will serve as the foundation of upcoming field work on the presence and regulation of this enzyme in the surface ocean.  



Use of Organic Phosphorus Sources by Coccolithophores

Coccolithophores are calcite precipitating phytoplankton that are dominant in many oligotrophic gyres of the oceans and they can form massive blooms visible from space.  They owe part of their ecological success to their ability to obtain phosphorus from organic compounds when inorganic P concentrations are vanishingly low. This is achieved through the activity of the zinc enzyme alkaline phosphatase, AP, which cleaves phosphate from various organic substrates.  We have studied the activity of this enzyme in the ubiquitous species Emiliana huxleyi and demonstrated that very small enzyme (and thus zinc) concentrations are necessary to provide the phosphate necessary for growth (Shaked et al. 2006).  We have now isolated and partially characterized this enzyme which has no homology to other known alkaline phosphatases (Xu et al. 2006).  MORE; YAN?  This work provides the basis for studying the expression of alkaline phosphatase, and, hence, the extent of P limitation, in the field.




Mercury Methylation

Our continuing work on the biogeochemistry of mercury which recently dealt with redox and photoredox processes (Lalonde et al. 2004; Amyot et al. 2005) is presently focused on mercury methylation.  Since methyl-mercury is the species accumulated in fish via the food chain, this is a key transformation in determining human exposure to mercury.  Yet it has received surprisingly little attention over the past 20 years. The two questions we are trying to answer are: 1) where is methylation occurring in the ocean and by what mechanism? and 2) what controls the rate of methylation by sulfate reducing bacteria in freshwater systems?  On the basis of previous field data, we have proposed that methyl mercury in the open ocean may originate from the deep sea, perhaps from hydrothermal vents (Kraepiel et al. 2003). We have indeed measured significant concentrations of methylmercury in some hydrothermal samples and shown that mercury methylation can be effected chemically at high pressure and temperature by reaction with yet unidentified trace organic compounds (Eileen Ekstrom’s doctoral Thesis, 2006). In terrestrial systems, we know that mercury methylation is effected by sulfate reducing bacteria. Using specific inhibitors and cobalt limitation as means to modulate the activity of vitamin B12, we have shown that the acetylCoA pathway is responsible for Hg methylation in sulfate reducing bacteria that oxidize their substrate to CO2 (complete oxidizers), but not in others that oxidize their substrate to acetate (incomplete oxidizers) (Ekstrom et al. 2003; Ekstrom and Morel, in review).  Our results shed light on the biochemical mechanisms of mercury methylation and show that the availability of trace metals such as cobalt may be important in controlling Hg methylation in anoxic waters and sediments.



The Role of Metals in Nitrogen Cycling in Soils.

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Previous Research Themes and Achievements

    •      I. Chemical Speciation in Natural Waters

    •      II. Metal-Microorganism Interactions

Over the years, the research activities in the Morel group have been focused principally on the related research themes of the chemical speciation of trace metals in natural waters, the interactions between trace metals and aquatic microorganisms, and the role of trace metals in controlling the biogeochemical cycles of bioactive elements. We have addressed such questions as: What are the chemical forms (the chemical species) of various metals in natural waters? How does the speciation of the metals affect their availability to microorganisms?  What metals are limiting or toxic to organisms under what conditions?  How do organisms affect the chemical speciation of the metals in their external and internal milieu? How do trace metals affect or control the uptake or utilization of carbon, nitrogen and phosphorus by the planktonic biota? To what extent do trace metals control productivity and phytoplankton assemblages in oceanic waters?  A central goal has been to relate the trace metal biochemistry and physiology of marine phytoplankton to the biogeochemical cycles of elements, including carbon, nitrogen and phosphorus, as well as trace metals.  As an extension of this work, we have addressed the related questions of inorganic carbon acquisition by marine phytoplankton and of the role of CO2 on plankton production and ecology.  Recently, we have begun to examine the interactions of trace element geochemistry and phytoplankton in the context of Earth history and evolution. We have also investigated various questions related to the biogeochemical cycling of arsenic and mercury.

