Equilibrium Models Concentration Ranges and Biological Functions of Metal Ions

Although equilibrium models will provide realistic representations of distributions of most chemical elements these are unlikely to hold for such metals which both have a number of biochemical functions and bind strongly to the various ligand sites of biomasses, in addition to being rather abundant in the environment (such as Cu). It can be estimated that "free" (i.e., just aquated) Cu2+ or Zn2+ ions will occur in plant sap at femtomolar or even lower concentrations at best, leaving their transport to coordinated (ligated) species which change ligand spheres several times during this process. There are specialized (metal-specific) transport and storage processes and speciation forms which

(a) Avoid that metal ions strongly coordinating to biomass (like Cu, Zn) will bind to whatever protein or apoprotein around, otherwise putting both its structure and function(s) into jeopardy (becoming thus toxic)

(b) Simultaneously give a reliable supply of these metal ions to enzymes which depend on such ions (say, Cu-dependent oxidases)

Strongly toxic elements like Cd, Hg or U will cause specific repulsion reactions in plants prompting them to

Fig. 2.7 Photograph (left), autoradiography (109Cd, T1/2 = 453 days) (center) and digitally contrast-enhanced version of cadmium distribution in sunflower (Helianthus annuus) seedlings (right). The root is cut off and laid to the left. Note that there are high Cd concentrations in the vasculae also in the leaves but Cd hardly leaks into the leaves

Fig. 2.7 Photograph (left), autoradiography (109Cd, T1/2 = 453 days) (center) and digitally contrast-enhanced version of cadmium distribution in sunflower (Helianthus annuus) seedlings (right). The root is cut off and laid to the left. Note that there are high Cd concentrations in the vasculae also in the leaves but Cd hardly leaks into the leaves

Fig. 2.8 Autoradiography of a young tomato plant; the radio-nuclide used for obtaining the picture was not specified (Feller 2005). Here, there is some concentration maximum in the upper root also, but the radionuclide does accumulate in the edges/tips of leaves while the vasculae are depleted with respect to radio-nuclide (reproduced with permission)

Fig. 2.8 Autoradiography of a young tomato plant; the radio-nuclide used for obtaining the picture was not specified (Feller 2005). Here, there is some concentration maximum in the upper root also, but the radionuclide does accumulate in the edges/tips of leaves while the vasculae are depleted with respect to radio-nuclide (reproduced with permission)

produce agents which trap them by chelation (induction of phytochelatins). Accordingly, these data (BCF, intra-and interspecific/organismal distributions) for Cd, Hg or U in plants or animals are more difficult to attribute to "purely chemical" features of transport, because here (and with As, Pb, Ni) there is superposition of

(a) Specific transport/quenching by chaperons (Tottey et al. 2005)

(b) Specific ligand properties in phytochelatins, with, e.g. Cd2+ affinity depending sensitively in sequence periodicity there (Dorcak and Krecel 2003), and - before it gets there - also

(c) Is due to an unlike level of phytochelatin induction by some given heavy metal in different kinds of plant

Thus, there is negative feedback in the environment/ metallome system, exactly tantamount to detoxification. For so-called semi-metals some of which also form "colloquial" complexes, like Sb, Bi, Te, the speciation pathway of biomethylation (Thayer 1995) will remove their electrophilic properties altogether, turning the cations into ligands (donors) of their own, whereas with other elements (Ge, Sn, Pb, Pt, Au, Cd or Hg) acceptor properties are substantially altered (see the data (c and x values, Table 2.3) for R2Sn2+, R3Sn+ and R3Pb+ species) but do not vanish. Of course, redox processes also influence acceptor properties (cp. the data for different oxidation states of V, Fe, Ce or Tl).

To a first approximation, in essential elements the number of different biological functions should correlate positively with the concentration of the corresponding element in biomass, but the amounts which are required to operate biocatalytic processes may be so small that corresponding differences will be unde-tectable if there is any other - e.g. osmoregulatory -function of that element. Osmoregulation, in particular in halophilic organisms, takes much larger quantities than both catalysis and chemical signalling (alkali metal ions, Ca2+) while no kind of biological function can be inferred from analytic data directly (for example, in humans, there are four or five essential elements with daily requirements <1 |imol (total) or <1 |g/kg FW, namely Se, V, Co, Mo (presumably +As)).

The corresponding equilibrium state of distributions of various metal ions usually differs from those of the soil substrate, hence entails some fractionation during transport as described before. So, fractionation can be compared to that effected by some single ligand, and Eq. 2.4 and its inversion are then used to define some "effective electrochemical ligand parameter" which describes the capability of some plant organ to fractionate among metals.

