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85-465: Umbilicaria esculenta , the rock tripe or Iwa-take , is a lichen of the genus Umbilicaria that grows on rocks. Two different types of polysaccharides are known to be the structural components, both a heteroglycan from the fungus and a glucan from the alga. U. esculenta lichens have a thallus attached to the substrate with a central holdfast. The thallus is also heteromerous (parts that are different in quality and number) and fully corticated (has

170-436: A holobiont . Many lichens are very sensitive to environmental disturbances and can be used to cheaply assess air pollution , ozone depletion, and metal contamination. Lichens have been used in making dyes , perfumes ( oakmoss ), and in traditional medicines . A few lichen species are eaten by insects or larger animals, such as reindeer. Lichens are widely used as environmental indicators or bio-indicators. When air

255-621: A keystone species in many ecosystems and benefit trees and birds . The English word lichen derives from the 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". In American English, "lichen" is pronounced the same as the verb "liken" ( / ˈ l aɪ k ən / ). In British English, both this pronunciation and one rhyming with "kitchen" ( / ˈ l ɪ tʃ ən / ) are used. Lichens grow in

340-674: A cortex and bark). Many of the Umbilicaria species are characterized by a veined or rugose thalline surface. Umbilicaria esculenta is a saxicolous lichen ; it grows on rocks and is known widely as the "rock tripe". It is usually found on at high altitude in East Asia . U. esculenta is considered a delicacy in China, Korea, and Japan where it is eaten in dishes, soups, or in salads. It has been used as starvation food by Native Americans and early settlers. This Lecanoromycetes -related article

425-406: A crustose lichen gets old, the center may start to crack up like old-dried paint, old-broken asphalt paving, or like the polygonal "islands" of cracked-up mud in a dried lakebed. This is called being rimose or areolate , and the "island" pieces separated by the cracks are called areolas. The areolas appear separated, but are (or were) connected by an underlying prothallus or hypothallus . When

510-413: A crustose lichen grows from a center and appears to radiate out, it is called crustose placodioid. When the edges of the areolas lift up from the substrate, it is called squamulose . These growth form groups are not precisely defined. Foliose lichens may sometimes branch and appear to be fruticose. Fruticose lichens may have flattened branching parts and appear leafy. Squamulose lichens may appear where

595-437: A few basic internal structure types. Common names for lichens often come from a growth form or color that is typical of a lichen genus . Common groupings of lichen thallus growth forms are: There are variations in growth types in a single lichen species, grey areas between the growth type descriptions, and overlapping between growth types, so some authors might describe lichens using different growth type descriptions. When

680-432: A few millimeters in thickness to pinnacles up to 15 cm high. Smooth biological soil crusts occur in hot deserts where the soil does not freeze, and consist mostly of cyanobacteria, algae, and fungi. Thicker and rougher crusts occur in areas where higher precipitation results in increased cover of lichen and mosses, and frost heaving of these surfaces cause microtopography such as rolling hills and steep pinnacles. Due to

765-407: A foliose lichen may branch, giving the appearance of a fruticose lichen, but the underside will be a different color from the top side. The sheen on some jelly-like gelatinous lichens is created by mucilaginous secretions. A lichen consists of a simple photosynthesizing organism, usually a green alga or cyanobacterium , surrounded by filaments of a fungus. Generally, most of a lichen's bulk

850-756: A gelatinous sheath of polysaccharides . These filaments bind soil particles throughout the uppermost soil layers, forming a 3-D net-like structure that holds the soil together in a crust. Other common cyanobacteria species are as those in the genus Nostoc , which can also form sheaths and sheets of filaments that stabilize the soil. Some Nostoc species are also able to fix atmospheric nitrogen gas into bio-available forms such as ammonia . Bryophytes in soil crusts include mosses and liverworts . Mosses are usually classified as short annual mosses or tall perennial mosses. Liverworts can be flat and ribbon-like or leafy. They can reproduce by spore formation or by asexual fragmentation , and photosynthesize to fix carbon from

