Introduction

Lichens have acquired a reputation as valuable monitors of environmental pollution. Pollution gradients have been related either to differential species sensitivity, creating characteristic floristic changes, or to the capacity of lichens to acquire and retain specific chemicals. These properties may be partly related to the nature of the lichen symbiosis and the structure of the resulting thallus. A brief review of lichen biology is provided to explain how the symbionts interact, what structural and physiological flexibility may exist and how far lichens react to the temporal and spatial heterogeneity of the natural, uncontaminated environment. Reference has been made to a number of valuable reviews that quote a wider literature.

Lichen biology

The lichen symbiosis consists of a fungal component, acting as the interface with the environment, and a photosynthetic component that is to a greater or lesser extent surrounded by the fungal tissues. Ascomycotina represent the main fungal group comprising the lichens, although Basidiomycotina and Deuteromycotina also occur frequently (Hawksworth, 1988a). More than 40% of the Ascomycotina are lichenized and some genera have both free-living and lichenized members. Whereas the photosynthetic component is usually either cyanobacterial or chlorophycean, thalli with both types of photobionts in different regions of the same thallus also occur (Hawksworth, 1988b; Tschermak-Woess, 1988). There is still doubt as to whether certain commonly lichenized chlorophycean algae, particularly the genus Trebouxia, exist in the free-living condition (Ahmadjian, 1993).