Therefore no longer qualitative statements are cast into a numerical framework but easily measurable values are linked to obtain information on complex stability or bond energies. Thus, by linear regression analysis, two new parameters are obtained which -once they are known for sufficiently many different metal ions, both essential and non-essential ones - in turn can be linked to this biochemical property of essentiality. Electrochemical ligand parameters for different complexes of the same metal ion are correlated with the respective complex formation constants to yield two parameters which are typical of a certain metal ion and way of bonding (denticity of ligand). So the redox potential of some metal complex or the electrochemical ligand parameter of its ligand are taken to determine - and in turn predict for yet other complexes - the complex formation constant -log kdiss. The regression Eq. 2.4 obtained in this manner provides some typical (intrinsic) binding stability (axis intercept, c) and a slope (x) which denotes the sensitivity of complex formation towards the (changing) kind of ligand (ligand sensitivity, x) (Fig. 2.1). Conversely, with den-ticity and electrochemical ligand parameter of ligand being known, a complex formation can be calculated by c and x of the central ion (recall that the electrochemical ligand parameter is obtained by dividing the actual change of redox potential in the Ru(II/III) couples by denticity (Lever 1990)). As a meaningful definition of c and x can only be obtained with sets of a equidentate ligands, the corresponding multiplying factor in the regression equation was skipped for simplicity. The result would be the same except for some alteration of both c and x values in multidentate ligands, but then Eq. 2.4 would be more difficult to handle.

Now, the situation regarding metal fractionation will be discussed for green plants, fungi and animals one after the other, giving rise to trophic networks and eventually geobiochemical cycles, including issues of ecological stoichiometry beyond C, N and P. Fungi can only degrade lignin, lignite and other kinds of polymeric phenols if they command suitable laccases, peroxidases, etc. (basidiomycete fungi) and thus require Fe(III), Cu and/or V. As these processes require dioxygen as the initial oxidant, they can only be achieved in an oxic milieu; even corresponding brown coals are considerably oxidized, producing O-rich solids or even oxalic acid. Thus it can be concluded there is Fe(III) with the corresponding c- and x-values (rather than those of Fe(II)) controlling bioavailability when the above ligands are present.

Both fungi and soil bacteria will simply oxidize and degrade substantial parts of soil organic matter to yield CO2, eventually reducing the ratio [C/N]red to values of 3-4 (Bardgett 2005). During this process, amino acids, hydroxamates and sometimes oxalate are formed. Now let us estimate which metal sequestration strategy, using bidentate agents, must be taken to adapt to [C/N]red » 3.5 in the soil and x2d » 16 (causing an enhanced uptake of Fe(III), Cu(II) and V(IV)) for doing such oxidative "crackdown" of phenolic polymers by fungi or other organisms. The result can be obtained from Fig. 2.16: the "best" ligand should have EL(L) = -0.03 V, that is, amino acids could achieve appropriate metal uptake from the sediment pretty well. This and the electrochemical ligand parameters of the N-donors imply values between -0.08 and -0.01 V for the N-free decomposition products of lignines or lignites. By this the "suitable" products of lignin degradation are determined: especially in exocellular processes formation of such products must be avoided which in turn would bind essential metals like Fe or Cu so strongly or in thus hardly soluble forms that retention outside the cell would become too large. The result of such ways into lignin/polyphenol degradation then would be metal ion starvation by the very products of the process. The optimal outcome of these is a product array providing minimum retention which has to be compared to the (hitherto far from complete) information on ligand-active compounds in aged humic substances or in lignins. The latter, as detected by IR- or 13C-NMR spectroscopies, include 3-ketoenolates (higher analogs of acac), 2-alkanoylphenolates, phenolpolycarboxy-lates, which provide optimum mobilization of the above metals if combined with amino acids or hydrox-amates. In soils, amino acids are produced/delivered either directly (grass roots, drying mosses) or produced from protein residues by soil-borne hydrolases while 3-ketoenolates like oxaloacetate are intermediates of the tricarboxylate cycle and provide additional ones upon decarboxylation. It can be concluded that the "strategy" of (basidio- or ascomycete) fungi for lignin degradation is not just the only one which ever evolved but also highly optimized: considering the frequent differences between "biochemistry" and "technical optimum" it is the more conceivable there was just one success as most similar reactions would yield materials from which the exoligninase-forming metals like Mn or Fe could no longer be retrieved.