935-488: A green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were found growing in a conjoined manner. Evidence that lichens are examples of successful symbiosis is the fact that lichens can be found in almost every habitat and geographic area on the planet. Two species in two genera of green algae are found in over 35% of all lichens, but can only rarely be found living on their own outside of

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1020-448: A host. Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen. Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves. It is thought that lichens may be even more complex symbiotic systems that include non-photosynthetic bacterial communities performing other functions as partners in

1105-459: A lichen can survive outside the lichen, the lichen symbiotic association extends the ecological range of both partners, whereby most descriptions of lichen associations describe them as symbiotic. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from

1190-519: A lichen. In a case where one fungal partner simultaneously had two green algae partners that outperform each other in different climates, this might indicate having more than one photosynthetic partner at the same time might enable the lichen to exist in a wider range of habitats and geographic locations. At least one form of lichen, the North American beard-like lichens, are constituted of not two but three symbiotic partners: an ascomycetous fungus,

1275-462: A millimeter). The cortex may be further topped by an epicortex of secretions, not cells, 0.6–1 μm thick in some lichens . This secretion layer may or may not have pores. Below the cortex layer is a layer called the photobiontic layer or symbiont layer . The symbiont layer has less densely packed fungal filaments, with the photosynthetic partner embedded in them. The less dense packing allows air circulation during photosynthesis, similar to

1360-440: A photosynthetic alga, and, unexpectedly, a basidiomycetous yeast. Phycobionts can have a net output of sugars with only water vapor. The thallus must be saturated with liquid water for cyanobionts to photosynthesize. Algae produce sugars that are absorbed by the fungus by diffusion into special fungal hyphae called appressoria or haustoria in contact with the wall of the algal cells. The appressoria or haustoria may produce

1445-428: A protective "skin" of densely packed fungal filaments, often containing a second fungal species, which is called a cortex. Fruticose lichens have one cortex layer wrapping around the "branches". Foliose lichens have an upper cortex on the top side of the "leaf", and a separate lower cortex on the bottom side. Crustose and squamulose lichens have only an upper cortex, with the "inside" of the lichen in direct contact with

1530-403: A single cortex wrapping all the way around the "stems" and "branches". The medulla is the lowest layer, and may form a cottony white inner core for the branchlike thallus, or it may be hollow. Crustose and squamulose lichens lack a lower cortex, and the medulla is in direct contact with the substrate that the lichen grows on. In crustose areolate lichens, the edges of the areolas peel up from

1615-476: A substance that increases permeability of the algal cell walls, and may penetrate the walls. The algae may contribute up to 80% of their sugar production to the fungus. Lichen associations may be examples of mutualism or commensalism , but the lichen relationship can be considered parasitic under circumstances where the photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Photobiont cells are routinely destroyed in

1700-764: A surface ( substrate ) like a thick coat of paint ( crustose ); have a powder-like appearance ( leprose ); or other growth forms . 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 they are not closely related to mosses or any plant. Lichens do not have roots that absorb water and nutrients as plants do, but like plants, they produce their own nutrition by photosynthesis . When they grow on plants, they do not live as parasites , but instead use

1785-422: A wide range of shapes and forms; this external appearance is known as their morphology . The shape of a lichen is usually determined by the organization of the fungal filaments. The nonreproductive tissues, or vegetative body parts, are called the thallus . Lichens are grouped by thallus type, since the thallus is usually the most visually prominent part of the lichen. Thallus growth forms typically correspond to

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1870-529: Is poikilohydric and does not have the ability to maintain or regulate its own water retention. This causes the biocrust's water content to change depending on the water in the surrounding environment. Due to biological soil crust existing in mostly arid and semi-arid environments with the inability to hold water, the crust is mainly dormant except for short periods of activity when the crust receives precipitation. Microorganisms like those that make up biological soil crust are good at responding quickly to changes in

1955-420: Is a stub . You can help Misplaced Pages by expanding it . Lichen A lichen ( / ˈ l aɪ k ən / LY -kən , UK also / ˈ l ɪ tʃ ən / LITCH -ən ) is a hybrid colony of algae or cyanobacteria living symbiotically among filaments of multiple fungi species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualistic relationship. Lichens are

