• Lichen;
  • anion competition;
  • toxicity


Phosphate and sulphite competed effectively with arsenate when samples of Umbilicaria muhlen-bergii (Ach.) Tuck, were incubated in arsenate solutions in the presence of increasing concentrations of the second anion. Equimolar amounts of phosphate had no effect on arsenate uptake while an 100-fold excess of the former caused almost complete cessation of arsenate uptake. By contrast, sulphate enhanced arsenate uptake by lichen samples while acetate had no effect.

In experiments on phosphate-arsenate competition it was possible, by regarding the latter as an inhibitor, to evaluate the kinetics of phosphate uptake in If. muhlenbergii. The Km of 660 μM and the Vmax. of 0.059 μmol min−1 indicated that the lichen took up some ten times more arsenate than phosphate per unit time. Studies of photosynthetic fixation of 14C revealed that a reduction in photosynthesis was related to the amount of arsenate accumulated. Conditions which enhanced arsenate uptake (e.g. stimulation by sulphate) resulted in greater photosynthetic inhibition, while those that reduced uptake (e.g. inhibition by phosphate) had an ameliorating effect. The ecological implications of these results are discussed.