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M. N. V. Prasad, PhD, is a Professor of Environmental Biology at the University of Hyderabad, India, and is the author, coauthor, editor, or coeditor of six books and more than 170 research papers on environmental botany and heavy metal stress in plants. Dr. Prasad is an elected Fellow of the Linnean Society of London, England, and the National Institute of Ecology, New Delhi, India; life member of the National Institute of Ecology and the Bioenergy Society of India; and a member of the International Allelopathy Society and the Indian Network for Soil Contamination Research.

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Recent findings on trace elements in the food chain and the environment

Trace elements are inorganic chemicals usually occurring in small amounts in nature. Trace element deficiencies and contamination are increasing due to the increasing industrialization of farming systems, industrial pollution, and other factors. In the right amounts, trace elements are beneficial; in fact, several trace elements are essential for human and animal health. Deficiencies can produce devastating health defects, while excess exposure or consumption can be harmful or even fatal. With chapters contributed by leading experts in their specialty areas, Trace Elements as Contaminants and Nutrients: Consequences in Ecosystems and Human Health:

• Uniquely consolidates information on plant and animal nutrient requirements, fortified foods, nutrient deficiencies, and excess exposure via air, water, and soil contamination

• Addresses areas for which there is a lack of information, such as bioavailability and uptake biochemistry, membrane biochemistry and transport mechanisms, enzymology, mode of action and toxicity, human health implications (efficiency and deficiency of trace elements), and biofortification

• Covers bioindication and biomonitoring as innovative biotechniques for controlling trace metal influences in the environment

• Incorporates information on specific elements, including zinc, iron, calcium, iodine, cadmium, lead, arsenic, mercury, selenium, and more

• Includes case studies

This is a seminal reference for scientists working in geochemistry, hydrology, analytical chemistry, environmental chemistry and biology, and separation science and technology; plant, soil, crop, agricultural, food, and water scientists; academic and regulatory professionals in these fields; and aid agencies and non-governmental organizations.

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Trace Elements as Contaminants and Nutrients

John Wiley & Sons

Chapter One

STEFAN FRNZLE and BERND MARKERT Department of Environmental High Technology, International Graduate School (IHI) Zittau, D-02763 Zittau, Germany

OTTO FRNZLE Christian-Albrechts-University Kiel, Ecology Centre, Olshausenstr. 40, D-24089 Kiel, Germany

HELMUT LIETH Wipperfrther Strasse 147, D-51515 Krten, Osnabrueck, Lower Saxony, Germany

1 INTRODUCTION

1.1 Analytical Data and Biochemical Functions

With ongoing improvement of analytic gear, it has already become commonplace to detect the vast majority of (stable) elements in biological samples [Garten, 1976; Markert, 1996, Lieth and Markert, 1990] as well as in soils [Kabata-Pendias and Pendias, 1984; Frnzle, 1990] or seawater [Nozaki, 1997]. The concentrations there may be considerably lower than in environmental compartments, down to the pico- or even femtomolar levels, because metal ions may not undergo bioaccumulation in plants or fungi [Lepp et al., 1987; Frnzle, 1993; Markert et al., 2003] in the same manner as unpolar organics. As a rule, there is "genuine" soil/plant bioconcentration (i.e., BCF [Biological Concentration Factor] > 1) for only a few metals (Mg, Zn, K) in green plants, with that of others being rare.