2040-442: Is a whitish coating on top of an upper surface. An epinecral layer is "a layer of horny dead fungal hyphae with indistinct lumina in or near the cortex above the algal layer". In August 2016, it was reported that some macrolichens have more than one species of fungus in their tissues. Lichens are fungi that have discovered agriculture A lichen is a composite organism that emerges from algae or cyanobacteria living among

2125-546: Is called the thallus . The thallus form is very different from any form where the fungus or alga are growing separately. The thallus is made up of filaments of the fungus called hyphae . The filaments grow by branching then rejoining to create a mesh, which is called being " anastomosed ". The mesh of fungal filaments may be dense or loose. Generally, the fungal mesh surrounds the algal or cyanobacterial cells, often enclosing them within complex fungal tissues that are unique to lichen associations. The thallus may or may not have

2210-411: Is greatly dependent on which microorganisms are most dominant in the specific forms of biocrust. Most research studies like that done by Canton et al. support that biological soil crust composed of large amounts of moss and lichens are better able to absorb water resulting in good soil infiltration. In comparison, biocrusts that aredominated by cyanobacteria is more likely to cause biological clogging where

2295-400: Is made of interwoven fungal filaments, but this is reversed in filamentous and gelatinous lichens. The fungus is called a mycobiont . The photosynthesizing organism is called a photobiont . Algal photobionts are called phycobionts . Cyanobacteria photobionts are called cyanobionts . The part of a lichen that is not involved in reproduction, the "body" or "vegetative tissue" of a lichen,

2380-452: Is one of the most dominant organisms found in biocrust and is fundamental to the crust's ability to reawaken from dormancy when rehydrated due to precipitation or runoff. Cyanobacteria have been found to outcompete the other components of biocrust when exposed to light and precipitation. Cyanobacteria are primarily responsible for the pigment and rejuvenation of the crust during environmental changes that result in short spurts of rehydration for

2465-452: Is usually determined by the photosynthetic component. Special pigments, such as yellow usnic acid , give lichens a variety of colors, including reds, oranges, yellows, and browns, especially in exposed, dry habitats. In the absence of special pigments, lichens are usually bright green to olive gray when wet, gray or grayish-green to brown when dry. This is because moisture causes the surface skin ( cortex ) to become more transparent, exposing

2550-403: Is very badly polluted with sulphur dioxide, there may be no lichens present; only some green algae can tolerate those conditions. If the air is clean, then shrubby, hairy and leafy lichens become abundant. A few lichen species can tolerate fairly high levels of pollution, and are commonly found in urban areas, on pavements, walls and tree bark. The most sensitive lichens are shrubby and leafy, while

2635-436: The apoplast . Secondary metabolites are thought to play a role in preference for some substrates over others. Lichens often have a regular but very slow growth rate of less than a millimeter per year. In crustose lichens, the area along the margin is where the most active growth is taking place. Most crustose lichens grow only 1–2 mm in diameter per year. Lichens may be long-lived , with some considered to be among

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2720-1288: The nitrogen cycle varies by crust composition because only cyanobacteria and cyanolichens fix nitrogen. Nitrogen fixation requires energy from photosynthesis products, and thus increase with temperature given sufficient moisture. Nitrogen fixed by crusts has been shown to leak into surrounding substrate and can be taken up by plants, bacteria, and fungi. Nitrogen fixation has been recorded at rates of 0.7–100 kg/ha per year, from hot deserts in Australia to cold deserts. Estimates of total biological nitrogen fixation are ~ 49 Tg/year (27–99 Tg/year). Soils in arid regions are slow-forming and easily eroded. Crust organisms contribute to increased soil stability where they occur. Cyanobacteria have filamentous growth forms that bind soil particles together, and hyphae of fungi and rhizines / rhizoids of lichens and mosses also have similar effects. The increased surface roughness of crusted areas compared to bare soil further improves resistance to wind and water erosion . Aggregates of soil formed by crust organisms also increase soil aeration and provide surfaces where nutrient transformation can occur. The effect of biological soil crusts on water infiltration and soil moisture depends on

