Speciation of Metals Before and During Root Uptake by Plants

Speciation forms of most trace metals (except the alkali metals) in soils will depend on geochemistry while that of Mo - unless in fully wet, reducing soils - will probably be [MoO4]2-, transporting it like borate with the sulfate ion transporter system. The other metals will be influenced by both geochemistry and the ligands delivered by roots or fungal mycelia to the soil. Whether citrate or malate (plants) or N donors (hydrox-amates, amino acids; fungi) will coordinate metals in the soil or ground water depends on the stability of other species already trapped (solid mineral phases, complexes with polymeric 1,2-diphenols (humic acids), carbonate, etc.) and thus on pH, redox potential and biological activity with the soil liquid. So, Strasburger and Sitte (1991) obviously oversimplify the situation in a manner negative to understanding when they state "the essential metals other than Mo are taken from the environment as cations".

More realistic, while and because admitting that there are still hypothetical steps and binding partners in their transport model, is the description of transport and bioaccumulation of metals given by Clemens et al.

(2002) (Fig. 2.6). Figure 2.6 shows that and how a transformation of ambient speciation forms takes place already outside the plant organisms by way of ligands delivered into the rhizosphere (root exsudate). There are several elements which may produce oxo- or thioanions depending on redox state and pH of ground water in various oxidation states, e.g. Cr, V and U. Chromate(VI) and vanadate(V) are readily taken up by plants but - e.g. in soybean or radish plants - undergo reduction to afford V(IV) and Cr(III), respectively, already within the roots. Therefore, only some small part of these metals does make it into the parts above the ground because complexes are too weak (Rehder 1991) to interact efficiently with other carriers after reduction and before admission to the xylem. Concerning Cr(III), one must not (mis-)take the kinetic inertness of many complexes for high thermodynamic stability: they are not. In addition, there is co-precipitation of Cr as a phosphate with Al(III), Fe(III) reducing xylem transmission furthermore.

Two different metal ions may be compared to each other using c, x and the electrochemical ligand parameter El(L) to compare complex stabilities for both ions linked to the same ligand (in the same manner). Like Ahrland et al. showed metal A to yield a more stable complex with a ligand 1 but metal B with ligand 2 (different binding preferences), the same may happen if A and B get into contact with biomasses of different taxo-nomical species or just different organs. The outcome will be fractionation of A and B (and the multitude of other metal ions) between these species, with a third taxonomical species likely to provide yet another pattern of fractionation (Markert (1996) discussed a total of 13, Garten (1976, 1978) even many more). From this, low correlation coefficients among abundances of A and B in a multitude of species are likely to result. Comparing abundances (or, more precisely, bioconcentration factors) among different species, thereby obtaining inter alia a correlation coefficient for the abundances of some pair of elements (which need not be metals) thus gives information on complexation properties of the corresponding biogenic materials: thus, average binding properties are measured. Except for some commercially purchased reference materials, Markert obtained his set of plants all from the same site (Grasmoor bog near Osnabrück, Lower Saxony, FR Germany, Fig. 2.22); accordingly soil background levels of the metals should be similar so that there is a linear relation between metal concentrations measured in photosynthetic organs and BCF values, at least as a first approximation. Like some chemically homogeneous ligand is distinguished by a certain electrochemical ligand parameter (unless it does form linkage isomers, of course), the fractionation behavior can be described by an effective electrochemical ligand parameter defined and calculated by rearranging Eq. 2.4 in order to extract EL(L) as follows:

gives

This effective electrochemical ligand parameter will pertain to the biomass of some species, at least to some organ such as a plant leaf as well as to coordination-chemical properties of dead biomasses such as marine POM. The data is extracted from abundance distributions of elements with respect to their ecochemical backgrounds, that is, from BCF values (most straightforwardly for aquatic plants and animals). Regressions to determine c and x from Eq. 2.4 were done by using data collections on complexation constants (mainly Furia 1972; Mizerski 1997; Izatt et al. 1971) and a list on electrochemical ligand parameters (Lever 1990).

Biomass and fluids contain substantial quantities of potential ligands with which most heavy and some light (Mg, Be, Ti, Al, Y) metals form rather stable complexes. Thus, uptake, state (speciation) and partition including "dumping" sites (removal as leaf or needle litter) of metals in plant biomass are mainly controlled by features of their coordination chemistry. Biomass contains various different ligand sites, including car-boxylate, amino-, imidazol- or polyphosphate moieties retained by polymers with mostly covalent backbones. Accordingly, any causal account for the Biological System of Elements must refer to coordination-chemical properties of biological ligands.