The presence of some chemical element in biomass, be it in substantial amounts, does by no means imply that it exerts some biochemical function. Several elements that are very abundant in the environment (particularly in soils) are not known to be essential for any kind of organism (e.g., Al, Ti) [Frnzle and Markert, 2007a,b]. Yet, besides the principal nonmetals C, H, O, N, S, and P, metals and other copious or trace elements were involved in biology quite apparently from the very beginnings of biological evolution [Beck and Ling, 1977; Kobayashi and Ponnamperuma, 1985a,b; Williams and da Silva, 1996; Frausto and Williams, 2001]-that is, from biogenesis itself-whereas during chemical evolution metal ions are (were) rarely required to afford crucial intermediates or catalyze transformations providing important structural features [Frnzle, 2007; Frnzle and Markert, 2002]. All living beings, even those that appear least advanced with respect to biological or biochemical complexity, share the requirement for at least seven metals (namely, K, Mg, Mn, Fe, Cu, Zn, Mo [or W in hyperthermophilic creatures]). These metals obviously differ considerably in their chemical properties; the latter is to be anticipated because metals in biology serve to effect or catalyze rather different transformations, causing them or just increasing selectivities of transformations. Nevertheless, the exact function is not yet established for all of these (7 + n) metals (neither for humans nor for any other species, irrespective of full genetic sequencing such as with Arabidopsis thaliana).

Moreover, there are fairly many cases of metals acting in bioinorganic chemistry which differ from the optimum catalysts as defined by all the experiences in inorganic and metal-organic catalytic chemistry [Frnzle, 2007]. In addition, there are biochemical processes transforming substantial amounts of matter in the biosphere which rely upon combinations of various metals. The most prominent example of this is photosynthesis: Mn (+ Ca) ions are located in the center of photosystem II, affording oxidation of water, with the electrons thus liberated being shuttled to chlorophyll and Rubisco (both containing Mg) in order to bind and reduce C[O.sub.2] or to restore chlorophyll as a neutral molecule, respectively. Hence, photosynthesis will occur in an efficient manner only if some stoichiometric relationship between Mg and Mn is kept within a green plant; the empirical value for green leaves Mg/Mn is close to 5 (stoichiometric, not mass, ratio), with conifer needles differing somewhat from this value.

The same reasoning on mandatory metal ratios holds for animal metabolism, too, here concerning, for example, the Mo/Mg and Cu/Mg ratios for combinations of oxidizing substrates (Mo, Cu in redox enzymes) and for storing energy from this process (Mg in kinases, NTPases). For the sake of efficient metabolism, living organisms must keep these ratios in their bodies rather constant throughout lifetimes. In those parts of their corresponding environments from which they retrieve metals (soil, ambient water, food organisms), the respective interelemental ratios most likely will differ from the demands of the organism under consideration and possibly even vary with time. Hence the organism need not just obtain several different metals by complexation but also has to achieve some fractionation among them. Because metal-biomass interactions, starting with sequestration from food or environment, depend on complexation of the metal ions to biomass or some carrier within or (with root exudate) outside the organism, this fractionation will be accomplished due to either unequal complex formation equilibria or selective transport across membranes. Moreover, the number of different sequestration agents in/around roots, fungal mycelia, or the guts of some animals is considerably smaller than that of different metal ions; accordingly, different metals are transported by the same carriers and compete for their binding sites [Duffield and Taylor, 1987].

Now, correlations among element abundances were produced for a number of plant species some time ago [Markert, 1996; see Section 2.1]. Here, abundances of element pairs were compared in 13 different plant species and correlated to each other. Conspicuously, abundances of chemically similar elements like P and As or Ca and Ba are not correlated, whereas the REE (rare earth elements) abundances are closely correlated once again. From the correlation analysis a so-called BSE has been established (Fig. 1, see also Section 2.1).

Accordingly, abundances of essential elements may but need not be positively correlated. In addition, there is a need for definition of binding properties of metal ions toward (different kinds of) biological material which can account for enrichments in certain samples. For this purpose, a general relationship that links concentrations or, more precisely, bioconcentration factors (BCF [values]) to complex stabilities of metals taken up by some organism must be constructed (and expressed by some quotient k', see below). This approach also can be extended to trophic chains. There are several reasons for this particular approach in biochemistry:

Metals interact with biological material by coordinating to it.

Not all metals cause pronounced biochemical effects (i.e., many ones are neither essential nor considerably toxic), hence can be expected just to follow chemical equilibria by speciation into biological material rather than being selectively enriched or expelled/retained/linked to certain "controlling" sites/molecules. Complex ligands using the same functional groups produce complexes of closely similar stabilities.