2805-416: The substrate . If a cyanobacterium is present, as a primary partner or another symbiont in addition to a green alga as in certain tripartite lichens, they can fix atmospheric nitrogen , complementing the activities of the green alga. In three different lineages the fungal partner has independently lost the mitochondrial gene atp9, which has key functions in mitochondrial energy production. The loss makes

2890-661: The amount of light reflected off of the surface) compared to nearby soils, which increases the energy absorbed by the soil surface. Soils with well-developed biological soil crusts can be over 12 °C (22 °F) warmer than adjacent surfaces. Increased soil temperatures are associated with increased metabolic processes such as photosynthesis and nitrogen fixation, as well as higher soil water evaporation rates and delayed seedling germination and establishment. The activity levels of many arthropods and small mammals are also controlled by soil surface temperature. The increased surface roughness associated with biological soil crusts increase

2975-420: The anatomy of a leaf. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by a haustorium . In crustose and foliose lichens, algae in the photobiontic layer are diffuse among the fungal filaments, decreasing in gradation into the layer below. In fruticose lichens, the photobiontic layer is sharply distinct from the layer below. The layer beneath

3060-969: The atmosphere. Biological soil crusts are formed in open spaces between vascular plants . Frequently, single-celled organisms such as cyanobacteria or spores of free-living fungi colonize bare ground first. Once filaments have stabilized the soil, lichens and mosses can colonize. Appressed lichens are generally earlier colonizers or persist in more stressful conditions, while more three-dimensional lichens require long disturbance-free growth periods and more moderate conditions. Recovery following disturbance varies. Cyanobacteria cover can recover by propagules blowing in from adjacent undisturbed areas rapidly after disturbance. Total recovery of cover and composition occurs more rapidly in fine soil textured, moister environments (~2 years) and more slowly (>3800 years) in coarse soil textured, dry environments. Recovery times also depend on disturbance regime, site, and availability of propagules. Biological soil crusts cover about 12% of

3145-859: The atmosphere. Lichens are often distinguished by growth form and by their photosymbiont . Crust lichens include crustose and areolate lichens that are appressed to the soil substrate , squamulose lichens with scale- or plate-like bodies that are raised above the soils, and foliose lichens with more "leafy" structures that can be attached to the soil at only one portion. Lichens with algal symbionts can fix atmospheric carbon, while lichens with cyanobacterial symbionts can fix nitrogen as well. Lichens produce many pigments that help protect them from radiation. Microfungi in biological soil crusts can occur as free-living species, or in symbiosis with algae in lichens. Free-living microfungi often function as decomposers, and contribute to soil microbial biomass. Many microfungi in biological soil crusts have adapted to

3230-489: The biocrust. A filamentous cyanobacterium called Microcoleus vaginatus was found to exist in a dormant, metabolically inactive state beneath the surface of the crust in periods of drought or water deficiency. When the biocrust eventually receives precipitation, it is able to perform hydrotaxis and appears to resurrect. In this stage, the M. vaginatus migrates upward to the surface of the crust when hydrated, to perform oxygenic photosynthesis. In this photosynthetic process,

3315-401: The capture of dust . These Aeolian deposits of dust are often enriched in plant-essential nutrients , and thus increase both the fertility and the water holding capacity of soils. The biological soil crust is an integral part of many arid and semi-arid ecosystems as an essential contributor to conditions such as dust control, water acquisition, and contributors of soil nutrients. Biocrust

3400-464: The cells in the short periods of time when the crust is hydrated and awakened from dormancy. Cyanobacteria are able to repeat this process over and over again in the event of rehydration in the future. The amount of time it takes for the greening process in biocrust to occur varies on the environmental conditions in which the biocrust lives. Biocrust can take anywhere from five minutes to 24 hours to awaken from dormancy. The crusts will only awaken if

