Importance of Actual World Knowledge within the Context of

55 2.2.3 Biological Methylation of Heavy Metals

Some elements including Hg, As, Sc, Te, Ti and Zn can undergo methylation by micro – organisms to form volatile molecules, such as CH3 Hg⁺, CH3Se and CH3 As and this can be a major route for losses of these elements from soils. Methylation is known to be brought about by both aerobic and anaerobic segments in aqueous environments. All biological methylation involves methyl cobalamin, a methylated derivative of B12 which contains CO. The rate at which biological methylation occurs depends on the condition, including temperature, redox and pH, but non – biological methylation can also occur (Davies, 1990).

The methylated forms of Hg are (CH3)2 Hg (most stable in alkaline conditions) and CH3

Hg+ (stable in neutral to acid soils). Lead is also thought to be methylated in the environment by both biological and abiotic mechanisms but the evidence is not conclusive. However, most organic-Pb compounds in the environment are probably derived from additions in petrol (Davies, 1990).

A soil plant relation of heavy metals is the major interrelationship affecting the dynamics of heavy metals between the soil and the plant. The soil-plant system is an open system subject to such, as contaminants, pesticides, and to losses such as the removal of metals in harvested plant material, leaching, erosion and volatilization (Davies, 1990).

56 shoot. Plant uptake of mobile ions present in the soil solution is largely determined by the total quantity of this ion in the soil. In the case of strongly adsorbed ions, adsorption is more dependent upon the amount of root produced. Mycorrhizae are symbiotic fungi which effectively increase the adsorptive area of the root and can assist in the uptake of nutrient ions, such as orthophosphates and micronutrients. Roots possess a significant CEC, due largely to the presence of carboxyl groups, and this may form part of the mechanism of moving ions through the outer part of the root to the plasmalem where active absorption occurs. Absorption of metals by plant roots can be by both passive and active (metabolic) processes. Passive (non-metabolic) uptake involves diffusion of ions in the soil solution into the root endodermis. On the other hand, active up-take place against a concentration gradient but requires metabolic energy and can therefore be inhibited by toxins. The mechanisms appear to differ between metals (Alloway, 1995).

Pb uptake is generally considered to be passive while that of Cu, Mo and Zn, is thought to be either active metabolic uptake or a combination of both active and passive uptake.

Absorption mechanism can vary for different metal ions, but ions which are absorbed into the root by the same mechanisms are likely to complete with each other. For example, Zn absorption is inhibited by Cu and H⁺ but not by Fe and Mn. Cu absorption is inhibited by Zn, NH⁴⁺, Ca and K (Alloway, 1996).

The rhizosphere is the zone about 1 -2mm wide between plant roots and the surrounding soil. It receives appreciable amounts of organic materials, mucilage, sloughed off cells and their lyocites (Alloway & Gala, 1984). These organic compounds give rise to intense microbiological and biochemical activity in the rhizosphere which

57 enables roots to mobilize some of the metals which are strongly adsorbed in the soil, by acidification, redox changes, or the formation of organic complexes.

Phenolic compounds and certain amino acids are known to be involved in the solubilisation of Fe³⁺ and Mn4+

Cereal deficiency in micronutrients such as Fe and Zn appear to have root exudates containing substances such as phytosiderophere-2-deoxymugineic acid which are effective in mobilizing these and other metals from sorption sites in the vicinity of the root. Mench and Martin (1990) showed that exudates with identical carbon contents from maize and tobacco extracted amounts of Mn, Cu, Cd and Fe which differ with the plant species ( Mench & Martin, 1990).

Tobacco root exudates increased the extraction of Cd but decreased that of Fe. Those of maize did not affect the concentrations of either of these metals. The uptake of metal from soils is greater in plants grown in pots of soil in the green house than from the same soil in the field. Devries and Tiller found the uptake of Cd by lettuce and onion bulbs grown in pots to be 6 and 25 times greater, respectively, than when grown in the same soil. This is probably due to differences in microclimate and soil moisture and to the roots of container grown, plants growing solely in contaminated soil; whereas those of field grown plant may reach down to less contaminated soil (Davies & Tiller, 1982).

Relative differences in the uptake of metal ions between plant species is genetically controlled and can be due various factors including surface area of evapo-transpiration.

The latter mechanism affects the mass flow of the soil solution in the vicinity of the root and the movement of the ions to the root absorbing surface.

58 E. Cal gave the general order of the transfer coefficients for most of the biologically important heavy metals. The transfer co efficient is the metal concentration in plant tissue above ground divided by the total metal concentration in the soil (Nickolson et al., 1998)^.

Although numerous soil and plant factors can affect the accumulation of metals in plants, the values given are intended guides to the order of magnitude of the transfer coefficients and root precise values.