I. Chemical Speciation in Natural Waters


We developed of a series of computer programs (the REDEQL and MINEQL series) to calculate chemical equilibrium in complex systems, including natural waters and man-made chemical systems.  These programs, as well as a number of derivative programs (including EPA’s MINTEQ) have been widely used in industry, government and academia for a variety of applications. The conceptual architecture of these programs is based on a re-discovery of Gibbs’s notion of chemical components and a general algebraic formulation of the chemical equilibrium problem. The FITEQL program uses the same formulation to solve the opposite problem: determining equilibrium constants from experimental equilibrium data. Key publications are: Morel and Morgan ES&T 1972; Westall et al, R. M. Parsons Technical Notes #18 and 19, 1977,1978. This conceptual approach to the problem of chemical equilibrium in complex systems is also utilized in the teaching text Principles of Aquatic Chemistry, Morel, Wiley 1983, and its sequel Principles and Applications of Aquatic Chemistry, Morel and Hering, Wiley 1993. The corresponding “tableau method” which bedeviled many a graduate student was eventually adopted by other authors, including Stumm and Morgan in the third edition of Aquatic Chemistry Wiley 1996. Principles of Aquatic Chemistry and Principles and Applications of Aquatic Chemistry together have sold more than 10,000 copies. A large but unknown number of copies of MINEQL, FITEQL and derivative programs have been distributed over the years.


We critiqued existing electrochemical and sequential extraction methods for measuring metal speciation in natural waters and  soils and proposed some new ones.  Westall et al. Anal. Chem 1979,  Waite and Morel Anal. Chem. 1983;1984; Hering et al. Marine Chemistry 1987; Nirel and Morel Water Res. 1990.


We performed experimental studies of solute adsorption on solid surfaces and implemented thermodynamic models to describe the data. By incorporating solutes into the solid phase, adsorption controls the geochemical cycles of many trace elements and compounds. The key to modeling adsorption is to quantify in a coherent thermodynamic formulation the relative roles of long-range electrostatic and short-range chemical interactions at solid surfaces. Besides providing some of the experimental data, our main contributions were to develop: 1) a method to include all sub-varieties of the so-called “Surface Complexation Model” into MINEQL (thus allowing, for example, the development of the “Triple Layer Model” used by the Stanford group in many publications); 2) the surface precipitation model (in which the surface phase is treated as a solid solution) for describing the transition between adsorption and precipitation; and 3) a coherent data base for adsorption of hydrous ferric oxide. More recently, we have extended this work to describe adsorption of solutes on permanently charged clays. Swallow et al. ES&T 1980, Farley et al. JCIS 1985, Dzombak and Morel JCIS 1986; J. Hydraulic Eng. AICHE 1987; Dzombak and Morel  Surface Complexation Modeling:  Hydrous Ferric Oxide; Wiley 1990; Kraepiel and Morel ES&T 1998, Kraepiel et al. JCIS 1999.


We carried out theoretical and experimental studies of the complexation of trace metals by inorganic and natural organic ligands in aquatic systems.  Our work entailed the modeling of complexation of trace metals by major anions upon mixing of freshwater with seawater, a quantitative description of metal complexation by humic substances, and the search for specific biogenic chelating agents.  The net effect of complexation on the cycling of trace elements is opposite to that of adsorption since it increases the solubility of elements and compounds. We showed the importance of sulfide binding upon mixing of wastewater with seawater in sewage outfalls and the role of chloride complexation of metals in estuarine systems.  We also made a thorough analysis of the “polyelectrolyte effect” (i.e., the long range coulombic interactions between metal and ligands) to quantify the complexation of metals by humates and approached the problem of identifying specific biogenic chelators by using pure cultures of microorganisms. In more recent work, we quantified the role of cysteine-rich polypeptides known as phytochelatins in the intracellular binding of trace metals in marine phytoplankton and studied the relation between phytochelatin concentrations and metal exposure in seawater. Morel et al. ES&T 1975; McKnight and Morel L&O 1979, 1980, Dzombak et al. ES&T 1986, Fish et al ES&T 1986, Hering and Morel ES&T 1988, Bartschat et al. ES&T 1992; Green et al. ES&T 1992; Ahner et al. PNAS 1994; L&O 1995a&b; 1997; Kraepiel et al. GCA 1997.

Complexation Kinetics

We studied the kinetics of complexation of metals by organic ligands in natural waters and demonstrated for the first time that some of these reactions can be exceedingly slow, despite the inherently fast kinetics of the underlying reaction steps. This counterintuitive result is explained by the rapid formation of intermediate metastable complexes that results in extremely slow approach to equilibrium. Hering and Morel ES&T 1988, 1990; GCA 1989.