Thus chemical ecology, coexistence and possibly mutual succession of different organisms which thrive on the same supporting soils are determined mainly by the complex-forming properties of the (chemical, mostly anionic) species delivered by roots or mycelia, respectively. In addition, other kinds of fungi (mykorrhiza) organize interaction and chemical exchanges of both metal ions and N or organic species while biochemical and MnO2-catalyzed degradations will alter identities, functional (donor) groups, thus EL(L) and selectivities of ligands in soil with time. MnO2 particles are thus efficient if combined with strong oxidants as to render permanganate oxidations even of rather refractory organics like glycine (^ HCHO + CO2 + NH4+) auto-catalytic, forming more manganese dioxide (Perez-Benito et al. 1987).

For the "original" species, these reflect the different demands due to different biochemical pathways and catalytic challenges in the various large groups of organisms. Nevertheless, all plants, fungi and soil bacteria can only successfully sequester cations from soil because there is a combination of two features:

(a) They use ligands which are typical for the realm of organisms and do not differ too much in EL(L).

(b) The c and x values are rather similar except for Cu and Ca among the essential metals; thus the "window of essentiality" (Fig. 3.1) is required for an effective co-existence of soil-dwellers even though they compete for the same resources at first glance: there simply is no option to escape competition by using metals as biocatalysts with very much "deviating" c2d- and x2d-values, no matter how abundant they may be (Al!).

For purposes of theoretical understanding, EL(L) obviously is the decisive criterion allowing to meet the different demands of green plants and of fungi, also regarding details in soil chemistry. When feeding by consumption of autotrophs, heterotrophs must overcome other chemical obstacles than to balance out (electron retrieval by) water oxidation and CO2 binding plus reduction which green plants have to do:

• Oxidation of organic matter has to be accomplished in a selective way, making use of Zn-, Mo- or Fe-dependent enzymes, leaving behind many different products besides CO2 and protons.

• Proportional to this oxidation energy can and must be stored by phosphorylation of carboxylate and sugar substrates which is done by Mg or Zn enzymes (kinases, phosphatases) mainly.

N transfer is metal-independent while N oxoanion reduction takes Mo once again. As the stoichiometric composition of the food of second- and higher-order consumers is fairly constant, the relative requirements of the metals which take part in the quantitatively dominant processes in an animal are constant also, and these metals may be acquired from food and from ambient water - or lost to the latter. This can work either way round, depending on the relationship between complex formation constants (and thus, on EL(L) once again). As shown by Fig. 2.11, this does not hold for phytoplankton: in eutrophic conditions, all C/N- (9 rather than 13), N/P- (<20 rather than 70) and thus eventually C/P ratios (about 150 instead of 900) are decreased. As Mg is implied in both photosynthesis and phosphorylation, C/Mg should not be changed by eutrophification whereas N redox demands increase, and thus C/Mo and Mg/Mo ratios should negatively respond to eutrophification, with an enhanced Mo level in the "eutrophic" alga. As evidenced by the altered CNP balance, (at least many) algae can adapt to this, accordingly must be able to absorb more Mo if exposed to (N; P)-increased conditions. Animals feeding on phytoplankton in turn have more Mo available, then. So it might very well occur that "trying" to keep the necessary stoichiometric ratios among essential metals in an aquatic animal which does feed on something particular needs EL(L)eff values which bring about thus feeble complexes that certain metals will be lost to water at least if the latter contains humic acids or other kinds of ligand-active DOM. Thus, "translating" the metal ratios of the food to the consumer's own demands given by bioinorganic chemistry, the key topic of (extended) ecological stoichiometry, is a kind of translation which, unlike a given plant or fungus is to thrive on a soil of certain composition, often brings about substantial losses to the environment. Of course, metals like the heavy alkaline earths or Zr which will not appreciably react with humic-type DOM will not undergo corresponding "leaching" from zooplankton.

Concerning zooplankton and larger aquatic animals, it is feasible to calculate some effective EL(L) in animals in the same way as for plants or bacteria: daphnia (D.magna) exhibit a decreasing abundance of metals like above (that is, Fe » Zn > Zr > Cu > Cd > Mn > Cr > Co > V > Mo) while for plant leaves it often is Ca > Mn > Fe » Al > Ba > Cu, in disaccord with both an expected accumulation by the Irving-Williams series giving stabilities of "normal" complexes. It can be seen from Table 2.3 that the "classical" (1953) IrvingWilliams series simply corresponds to the sequence of increasing c values for bidentate behaviour: Ba (0.45) » Sr (0.55) < Ca (0.73) < Mn(II) (3.01) < Mg (3.94) < Fe(II) (4.20) < Co(II) (5.48) < Ni (6.65) < Cu (9.04) >> Zn (5.15). Accordingly the Irving-Williams series is observed without any permutations if c dominates in

