By-Roya Minnis & Makiko Takanashi
What are Lichens?
Lichens are mutualistic associations between fungi and algae. The fungus is an ascomycote or a basidiomycote while the alga is a green alga. Lichens are commonly encrusted on exposed rocks and trunks of trees; they also hang from trees in wet forests. It is believed that the alga contributes organic food from photosynthesis, while the fungus is water and mineral salts. The fungus can also conserve water enabling some lichens to grow in dry conditions where no other plants exist.
Ecological Associations
SOIL BUILDERS by their petrolytic action of rock disintegration." Lichens are the first plants" to colonize bare rocks. They cling to the bare rock living mostly an air & water and as they grow they have an etching effect on the surface. The rock dust eroded by lichens and weather gathers in crevices & ledges along with wind-borne pollen & other organic martial to form soil in which mosses, firns and flowering plants may grow. Lichens are found on seashores, mountainpeaks, but not common in cities too much pollution.
Lichens are a group of organisms which are among the most highly sensitive to air pollution." Lichens are particularly susceptible to acid rain (the absence of a cuticle is a key factor) and increase in amounts of trace and essential elements to toxic levels have been attributed to it. Lichens have been described as most successful example of mutualism." It consists of a mycobiont (fungal partner) and a photobiont (photosynthetic partner). When the accumulation of metals in the lichen reaches toxic levels, the chlorophyll in the photobiont is converted to phaeophytin, oxidation processes destroy the chlorophyll, all-wall permeability changes, the fungal layer fades away and K efflux occurs, disturbing the stable structure of the lichen thallus. When there is an acid precipitation the pH of the substrate falls. This in turn affects the availability of nutrients to the plants as the solubility of the nutrients is directly related or proportional to the availablity of the nutrient in ionic form that can then be utilized by the plant.
References:
TRAGER,W.(1970) Symbiosis Selected Topic in Biology New York Van Nostrand Reinhold Co.