As a rule, aquatic organisms (plants) derive their demands, e.g. of metal ions directly from the solution. Thus, the argument on BCF/complex stability relationships fully holds for terrestrial organisms only. Yet, there are hints which suggest that both algae, bacterial biofilms did hold foot on solid land far earlier than hitherto assumed, that is, much before large amounts of dioxy-gen cumulated in the atmosphere, only then allowing for substantial nitrification by either lightning or biochemistry. Thus nitrate and in turn hydroxamates were unlikely to be used before the onset of the Paleozoic.

Whatever be the reason for the Eq. 2.21 to apply, the relationship can be used to analyze some feature of metabolic thermodynamics, namely, the minimum amount/share of nitrogen which is retained in some organ when releasing ligands to soil directly. This minimum value of N/C or maximum of C/N in respective organisms does exist because the values from which Eq. 2.21 was derived are minimum redox potentials at which the required oxidant is no longer detectable. Accordingly, delivery of substantial amounts of the sequestering ligands in Table 2.2 requires the ambient redox potential to be higher and thus, C/N to be smaller notwithstanding possible differences of C/N between subterraneous organs of some organism and its total composition. Except for thick old roots which do no longer substantially contribute to element uptakes from soil but rather behave as a kind of mechanical anchor and the C/N ratios (about 200) of which resemble that of other kinds of wood (Sterner and Elser 2002), the latter differences are but fairly small. Notably, this argument does not depend on any assumption concerning the manner of sequestration of any metal ions required to make these sequestering ligands, e.g. that of Mo contained and active in nitrate reductase. In addition, but some of the mentioned ligands contain nitrogen at all, leaving aside, e.g. hydroxicarboxylates or 1,2-diphenols, hence there is no direct coupling other than that from metabolism.

Optimum availability of some metal in turn depends on the differences among electrochemical ligand parameters of N-containing and N-free ligands present in soil. Thus, given a couple of such (different) ligands can be produced at sufficiently high redox potentials, availability of specific metals (including some of little use to both plants and fungi, such as Cd) will peak somewhere (Franzle 2008). This might shape an ecosystem, especially if it gets full circle: soil-contacted organs of different creatures - including mykorrhiza -will then "cooperate" to stabilize extraction of the set of metals needed to produce sufficient amounts of sequestration ligands as well as permitting biochemistries including large amounts of N. For example, Fe(III) can be extracted from an oxidizing soil by means of diphenols or hydroxamates or combinations thereof. For Fe(III), x2d = 21.39, with a difference of EL(L) between diphenols and hydroxamates of +0.21 V. Then, C/Nred (reduced denotes the numbers/molar concentrations of the corresponding ligands; that is, corrected for the intraligand C/N ratio) should be very large in soil. Anyway, it will work only with both nitrate and dioxygen present in soil.

Roots (or mycelia) tend to deliver ligands themselves to the soil which bind metal ions from soil solution

Fig. 2.5 Malcolmia maritima, the color of flower unaltered by metal exposition. Concerning isolated anthocyanes (probably oenin) from grape (Vitis vinifera), the red pigment does not apparently react with Fe(III) in either water or ethanol but turns dark-purple with Cu(II) and yields a blue precipitate with Pb(II) acetate (Anderson 1924). It also behaves as a pH indicator. The yellowish-green colour observed in live plants thus can be due to superposition of some chlorosis and formation of blue to blue-red (i.e. purple) complexes; possibly some third ligand such as an amino acid also contributes

Fig. 2.5 Malcolmia maritima, the color of flower unaltered by metal exposition. Concerning isolated anthocyanes (probably oenin) from grape (Vitis vinifera), the red pigment does not apparently react with Fe(III) in either water or ethanol but turns dark-purple with Cu(II) and yields a blue precipitate with Pb(II) acetate (Anderson 1924). It also behaves as a pH indicator. The yellowish-green colour observed in live plants thus can be due to superposition of some chlorosis and formation of blue to blue-red (i.e. purple) complexes; possibly some third ligand such as an amino acid also contributes or even mineral/clay phases around and render them better accessible to the plant or fungus. Like soil solutions, hydroponic culture solutions contain chelators and other ligands, making speciation of dissolved metals too complicated to be grasped other than by hydro-geochemical models like MINTEQ+. Often complexation reactions yield intensely coloured complexes (Riedel et al. 2004); this also can happen when metal ions penetrate into biological matter, e.g. causing flowers to change colour if the plants are grown on metal-rich soils (Strasburger and Sitte 1991). For example, in the brassicacea Malcolmia maritima (Virginia stock) there are small amounts of red anthocyanes dyeing the flowers light rose to bluish (Fig. 2.5), but in presence of Cu, Zn or Pb their colors change to yellowish-green upon M2+ complexation. This change of flower colors is practically used in ore prospection.

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