3485-632: The central Brooks Range of northern Alaska have been given a maximum possible age of 10,000–11,500 years. Unlike simple dehydration in plants and animals, lichens may experience a complete loss of body water in dry periods. Lichens are capable of surviving extremely low levels of water content ( poikilohydric ). They quickly absorb water when it becomes available again, becoming soft and fleshy. Biological soil crust Biological soil crusts are most often composed of fungi , lichens , cyanobacteria , bryophytes , and algae in varying proportions. These organisms live in intimate association in

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3570-445: The characteristic cortex of the lichen thallus, and could also be important for its shape. The lichen combination of alga or cyanobacterium with a fungus has a very different form (morphology), physiology, and biochemistry than the component fungus, alga, or cyanobacterium growing by itself, naturally or in culture. The body ( thallus ) of most lichens is different from those of either the fungus or alga growing separately. When grown in

3655-543: The climate, soil, and disturbance conditions. For example, biological soil crusts are more dominated by green algae on more acidic and less salty soils, whereas cyanobacteria are more favored on alkaline and haline soils. Within a climate zone , the abundance of lichens and mosses in biological soil crusts generally increases with increasing clay and silt content and decreasing sand. Also, habitats that are more moist generally support more lichens and mosses. The morphology of biological soil crust surfaces can range from smooth and

3740-466: The conditions are conducive to the biocrust. Biocrust influences a soil's microtopography, carbohydrate content, porosity, and hydrophobicity which are the major contributing factors to soil hydrology. The relationship between biocrust and soil hydrology is not fully understood by scientists. It is known that the biocrust does play a role in the absorption and retention of moisture in the soil. In arid and semi-arid environments biocrust can cover over 70% of

3825-435: The course of nutrient exchange. The association continues because reproduction of the photobiont cells matches the rate at which they are destroyed. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs ( haustoria ) similar to those produced by pathogenic fungi that feed on

3910-891: The crust microtopography. A recent study in China shows that biocrusts have been an import factor in the preservation of sections of the Great Wall built using rammed earth methods. Biological soil crusts are highly susceptible to disturbance from human activities. Compressional and shear forces can disrupt biological soil crusts, especially when they are dry, leaving them to be blown or washed away. Thus, animal hoof impact, human footsteps, off-road vehicles , and tank treads can remove crusts, and these disturbances have occurred over large areas globally. Once biological soil crusts are disrupted, wind and water can move sediments onto adjacent intact crusts, burying them and preventing photosynthesis of non-motile organisms such as mosses, lichens, green algae, and cyanobacteria, and of motile cyanobacteria when

3995-477: The crusts, increased trapment of nutrient-rich dust, as well as increased concentrations of micronutrients that are able to chelate to the negatively charged clay particles bound by cyanobacterial filaments. The increased nutrient status of plant tissue in areas where biological soil crusts occur can directly benefit herbivore species in the community. Microarthropod populations also increase with more developed crusts due to increased microhabitats produced by

4080-404: The cyanobacteria carries with it a green-blue photosynthetic pigment to the surface of the crust. When inevitably there is a period of insufficient water again, the M. vaginatus is able to return to a dormant state, migrating back down into the crust and bringing the pigment with it. This process goes along with the rapid turning on of metabolic pathways allowing metabolic functions to occur within

4165-443: The dominant crust organisms, soil characteristics, and climate. In areas where biological soil crusts produce rough surface microtopography, water is detained longer on the soil surface and this increases water infiltration. However, in warm deserts where biological soil crusts are smooth and flat, infiltration rates can be decreased by bioclogging . The darkened surfaces of biological soil crusts decreases soil albedo (a measure of

4250-493: The earth's landmass. They are found on almost all soil types, but are more commonly found in arid regions of the world where plant cover is low and plants are more widely spaced. This is because crust organisms have a limited ability to grow upwards and cannot compete for light with vascular plants. Across the globe, biological soil crusts can be found on all continents including Antarctica. The species composition and physical appearance of biological soil crusts vary depending on

