Lichens metal complexes

Iskandar, I. K. Syers, J. K. (1972). Metal-complex formation by lichen compounds. Journal of Soil Science, 23, 255-65. 

The fact that soluble metal complexes are formed when lichen compounds react with minerals and rocks in the laboratory is particularly significant. If these complexes are formed under field conditions, and there is no obvious reason to believe that they should not, then being soluble, the products of biogeochemical weathering can be removed from the site of weathering. 

Rapid formation of soluble colored complexes when lichen compounds or ground lichen thalli were shaken with water suspensions of minerals and rocks was reported by Schatz (1963) and Syers (1969) no information was obtained, however, for the amounts of cations complexed. Of the six lichen compounds investigated by Iskandar and Syers (1972), four formed soluble, colored complexes with biotite. The extract obtained from the interactions of lecanoric acid, a depside of the orcinol type, and biotite was reddish-yellow in color. The formation of a reddish-yellow complex when lecanoric acid is allowed to react with calcium salts is used as a specific color test for this acid (Culberson, 1969). Because a complex may be colorless or adsorbed by the silicate phase, the fact that a lichen compound does not form a colored complex does not necessarily indicate that complex formation has not occurred. Iskandar and Syers (1972) showed that significant amounts of Ca, Mg, Fe, and Al were complexed by lichen compounds. In general, greater amounts of divalent than trivalent cations were complexed. For a particular cation, a similar amount was released from the silicates by solutions of the lichen compounds and by solid lichen compounds. These findings suggest that lichen compounds are sufficiently soluble in water to form soluble metal complexes. 

A metal-complexing action may be involved in the accumulation by lichens of radioactive cations such as "Sr. Subbotina and Timofeev-Resovkii (1961) reported high accumulation coefficient values for crustose lichens suspended in aqueous solutions of the radioactive isotopes of several metals. Schulert (1962) suggested that a chelation mechanism was involved in the accumulation of Sr by lichens and Tuominen (1967) showed that the uptake of Sr by Cladonia alpestris was a physicochemical process and was not metabolically controlled. 

Early studies of metal uptake by lichens presented metal sorption as an ion exchange process, and reported metal accumulation in terms of relative uptake capacity, or the capacity of one metal to displace another as an adsorbed complex. Competitive uptake studies by Puckett et al. (1973)... 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

Spectroscopic and bulk aqueous complexation studies of major lichen acids, including derivation of the structures and stabilities of metal-organic complexes and precipitates involving a wide range of elements. 

Takani, M., Yajima, T., Masuda, H. Yamauchi, O. (2002). Spectroscopic and structural characterization of copper(II) and palladium(II) complexes of a lichen substance usnic acid and its derivatives. Possible forms of environmental metals retained in lichens. Journal of Inorganic Biochemistry, 91, 139-50. 

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