Complex formation usually occurs close to chemical equilibrium under physiological conditions and can be described by perturbation theory because there are only small effects on metal ions brought about by electronic properties and energies.

Any living being thus must cope with the endeavor to

obtain the essential elements in a mixture that meets the demands of the corresponding organism while being constantly connected to an environment (e.g., soil, surrounding water) that usually does not match the respective demands directly (cf. Liebig minimum principle),

keep away toxic elements, in some cases even, and

maintain a certain ratio between different essential elements involved in catalyzing the same biochemical transformation (e.g., Mn and Mg in photosynthesis).

There are several matters that render this a complicated endeavor: The relative stabilities of (chelate) complexes of (divalent) metal ions usually change according to a certain sequence, referred to as Irving-Williams sequence [Irving and Williams, 1953], which itself is not related to the specific amounts required by an organism, nor to the abundances of metals in soil or fresh water (or the inverse behavior, thereby permitting organisms to retrieve similar amounts of Mn, Cu, Zn, and so on in spite of their highly different complex stabilities). Yet, the Irving-Williams sequence rather is something like a rule-of-thumb. With ligands other than dicarboxylates, amino acids, or phenol(-ic) carboxylates, there are (often several) "inversions" of the stability series. Accordingly, living beings might select appropriate metals by producing and delivering suitable ligands, with the above ones not being the only ones that could be produced in substantial amounts and given away, for example, by roots or mycelia. On the other hand, soil organic matter (SOM) or aquatic organics (DOM) contain certain ligand functions capable of retaining metals from transfer into living beings. Thus there is some competition for the metals between plant or fungus and soil. The data that are derived from analyses of (plant) biomass (e.g., Markert [1996]) thus correspond to some superposition of effects.

Therefore, organisms have to change concentrations of the metal ions encountered in their environment/food actively in a typical way; for example, wood-degrading (basidiomycete) fungi need much more Fe and Cu, sometimes also V, to accomplish oxidative degradation of lignin than green plants require for sustaining their unlike biochemistry. The latter in turn have larger demands for Mg and Mn owing to photosynthesis. Plants and fungi might-and do-deliver different ligands to cope with this: citric, malic, and oxalic acids in green plants and peptides; hydroxamates and sometimes amino acids in fungi (and also in soil bacteria) [Kaim and Schwederski, 1993; Farago, 1986; Haas and Purvis, 2006]. How, then, do these ligands compare with respect to metal binding affinities and selectivities to the former ones?

Complex stabilities depend on the extent of metal ion-ligand interactions; at the same time, due to orbital interactions, the more the energy levels of the central metal ions are changed, the stronger the interactions become. In metal ions that are susceptible to redox reactions, the shift of orbital energies can be detected directly by change of redox potential of the altered complex. Hence there should be a relationship between complex stability and the potential shift caused by some ligand in a standard system (the so-called electrochemical ligand parameter [E.sub.L](L) [Lever, 1990]), with the latter providing a measure for binding capability [Frnzle, 2007]. This argument from perturbation theory can be represented in the following equation, with complex stabilities at a given metal ion taken from experiment or literature (aqueous medium, 25C, I about 0.2 M/kg) correlated to the above electrochemical ligand parameter [E.sub.L](L) by linear regression analysis:

-log [k.sub.diss] = x [E.sub.L](L) + c, (1)

Here, x is the slope parameter of the correlation between (logarithmic) complex formation constant (taken, e.g., from Furia [1972] and Moeller et al. [1965]) and the electrochemical ligand parameter [Lever, 1990; Frnzle, 2007; Frnzle and Markert, 2007a] for a given metal ion-say, in a series of Zn(II) complexes with bidentate ligands (e.g., glycinate, oxalate, lactate, ethylene diamine, and aminomethanephosphonate); then c = 5.15 and x = +8.69 for [Zn.sup.2+] (tables for >50 different metal ions were published elsewhere, e.g., Frnzle and Markert [2007a]) while c gives the axis intercept at [E.sub.L](L) = 0. After rearrangement of the above Eq. (1), one obtains

