The response of lichens to atmospheric deposition with an emphasis on the Arctic

The effects of contaminants on arctic lichens are a major concern, because lichen-dominated ecosystems occur extensively in arctic regions. Lacking roots or other absorptive organs, lichens are dependent primarily on atmospheric sources of nutrients and therefore readily accumulate atmospheric contaminants. Because they lack stomatal control of gas exchange, deposition of gaseous pollutants to lichens occurs across their entire surface, and this may exceed deposition occurring to vascular plants. Pollutants such as SO2 can vastly reduce species richness and abundance of lichens and cause shifts from predominantly sexual to asexual reproduction. In surviving species, modifications of external morphology reflecting ultrastructural changes may occur. Experimental studies with arctic and other lichen species have documented a wide range of responses to various contaminants, including K+ leakage due to alteration of membrane permeability, changes in chlorophyll fluorescence characteristics, reduced nitrogen fixation, and changes in enzyme activity.