4335-542: The easiest ways to maintain and improve biological soil crusts. Protecting relic sites that have not been disturbed can serve as reference conditions for restoration. There are several successful methods for stabilizing soil to allow recolonization of crusts, including coarse litter application (such as straw) and planting vascular plants, but these are costly and labor-intensive techniques. Spraying polyacrylamide gel has been tried, but this has adversely affected photosynthesis and nitrogen fixation of Collema species and thus

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4420-640: The edges lift up. Gelatinous lichens may appear leafy when dry. The thallus is not always the part of the lichen that is most visually noticeable. Some lichens can grow inside solid rock between the grains ( endolithic lichens ), with only the sexual fruiting part visible growing outside the rock. These may be dramatic in color or appearance. Forms of these sexual parts are not in the above growth form categories. The most visually noticeable reproductive parts are often circular, raised, plate-like or disc-like outgrowths, with crinkly edges, and are described in sections below. Lichens come in many colors. Coloration

4505-400: The environment even after a period of dormancy such as precipitation. Desiccation can lead to oxidation and the destruction of nutrients, amino acids, and cell membranes in the microorganisms that make up biological soil crust. However, the biological soil crust has adapted to survive in very harsh environments with the aid of cyanobacteria . Cyanobacteria have evolved the ability to navigate

4590-474: The extreme conditions of their surrounding environment by existing in a biocrust. A trait of the biological soil crust community is that it will activate from a dormant state when it is exposed to precipitation transforming from a dry, dead-looking crust to an actively photosynthetic community. It will change its appearance to be vibrant and alive to the naked eye. Many crusts will even turn different shades of dark green. The cyanobacterium Microcoleus vaginatus

4675-441: The filaments ( hyphae ) of the fungi in a mutually beneficial symbiotic relationship. The fungi benefit from the carbohydrates produced by the algae or cyanobacteria via photosynthesis . The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungi, which also gather moisture and nutrients from the environment, and (usually) provide an anchor to it. Although some photosynthetic partners in

4760-421: The fungi completely dependent on their symbionts. The algal or cyanobacterial cells are photosynthetic and, as in plants, they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Phycobionts (algae) produce sugar alcohols ( ribitol , sorbitol , and erythritol ), which are absorbed by the mycobiont (fungus). Cyanobionts produce glucose . Lichenized fungal cells can make

4845-501: 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. The long life-span and slow and regular growth rate of some species can be used to date events ( lichenometry ). Lichens are

4930-460: The fungus living inside the lichen; thus they are not considered to be part of the lichen. Moisture makes the cortex become more transparent. This way, the algae can conduct photosynthesis when moisture is available, and is protected at other times. When the cortex is more transparent, the algae show more clearly and the lichen looks greener. Lichens can show intense antioxidant activity. Secondary metabolites are often deposited as crystals in

5015-472: The green photobiont layer. Different colored lichens covering large areas of exposed rock surfaces, or lichens covering or hanging from bark can be a spectacular display when the patches of diverse colors "come to life" or "glow" in brilliant displays following rain. Different colored lichens may inhabit different adjacent sections of a rock face, depending on the angle of exposure to light. Colonies of lichens may be spectacular in appearance, dominating much of

5100-708: The intense UV radiation present in areas where biological soil crusts occur, biological soil crusts appear darker than the crustless soil in the same area due to the UV-protective pigmentation of cyanobacteria and other crust organisms. Biological soil crusts contribute to the carbon cycle through respiration and photosynthesis of crust microorganisms which are active only when wet. Respiration can begin in as little as 3 minutes after wetting whereas photosynthesis reaches full activity after 30 minutes. Some groups have different responses to high water content, with some lichens showing decreased photosynthesis when water content

5185-400: The intense light conditions by evolving the ability to produce melanin , and are called black fungi or black yeasts . Fungal hyphae can bind soil particles together. Green algae in soil crusts are present just below the soil surface where they are partially protected from UV radiation. They become inactive when dry and reactivate when moistened. They can photosynthesize to fix carbon from

