Crustose lichen
s form a crust that strongly adheres to the substrate, making separation from the substrate impossible without destruction. The basic structure of crustose lichens consists of a cortex layer, an algal layer, and a medulla. The upper cortex layer is differentiated and is usually pigmented. The algal layer lies beneath the cortex. The medulla fastens the lichen to the substrate and is made up of fungal hyphae. The surface of crustose lichens is characterized by branching cracks that periodically close in response to climatic variations such as alternate wetting and drying regimes.
Subtypes
- Powdery – considered as the simplest subtype due to the absence of an organized thallus.
- Endolithic – grows inside the rock, usually in interstitial spaces between mineral grains. The
- Epilithic – grows on top of the rock without penetrating the rock substrate.
- Epiphloeodal – grows only on the surface of plants.
- Endophloeodic – grows underneath the cuticle of leaves or stems.
- Squamulose – has a scale-like appearance resulting from partial separation from substrate.
- Peltate – similar to squamulose, but thallus remains attached near the central area.
- Bullate – has an extremely inflated appearance.
- Effigurate - has radially arranged marginal lobes that are prolonged.
- Lobate – characterized by a thallus that radially arranged with lobes that are partially raised.
- Suffruticose – clusters of coralloid cushions.
Structure
Growth
In general, lichens do not grow very quickly. Annual growth rates vary among different growth forms. Crustose lichens have the lowest rates of growth. The diameter and area of the thallus exponentially increase, provided that organic substances are distributed uniformly in all parts of the lichens. However, as the thallus increases in size, the circumference also increases, which corresponds to a larger increase in volume. As a result, movement and uniform distribution of organic substances become more difficult. The growth of crustose lichens is dependent on several factors, including moisture levels, sunlight, and temperature. High rates of precipitation and high moisture levels promote the growth of crustose lichens. Crustose lichens are more prevalent in areas with higher precipitation. A similar trend is observed when aridity is taken into account. Crustose lichens prefer sites of lower aridity. The amount of sunlight that lichens receive determines the rate at which photosynthesis occurs. Moreover, surface area also influences photosynthetic rates. In high sunlight conditions, foliose lichens with broad lobes are prevalent. In comparison, crustose lichens have less surface area than foliose lichens and will tend to have slower photosynthetic rates. Generally, higher levels of sunlight promote growth of the crustose lichens. Extreme temperatures are unfavorable for the growth of crustose lichens. Temperatures below 0 °C can result in cessation of growth and thalli freezing. Annual growth rates for the Rhizocarpon subgenus show a correlation with annual and winter mean temperatures, but not with mean summer temperatures. Unfortunately, little faith can be put in these correlations because they use unvalidated measures of unknown accuracy and precision and measurement of growth was done along a single diameter. Since thallus growth along any radius might not match growth along any other radius it is unclear if these correlations are meaningful. Various publications can be consulted to see that there is tremendous within thallus variation in lateral growth Those valid criticisms of lichenometric dating have yet to be answered.Photosynthesis
Photosynthetic rates vary among lichen growth forms due to differences and variations in thalli thicknesses. Irregular thicknesses in crustose lichens result in greater variation in photosynthetic rates relative to more uniformly thick forms such as foliose lichens.Distribution and habitat
Crustose lichens can be found in a wide range of areas. They can be found, among others, together with epiphytic algae and liverworts, living on the surfaces of leaves of tropical evergreen trees and shrubs. They also thrive in carbonate-rich karst areas. In southern China, it has been estimated that 5-30% of rock outcrops in bare karst areas and 30-70% in forest karst areas are covered with crustose lichens. Crustose lichens also flourish in extreme environments. Various species of crustose lichens, including Biatora granulosa and Lecidea uliginosa, were found covering recently-burned surfaces caused by a subarctic forest fire in an area near the Great Slave Lake. Crustose lichens also grow in areas of high elevations, such as the western Himalayan region. Concentrations of terricolous crustose lichens were highest in areas of higher elevation, relative to other foliose and fruticose lichens. In areas of high pollution, the majority of lichens are killed and are the first plants to disappear in cities due to their high sensitivity to atmospheric pollutants. Nonetheless, surrounding the central area of cities in which most plants cannot thrive, crustose lichens Physcia or Xanthoria have been found growing, although they do fall short of natural development and size. The crustose lichen Lecanora conizaeoides is another highly resilient species, and remarkably seems to only grow in industrial areas of the United Kingdom.Ecological, economic, environmental, and geological significance
Approximately 200 different compounds, which have not been found in any other symbiotic species, have been extracted from lichens in the past 50 years. Lichenin is one of the more important compounds which can be extracted from lichens. Lichenin is composed of multiple similar carbohydrates, and is the reserve cellulose of lichens. Lichenin is identical chemically to amylose from starch grains, although it differs physically. The properties of lichenin support the use of lichens as raw materials in pastries, confectionaries, food, and the production of alcohol. Lichen is also used as a food for certain invertebrates such as mites, caterpillars, earwigs, black termites, and slugs and snails. The snails Chondria avenacea and Pyramidula rupestris feed on crustose forms such as Verrucaria and Protoblastenia.Saxicolous crustose lichens play an important role in the weathering of rocks. Repeated contraction and expansion of thalli occurs in response to alternate periods of wetting and drying, resulting in the breakdown of rock fragments and removal of mineral grains from the rock surfaces. Crustose lichens also chemically weather rocks through hydrolysis. In a study conducted by Kitagawa and Watanabe, the crustose genus Porpidia altered minerals, specifically biotite in granite. Furthermore, vermiculite-like minerals were formed as a result of biotite alteration through hydrolysis. Crustose lichens living in karst areas have substantial influence on carbon dioxide flux at the boundary between the lithosphere and atmosphere because they increase the rates of corrosion of carbonate rocks in these areas. Some species of crustose lichens exhibit antibiotic properties. Lepraria chlorina contains substantial amounts of vulpinic acid, which is a chemical that has anti-inflammatory properties. Crustose lichens may also be used for dating rock surfaces, through a technique called lichenometry. As soon as a rock is exposed to the Earth’s atmosphere, spores of various organisms are driven into the crevices on the surface. The majority of these spores die under the extreme conditions of a rock surface, an area where water evaporates rapidly and daily fluxes in temperatures are quite large. The spores of some crustose lichens, however, can develop on these surfaces. Eventually the crustose spores form small and round thalli and increase in diameter yearly. When lichens are used for dating a rock surface, only the diameters of the largest thalli of one species are measured, as there is an assumption that only they began development when the surface was initially exposed. The age of exposure of a rock surface is then extrapolated from records.