Photochemical Redox Cycling of Iron

We postulated and first demonstrated the importance of the Fe photocycle in surface waters both for the dissolution of Fe oxides and for the formation of Fe(II) from Fe(III) complexes in solution. Waite and Morel JCIS 1984 and ES&T 1984; Hudson et al. Mar. Chem. 1992; Voelker et al ES&T 1997. The earliest description of these process is actually given in Principles of Aquatic Chemistry 1983. Related work on the (photo)redox cycle of mercury is described below


II. Metal-Microorganism Interactions

Method Development

We designed a “chemically defined” growth medium for studying the trace metal physiology of marine phytoplankton, the Aquil medium which is now widely used; see Morel et al. J. Phycol 1979; Price et al. Biolog. Oceanogr.1988/1989; Sunda et al. 2005. Development of methods for distinguishing extracellular and intracellular concentrations of metals and for simultaneous measurement of Fe reduction and uptake in cultures: Hudson and Morel L&O 1989; Shaked et al. L&O 2004; Tang and Morel Mar. Chem. 2005.

The Free Ion Model and A Kinetic Framework

An important outcome of our early work is the conceptual development and experimental verification of the “Free Ion Activity Model” (FIAM) for the effects of metals on aquatic organisms.  This model links the biological availability and effects of essential and toxic metals to their chemical speciation.  It is the basis of most modern work on the interactions of trace metals and aquatic organisms and is also now incorporated in various EPA rules based on the FIAM model, or its extension the BLM (Biotic Ligand Model).  Cu toxicity: Anderson and Morel L&O 1978, Morel et al 1978, Rueter and Morel L&O 1981; Zn limitation: Anderson et al. Nature 1978; Fe limitation: Anderson and Morel L&O 1982; Cd toxicity: Foster and Morel L&O 1982, Harrison and Morel J Phycol. 1984.  We subsequently modified the free ion model and provided a kinetic framework for analyzing the interactions of trace metals and microorganisms. This work lead to the general hypothesis that the acquisition of essential trace elements by planktonic organisms in oligotrophic oceanic waters is limited by the kinetics of diffusion and chemical reactions with uptake molecules at the cell surface.  The net result is a general hypothesis of co-limitation of growth by major and trace nutrients that is modulated by the size of the organisms. Hudson and Morel L&O 1990, DSR 1993; Morel et al. L&O 1991.

The Role of Iron in Primary Production and the Ecology of Marine Phytoplankton

Following the laboratory studies mentioned above (and others such as Rich and Morel L&O 1990), we engaged in field work, particularly to test the “Iron Hypothesis” according to which High Nutrient Low Chlorophyll (HNLC) regions of the oceans are Fe limited. We resolved the apparent conflict between the Fe hypothesis and alternative explanations by showing that in the Equatorial Pacific large diatoms are limited by Fe while cyanobacteria are controlled chiefly by grazing.  This “ecumenical hypothesis” which was criticized at the time of IRONEX-1 was then confirmed by IRONEX-2 and has stood the test of time. Morel et al. Oceanography 1991; Price et al. DSR 1991, L&O 1994. Recently, we have begun reexamining the questions of the mechanism of Fe uptake and storage in marine phytoplankton and of the bioavailability of Fe compounds in the sea using new kinetic and molecular biology tools. We have established the central importance of Fe(III) reduction by cell surface enzymes for Fe uptake, characterized an Fe storage protein in Trichodesmium (the dominant N2-fixing cyanobacterium in the oceans) and shown that the Fe bound in such protein is available to marine phytoplankton.  Kustka et al. L&O 2005; Shaked et al. L&O 2005; Castruita et al. AEM 2006.