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5270-441: The laboratory in the absence of its photobiont, a lichen fungus develops as a structureless, undifferentiated mass of fungal filaments ( hyphae ). If combined with its photobiont under appropriate conditions, its characteristic form associated with the photobiont emerges, in the process called morphogenesis . In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either

5355-409: The lichen contacts the environment, is called a cortex . The cortex is made of densely tightly woven, packed, and glued together ( agglutinated ) fungal filaments. The dense packing makes the cortex act like a protective "skin", keeping other organisms out, and reducing the intensity of sunlight on the layers below. The cortex layer can be up to several hundred micrometers (μm) in thickness (less than

5440-601: The lifeform that first brought the term symbiosis (as symbiotismus ) under biological context. Lichens have since been recognized as important actors in nutrient cycling and producers which many higher trophic feeders feed on, such as reindeer, gastropods, nematodes, mites, and springtails. Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but are not plants . They may have tiny, leafless branches ( fruticose ); flat leaf-like structures ( foliose ); grow crust-like, adhering tightly to

5525-420: The most tolerant lichens are all crusty in appearance. Since industrialisation, many of the shrubby and leafy lichens such as Ramalina , Usnea and Lobaria species have very limited ranges, often being confined to the areas which have the cleanest air. Some fungi can only be found living on lichens as obligate parasites . These are referred to as lichenicolous fungi , and are a different species from

5610-539: The oldest living organisms. Lifespan is difficult to measure because what defines the "same" individual lichen is not precise. Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the "same" lichen, and two lichens can merge, then becoming the "same" lichen. One specimen of Rhizocarpon geographicum on East Baffin Island has an estimated age of 9500 years. Thalli of Rhizocarpon geographicum and Rhizocarpon eupetraeoides / inarense in

5695-414: The photobiont "leak" out the products of photosynthesis, where they can then be absorbed by the fungus. It appears many, probably the majority, of lichen also live in a symbiotic relationship with an order of basidiomycete yeasts called Cyphobasidiales . The absence of this third partner could explain why growing lichen in the laboratory is difficult. The yeast cells are responsible for the formation of

5780-434: The photosynthetic partner tend to be dark grey, brown, or black. The underside of the leaf-like lobes of foliose lichens is a different color from the top side ( dorsiventral ), often brown or black, sometimes white. A fruticose lichen may have flattened "branches", appearing similar to a foliose lichen, but the underside of a leaf-like structure on a fruticose lichen is the same color as the top side. The leaf-like lobes of

5865-442: The plant's surface as a substrate. Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. They are abundant growing on bark, leaves , mosses, or other lichens and hanging from branches "living on thin air" ( epiphytes ) in rainforests and in temperate woodland . They grow on rock, walls, gravestones , roofs , exposed soil surfaces, rubber, bones, and in

5950-432: The pores of the soil are obstructed by the cyanobacteria responding to the presence of moisture by awakening from their dormancy and swelling. The darkening of the soil surface by biocrust can also raise the soil temperature leading to faster water evaporation. There is limited research, however, that indicates that the rough surface of cyanobacteria traps water runoff and lichen in cyanobacteria-dominant biocrust increase

6035-408: The porosity of the soil allowing for better infiltration than soil that does not have any biocrust. The type of soil and its texture is also a major determining factor in the biological soil crust's relationship with water retention and filtration. Soils with a large presence of sand (less soil and clay) have high levels of water retention in their surface levels but have limited downward movement of

6120-423: The presence of biological soil crusts may slow the establishment of invasive plant species such as cheatgrass ( Bromus tectorum ). Biological soil crusts do not compete with vascular plants for nutrients, but rather have been shown to increase nutrient levels in plant tissues, which results in higher biomass for plants that grow near biological soil crusts. This can occur through N fixation by cyanobacteria in

6205-408: The probability that plant seeds will be caught on the soil surface and not blown away. Differences in water infiltration and soil moisture also contribute to differential germination depending on the plant species. It has been shown that while some native desert plant species have seeds with self-burial mechanisms that can establish readily in crusted areas, many exotic invasive plants do not. Therefore,

