The Control of Cadmium Uptake in the Lichen Genus Peltigera

Abstract

Intra- and extracellular Cd uptake in the lichen genus Peltigera were investigated, and intracellular uptake found to display Michaelis-Menten kinetics. Compared with values reported for Zn uptake in free-living algae and fungi, Peltigera had low affinities for Cd and low maximum uptake rates. Intra-, and to a lesser extent extracellular uptake rates were temperature dependent. When lichens were incubated concurrently with Cd and equimolar concentrations of a range of other cations, most were found to reduce both extra- and intracellular Cd uptake, implying that the Cd uptake systems had low specificities. Mg, though not a strong competitor for extracellular Cd uptake, inhibited intracellular Cd uptake to a similar extent to borderline elements, and it is postulated that intracellular Cd uptake occurs by a system which normally transports Mg. Although concurrently-supplied cations reduced Cd-induced inhibitions of photosynthesis, the reductions were not proportional to the effect of the cations on intracellular Cd uptake. This indicated that other cations affected the toxicity of Cd to photosynthesis by some means in addition to reducing Cd uptake. Intracellular Cd uptake was light-stimulated, suggesting that a close relationship existed between metal uptake and metabolism. The rate of intracellular Cd uptake in the dark was probably not directly linked to the supply of respirable reserves, as it was unaffected by prolonged storage in the dark, and was not increased by adding glucose. It is hypothesized that light-stimulated Cd uptake represents active entry into algal cells, but with uptake in the dark it is not clear which symbiont is involved, and whether energy is required.