Roles of Zn, Co and Cd in Phytoplankton Physiology and Ecology

We demonstrated that Co and Cd can replace Zn as essential elements for the growth of marine phytoplankton, and showed that these metals can all serve as catalytic centers in carbonic anhydrase (CA). This enzyme catalyses the reversible transformation of CO2 into bicarbonate and is important in inorganic carbon acquisition for photosynthesis. This led to the discovery and characterization of two novel classes of carbonic anhydrases: the d class of CAs which contain either Zn or Co as their metal center and the z class of CAs which are the first and only known cadmium enzymes.  We have now shown that cadmium CAs are ubiquitous in marine waters. Despite the extremely fast kinetics of CAs, these enzymes are needed at high cellular concentration because of their very low affinity for their inorganic carbon substrate.  As a result CAs represent a major metal requirement in marine phytoplankton. This work provides an explanation for the nutrient-like behavior of Cd in seawater. We also showed that the silica frustule of diatoms serves as a proton buffer for their external CA, thus establishing a biochemical role for silica in phytoplankton. Price and Morel Nature 1990; Morel et al. Nature 1994; Lee et al L&O 1995; Lee and Morel MEPS 1995; Yee and Morel L&O 1996; Roberts et al. J. Phycol 1997; Cullen et al Nature 1999; Cox et al. Biochemistry 2000; Lane and Morel PNAS 2000; Milligan and Morel Nature 2002; Lane et al. Nature 2005; Park et al. Env. Microbiol. 2007.

 C4 Photosynthesis in Diatoms and Role of CO2 in Phytoplankton Ecology

Our work on Zn, Co, Cd in phytoplankton lead us to study the function of carbonic anhydrases (CA) and the mechanism of inorganic carbon acquisition in these organisms. We have discovered that diatoms employ a C4 photosynthetic pathway.  This is a remarkable result since diatoms evolved some 150 Myr ago while C4 photosynthesis in higher plants is thought to have appeared much later.  The implications is that diatoms have evolved in response to the long term decrease in pCO2 in the Earth’s atmosphere and that inorganic carbon availability may be limiting their rate of photosynthesis. Our field studies have confirmed the importance of pCO2 in the growth and ecology of marine phytoplankton.  Because of the possible importance of a feedback of marine primary production on the present increase of atmospheric CO2 the continuation of this work is a major ongoing research theme in the group.  Tortell et al. Nature 1997; Reinfelder et al. Nature 2000; Lane and Morel Plant. Phys, 2000; Riebesell et al. Nature 2000; Tortell and Morel L&O 2002; Tortell et al. L&O 2000, MEPS 2002; Morel et al. Funct. Plant Biol. 2002; Reinfelder et al. Plant Phys. 2004.

Extracellular Reductases

We documented the activity of extracellular metal reductases in marine phytoplankton.  This work which received only modest attention at the time is now the basis of new work on the mechanisms of Fe acquisition by phytoplankton (see above). Jones et al. J. Phycol 1987; Jones and Morel Plant Phys. 1988.

We also discovered extracellular amine and amino acid reductases, which provide NH4+ for uptake and produce H2O2. Palenik et al. L&O 1987; Palenik and Morel L&O 1988; 1990; MEPS 1990; AEM 1991.  In this work as well as in other work on urease (Price and Morel L&O 1991), we demonstrated that trace metals are important in the nitrogen nutrition of marine phytoplankton.

Recently we have examined the question of acquisition of phosphate from organic compounds by phytoplankton --particularly coccolithophorids-- and the role of metals in this process.  This work has lead to the discovery of a novel alkaline phosphatase (a zinc enzyme that hydrolyses the phosphate group from organic moieties) with no homology to other known phosphatases.  As a result of the high affinity of this enzyme for its organic substrates and its fast kinetics the corresponding zinc requirements are quite low and there is no evidence for replacement by another metal. Shaked et al. 2005; Xu et al. 2006.

Arsenate Respiration

We discovered, and studied in cultures and in the field the respiration of arsenate by bacteria. Ahmann et al. Nature 1994, ES&T 1997; Newman et al. AEM 1997, Arch. Microb. 1997, Geomicrobio. J. 1998

Metal-Microorganism Interactions: The global Geochemical Cycle of Mercury

 We have modeled the global cycle of mercury and studied some key transformations such as photoreduction of Hg(II), oxidation of Hg(0) and microbial methylation of Hg(II).  In addition, on the basis of new data showing no change in Hg concentration in Pacific tuna over thirty years, we have hypothesized that methylmercury in the open oceans may be formed at the ocean bottom and may not be influenced by pollution. We are presently studying the possible importance of hydrthermal inputs of methylmercury in the oceans. Mason et al. GCA 1994; Water Air Soil Pollut. 1995a&b; ES&T 1996; Amyot et al. ES&T 1997; Morel et al;. Annual Review Ecol. and Systematics 1998; Jay et al. ES&T 2000; Lalonde et al. ES&T 2001; Ekstrom et al. AEM 2003; Kraepiel et al. ES&T 2003: Lalonde et al. 2004; Amyot et al. 2005.

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Princeton University
Department of Geosciences