Fig. 2.9 Green plants (left, represented by beech tree) and fungi (right center) secernate ligands with quite different electrochemical ligand parameters which accordingly display different metal selectivities fulfilling the demands of either heterotrophic or photosynthetic group of organisms. Fungi thus enrich Fe, Cu and V for haem-based peroxidases, laccases and V-dependent haloperoxidases involved in wood degradation: Even the simple molecular anion amavadin (Nawi and Reichel 1987; Rehder 1991), presumably just a kind of transport form for vanadium in fungus Amanita muscaria, exhibits a variety of versatile catalytic features in oxidative cleavage of aromatic and other C ring systems, phenol functionalization etc. Green plants in turn get better access to Mg, Mn (photosynthesis) and Ca for cell cycle regulation, etc. Zn is required by both plants and fungi but its complexes with bidentate low-EL(L)-ligands are rather weak which is compensated by plants by delivering the (tridentate) citrate the Zn complex of which is somewhat more stable. In the same manner citrate additionally enhances uptake of Fe and Cu while no advantage is gained by citrate delivery for either V (required by many fungi) or Mg, and for Mn(II) the situation even is reverse: complexes of some bidentate ligands are more stable than the citratomanganese(II) species. Positions where the ligands are written correspond to their El(L) values (cp. Table 2.2) while the abscissa gives the complex stability constants for bidentate ligands with the metal ions at the right side obtained from Table 2.2 and Eq. 2.4. Concentrations of "free" Fe(III) ions in a soil solution are exceedingly small (Scheffer et al. 1998; Sigg and Stumm 1994). For bidentate ligands with EL(L) > -0.12 V, the vertical arrangement of the lines for divalent cations reproduces the Irving-Williams (Irving and Williams 1953) series: Ca < Mg » Mn(II) << Cu(II) > Zn. Oxalate is produced by roots of many plants while 1,2-diphenols are prominent in humic matter (Scheffer et al. 1998)

general binding behaviour, that is, for ligands having EL(L) » 0 V such as amino acids (cp. Figs. 2.1 and 2.9) while other values of electrochemical ligand parameters bring about the frequent (Irving and Williams 1953; Sigel and McCormack 1970) "permutations" among positions of certain element pairs in the series. As outlined before, EL(L) << 0 V is frequent in biological material and both its soluble and polymeric components, hence here occur similar permutations also. When does this happen? The limiting condition is given by for an individual ion M

for some second ion M', accordingly

Some "deviation" from the Irving-Williams series results if differences of ligand sensitivities get larger than those of intrinsic bond stabilities:

Fig. 2.10 Chemical processes which can alter speciation of various elements differ among different soil layers: various sediment layers can control passage of heteroelements into deeper soil or sediment strata by chemical reactions, with phenols, phosphate or As(V) retained in corresponding layers whereas other kinds of transformation either directly invoke biological activity (e.g. biomethylation), not to be mimicked by simple element-

organic chemistry except for As (Cadet reaction) and possibly Co. MnO2 is a potent redox catalyst capable of oxidizing Cr(III) to yield chromate(VI), Ce(III) to Ce(IV), phenols to catechols and muconic acids, etc. given there is O2 while Aspergillus and other fungi can mediate biomethylation of As, Sb and many metals. If neutral species like As(CH3)3 arise by this process, they may diffuse out and escape into the atmosphere

Fig. 2.10 Chemical processes which can alter speciation of various elements differ among different soil layers: various sediment layers can control passage of heteroelements into deeper soil or sediment strata by chemical reactions, with phenols, phosphate or As(V) retained in corresponding layers whereas other kinds of transformation either directly invoke biological activity (e.g. biomethylation), not to be mimicked by simple element-

organic chemistry except for As (Cadet reaction) and possibly Co. MnO2 is a potent redox catalyst capable of oxidizing Cr(III) to yield chromate(VI), Ce(III) to Ce(IV), phenols to catechols and muconic acids, etc. given there is O2 while Aspergillus and other fungi can mediate biomethylation of As, Sb and many metals. If neutral species like As(CH3)3 arise by this process, they may diffuse out and escape into the atmosphere or

implying that the following value of electrochemical ligand parameter must be surpassed:

When Zn is more enriched (larger BCF value, and k' >

0) in some biomass than Cu, Eq. 2.20 implies EL(L)eff

< -0.30 V, with BCF ,m > BCF „., in the same sam' Mn(II) Co(II)

ple (Daphnia magna) a corresponding value of <-0.27 V is obtained (Franzle and Markert 2000a, b). There are bioligands - phosphorylated species - which are capable to explain such extents of "anomalous" fractionation without invoking active transport; however, complex formation constants get rather low in either pair of divalent metal ions (k » 103). Fractionations of this kind thus would appear to imply rather high environmental concentrations of such ions, much into toxic levels as far as Cu or Co are concerned, but, in passing several transport steps and kinds of membranes, similar amplification of complex stability differences might take place - to be expressed in the k' parameter - like observed in certain plants (e.g. Ericaceae). However, we consider aerobic organisms here while the +II oxidation states of both cobalt and manganese will not be maintained in oxidizing conditions at neutral or higher pH values when ligands like amino acids are present.

