A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi in a symbiotic relationship. The combined lichen has properties different from those of its component organisms. Lichens come in many colors, sizes, and forms. The properties are sometimes plant-like, but lichens are not plants.
Lichens may have tiny, leafless branches (fruticose), flat leaf-like structures (foliose), flakes that lie on the surface like peeling paint (crustose), or other growth forms. Lichens do not have roots that absorb water and nutrients as plants do, but like plants, they produce their own food by photosynthesis. When they grow on plants, they do not live as parasites, but instead use the plants as a substrate.
Pronunciation and etymology
In American English, "lichen" is pronounced the same as the verb "liken" (/ˈlaɪkən/). In British English, both this pronunciation and one rhyming with "kitchen" (/ˈlɪtʃən/) are used.
English lichen derives from Greek λειχήν leichēn ("tree moss, lichen, lichen-like eruption on skin") via Latin lichen. The Greek noun, which literally means "licker", derives from the verb λείχειν leichein, "to lick".
Macro vs micro
A macrolichen is a lichen that is either bush-like or leafy; all other lichens are termed microlichens. Here, "macro" and "micro" do not refer to size, but to the growth form. Common names for lichens may contain the word moss (e.g., "reindeer moss", "Iceland moss"), and lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant.
Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. Lichens are abundant growing on bark, leaves, mosses, on other lichens, and hanging from branches "living on thin air" (epiphytes) in rain forests and in temperate woodland. They grow on rock, walls, gravestones, roofs, exposed soil surfaces, and in the soil as part of a biological soil crust. Different kinds of lichens have adapted to survive in some of the most extreme environments on Earth: Antarctica, hot dry deserts, rocky coasts, and toxic slag heaps. They can even live inside solid rock, growing between the grains.
It is estimated that 6% of Earth's land surface is covered by lichen. There are about 20,000 known species of lichens. Some lichens have lost the ability to reproduce sexually, yet continue to speciate. Lichens can be seen as being relatively self-contained miniature ecosystems, where the fungi, algae, or cyanobacteria have the potential to engage with other microorganisms in a functioning system that may evolve as an even more complex composite organism.
Lichens may be long-lived, with some considered to be among the oldest living things. They are among the first living things to grow on fresh rock exposed after an event such as a landslide.
Lichens in Antarctica
Three main types of lichens exist in Antarctica:
- Crustose lichens, which form a thin crust on the surface of the substrate they grow on.
- Foliose lichens, which form leaf like lobes.
- Fruticose lichens which have a shrubby growth habit.
Lichens have very slow rates of growth. In the most favorable of conditions in the northernmost areas of coastal Antarctica, growth rates reach 1 cm or more per 100 years. In the harsher environment of mainland Westarctica, growth is much slower, and may be as little as 1 cm per 1000 years for Buellia frigida.
Lichens can be found growing in most areas of the Antarctic capable of supporting plant life. Currently four general distributional patterns of lichens are known. These are: species confined to the Maritime Antarctic; those found in the Peninsula and extending to the Lesser Antarctic; those with a Circum Antarctic distribution; and those with very disrupted or Disjunct distribution patterns.
The Maritime Antarctic lichens are restricted to the northern Antarctic Peninsula and nearby islands. Many of the lichens found in Antarctica are restricted to this area. A number of the lichen species found here are also found in the subantarctic islands and the colder parts of the southern continents, and may represent a southern extension of these populations. This area has the greatest species diversity in Antarctica.
Lichens have been collected from as far south as 86°30'.
Adaptations to Antarctica
Lichens have a number of adaptations that enable them to survive in Antarctica. They are able to exhibit net photosynthesis while frozen at temperatures as low as -20°C. They can absorb water from a saturated atmosphere when covered by snow. Additionally, snow cover affords protection from the elements and most growth appears to occur when they are buried beneath at least a thin protective layer of snow.
Effect of climate change on Antarctic lichen
According to a 2017 study published by Swedish University of Agricultural Sciences, short periods of increased temperature can push Antarctic lichens to their physiological limits, potentially threatening their survival. The observed effect is significant enough to lead the study group to conclude that a continued increase in temperature will lead to a severe species loss among Antarctic vegetation.
Lichen species native to southern latitudes appeared unable to adapt to rising temperature and were more likely to die out as opposed to lichen species which were native to northern latitudes, but had colonized Antarctic habitats. Those lichen were more readily adaptable to rising temperature.
The most common historical human use for lichens is for dye. However they have been utilized in other ways, especially as a food source.
As a food
There are records of lichens being used as food by many different human cultures across the world. Lichens are eaten by people in North America, Europe, Asia, and Africa, and perhaps elsewhere. Often lichens are merely famine foods eaten in times of dire needs, but in some cultures lichens are a staple food or even a delicacy. Two problems often encountered with eating lichens is that they usually contain mildly toxic secondary compounds, and that lichen polysaccharides are generally indigestible to humans. Many human cultures have discovered preparation techniques to overcome these problems. Lichens are often thoroughly washed, boiled, or soaked in ash water to help remove secondary compounds.
In the past Iceland moss (Cetraria islandica) was an important human food in northern Europe and Scandinavia, and was cooked in many different ways, such as bread, porridge, pudding, soup, or salad. Wila (Bryoria fremontii) was an important food in parts of North America, where it was usually pitcooked.
Reindeer lichen (Cladina spp.) is a staple food of reindeer and caribou in the Arctic. Northern peoples in North America and Siberia traditionally eat the partially digested lichen after they remove it from the rumen of caribou that have been killed. It is often called 'stomach icecream'. Rock tripe (Umbilicaria spp. and Lasalia spp.) is a lichen that has frequently been used as an emergency food in North America. One species of Umbilicaria, Iwa-take (U. esculenta), is used in a variety of traditional Korean and Japanese foods. It is quite expensive, and is collected off the sides of cliffs. In India, and other centers of curry powder production, garam masala sauce contains certain lichens used as bulking agents.
Very few lichens are poisonous. Poisonous lichens include those high in vulpinic acid or usnic acid. Most (but not all) lichens that contain vulpinic acid are yellow, so any yellow lichen should be considered to be potentially poisonous.
As an alcoholic drink
As early as the mid-1700s, monks in the Ussolka Monastery in Siberia used lichens in place of hops to brew beer, and in the early 19th century, a Frenchman developed a process to turn lichens into alcohol.
In 1867, after an abnormally cold, wet summer in Sweden, harvests failed and no potatoes or grain were available for making vodka. Sten Stenberg, a professor of in chemistry from Stockholm, discovered a way to brew vodka from lichens. He boiled clean, dry lichens with sulfuric acid or nitric acid for 4-5 hours, which turned most of the lichen bulk into glucose. He then neutralized the acid broth with chalk, added a large helping of baker's yeast, distilled the mix, and bottled the brew.
Stenberg established a large distillery, and by 1871, more than 250,000 pounds of reindeer lichens had been brewed into 1,500 gallons of spirits. The industry was short lived, however. By 1884, Swedish distillers had exhausted the lichen supply and once grain harvests returned to their normal levels, consumers lost interest.