[IT[L[E.sub.L](L).sub.eff] = [+c - log [k.sub.diss]]/x (2)

to define fractionation behavior via an effective electrochemical ligand parameter. Thus a large, if not comprehensive, set of parameters were produced by the first author (e.g., Frnzle and Markert [2007a]) which permit to estimate hydrolytic complex formation stabilities, with [E.sub.L](L) being typical of a given ligand (donor) moiety. When different ligands are present, speciation or distribution/partition may also be inferred from the above equation. In soil, kinds and properties of ligands (SOM) change upon humification, and so do the ligand affinities of the metal ions. Then soil composition (C/N ratios, etc.) and speciation of N-free versus nitrogeneous ligands control which metals will be passed into green plants or fungi, respectively.

2 MATERIALS AND METHODS

2.1 Data Sets of Element Distribution Obtained in Freeland Ecological Studies: Environmental Analyses

The first data on element (abundance) correlations among green plant species later on to be used in this study were obtained at Grasmoor (literally, "grassy bog") natural reserve near Osnabrck, Lower Saxony, Germany around 1990 (Figs. 2a and 2b).

The original abundance correlations were derived from these data for a total of 13 plant species [Markert, 1996] and called the Biological System of Elements (Figs. 1 and 3), with the name "Biological System of Elements" alluding to the chemical periodic system of elements (cf. Railsback [2003]). From these data already the clear-cut limits of that analogy become apparent: There is nothing like (some) "biological/ biochemical group[s] of elements" which could directly be related/compared to the chemical groupings of the PSE. While abundances of most REEs among each other and with Al correlate strongly, there are almost no relationships, for example, for distributions of P and As or of Ca and Ba. Recently, when converting these data from mere correlations of abundances into a kind of parameter which describes both (a) the general binding features of some kind of bioorganic ligand system/tissue to retain and accumulate metals and (b) their capacity of fractionation among the latter, comparative data for the same species at other sites (Betula pendula) and for quite different (including aquatic) plants were calculated (Table 2). For essential elements, this is expected to correspond to the demands that relatively differ among different plant species, with distributions/BCF values of certain nonessential elements to be used as a benchmark for this. The focus of interest thus shifted from mere comparison to producing a scale for the underlying biochemistry; for the latter purpose, bioconcentration factors soil/photosynthetic organs (neglecting the extent of bioavailability of metals in certain soils) are used to get an idea on bioinorganic (metal ion) concentration processes and eventually characterize fractionation inside plants and their corresponding rhizospheres (see below; k' index).

2.2 Conversion of Data Using Sets of Elements with Identical BCF Values

The distribution of the essential metal Mg among the 13 species is highly correlated with those of (other essential elements italicized) Al (r = +0.60), Ca (0.71), Cs (0.74), Cu (0.61), Eu (0.74), K (0.79), N (0.70), Pb (0.66), [Si.sup.3] (0.71), Sc (0.66), and Sr (0.63), but not Mn (-0.13; in spite of the coupling between Mn [PS II] and Mg [chlorophyll, rubisco] in photosynthesis), Zn (-0.09), V (-0.73), or Ba (+0.27) [Markert, 1996]. There are positive abundance correlations between Mg and the REEs La, Ce, and Dy, an even more strongly positive one (r = +0.738) with Eu, and negative correlations of Mg abundances with those of Y, Gd, Tb, and Ho through Yb, whereas occurrences of Pr, Nd, and Lu (which latter, strictly speaking, is not a REE anymore as the filled [f.sup.14] orbital set is no longer activated by oxidation or certain coordination effects) in the 13 plant species investigated by Markert are not at all correlated to the abundances of Mg.

(Continues...)

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