In any case, such values of electrochemical parameters for ligands relevant in biology almost from biopoiesis onwards allowed for broad and differentiated use of now essential elements much like in recent biomasses also >2.3 bio. years ago, without the necessity to have specialized transport systems at hand. Possibly, phosphate-containing regulating systems and transporters - besides the more advanced polyphenols, hydroxamates or proteins (chaperons) - involved in transportation of, e.g. Fe are relics of those days. Possible items of such transport include all of the more general essential elements except for Co and V. The edges to the central "window of essentiality" in the c/x mapping suggest that changes of accessible oxidation states in redox-active elements did not thoroughly change the patterns or conditions of essentiality, especially concerning some transition metals close to these edges in their now familiar oxidation states (Mn, Mo, V, Co). Others close to or within this area after undergoing oxidation did never attain biocatalytic functions as far as can be assessed (Eu, Ce), while Mo and Ni were shifted to these edges.

Along a limnetic trophic chain, concentrations of many - including essential - metal ions tend to decrease rather than increase up the trophic chain (Franzle and Markert 2002b); elements with highly negative ligand sensitivities x2d such as Sr, Ba, La, Pr, Nd or Zr (-9 > x2d > -46), but also Ni (x2d = +8.85), tend to be biomagni-fied by daphnia (two different species) eating phyto-plankton followed by biodilution with the daphnia being consumed by fishes whereas there is no instance of the inverse set of biomagnification behaviors (Table 2.11):

For "switching" purposes (cell signalling, enzyme activation) no species can be used which is very abundant in the environment. An example is the role of Na in enzyme activation in terrestrial animals; this presumably would not work in seawater. Activation of metabolism by hormones containing iodinated tyrosines - once again typical of vertebrates - depends on the same condition even though the I abundance in OW is rather limited. There, however, are cases where there is biodilution in either stage of the trophic chain, notably with generally essential elements like Mn, Fe, Zn and I. Ce and Cr are depleted by fishes after eating zooplankton which latter would not alter the concentrations seen in its own (phytoplankton) food. Zn and Fe(II) which is the only water-soluble oxidation state, additionally enriched by photoreduction of Fe(III) in - even oxygen-saturated - freshwater (Franzle et al. 2005) and thus is far more bioavailable than the higher oxidation state. Both Fe2+ and Fe(III) have positive x2d values (Table 2.3) while BCF - and BCF, , ., alga/daphnia daphnia/

fish-values for Cu or Al are regrettably absent from the above set of data. The oxidation state of Mn during biotransfer is uncertain.

Conceivably the number/volume density of metal ion binding site decreases when changing from daphnia to (planktivorous) fish while EL(L)eff for biomagnifica-tion in fish should be close to 0 V. As discussed before, EL(L)eff = -0.25 V for biomagnifications of metals in D.magna which is far lower than that for both fishes and many kinds of aquatic plants including phytoplankton. Through the trophic chain, nitrogen is effectively retained in the consuming organisms, with Ni (urease; or Mn in animals), Mo (redox processes) and Mn involved in closing the internal N cycles while a larger part of C is oxidized up to CO2. As a result, the C/N ratio familiar from ecological stoichiometry steadily decreases from phytoplankton via microcrustaceans up to fish (Fig. 2.11, from Franzle et al. 2005):

It is necessary to better analyze the differences in EL(L) between daphnia and "typical" limnetic phyto-plankton. The corresponding value for aquatic plants is distinctly higher than for D.magna yet it is negative, and bidentate ligands are involved as evidenced by the distributions of the above, mainly essential, metal ions. Hence metals with negative ligand sensitivity are enriched by phytoplankton whereas Fe or Zn undergo depletion with respect to their algal concentrations. The situation resembles that in uptake of metal ions from the rhizosphere by green plants, fungi or local bacteria.

Table 2.11 Biomagnification of elements in simplified trophic chains in freshwater (Franzle and Markert 2002)
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