6290-506: The soil as part of biological soil crusts . Various lichens have adapted to survive in some of the most extreme environments on Earth: arctic tundra , hot dry deserts , rocky coasts , and toxic slag heaps. They can even live inside solid rock, growing between the grains ( endolithic ). There are about 20,000 known species. Some lichens have lost the ability to reproduce sexually, yet continue to speciate . They can be seen as being relatively self-contained miniature ecosystems , where

6375-400: The soil not being covered by plants, indicating that the relationship between soil, water, and biocrust is extremely pertinent to these environments. Biocrusts has been shown to increase infiltration of water and pore space (or porosity) in soil but the opposite may occur depending on the type of biocrust. The effect biocrust has on water infiltration and the amount of water retained in the soil

6460-455: The soil remains dry. This kills the remaining intact crust and causes large areas of loss. Invasive species introduced by humans can also affect biological soil crusts. Invasive annual grasses can occupy areas once occupied by crusts and allow fire to travel between large plants. In contrast, previously, it would have just jumped from plant to plant and not directly affected the crusts. Climate change affects biological soil crusts by altering

6545-412: The substrate and appear leafy. In squamulose lichens the part of the lichen thallus that is not attached to the substrate may also appear leafy. But these leafy parts lack a lower cortex, which distinguishes crustose and squamulose lichens from foliose lichens. Conversely, foliose lichens may appear flattened against the substrate like a crustose lichen, but most of the leaf-like lobes can be lifted up from

6630-438: The substrate because it is separated from it by a tightly packed lower cortex. Gelatinous, byssoid, and leprose lichens lack a cortex (are ecorticate ), and generally have only undifferentiated tissue, similar to only having a symbiont layer. In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called cephalodia . Pruinia

6715-410: The surface of the visual landscape in forests and natural places, such as the vertical "paint" covering the vast rock faces of Yosemite National Park . Color is used in identification. The color of a lichen changes depending on whether the lichen is wet or dry. Color descriptions used for identification are based on the color that shows when the lichen is dry. Dry lichens with a cyanobacterium as

6800-492: The surface they grow on (the substrate ). Even if the edges peel up from the substrate and appear flat and leaf-like, they lack a lower cortex, unlike foliose lichens. Filamentous, byssoid, leprose, gelatinous, and other lichens do not have a cortex; in other words, they are ecorticate . Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, differentiated by having different densities of fungal filaments. The top layer, where

6885-472: The symbiont layer is called the medulla . The medulla is less densely packed with fungal filaments than the layers above. In foliose lichens, as in Peltigera , there is usually another densely packed layer of fungal filaments called the lower cortex. Root-like fungal structures called rhizines ( usually ) grow from the lower cortex to attach or anchor the lichen to the substrate. Fruticose lichens have

6970-794: The timing and magnitude of precipitation events and temperature . Because crusts are only active when wet, some of these new conditions may reduce the amount of time when conditions are favorable for activity. Biological soil crusts require stored carbon when reactivating after being dry. Suppose they do not have enough moisture to photosynthesize to make up for the carbon used. In that case they can gradually deplete carbon stocks and die. Reduced carbon fixation also leads to decreased nitrogen fixation rates because crust organisms do not have sufficient energy for this energy-intensive process. Without carbon and nitrogen available, they cannot grow nor repair damaged cells from excess radiation. Removing stressors such as grazing or protecting them from disturbance are

7055-470: The uppermost few millimeters of the soil surface, and are the biological basis for the formation of soil crusts. Cyanobacteria are the main photosynthetic component of biological soil crusts, in addition to other photosynthetic taxa such as mosses, lichens, and green algae. The most common cyanobacteria found in soil crusts belong to large filamentous species such as those in the genus Microcoleus . These species form bundled filaments that are surrounded by

7140-408: The water. Soils that were less than 80% sand had greater infiltration due to biocrust creating soil aggregates. Other factors like plant roots may play a role in water retention and soil moisture at depths below the soil crust. The presence of biological soil crust cover can differentially inhibit or facilitate plant seed catchment and germination . The increased micro-topography generally increases

7225-446: Was greater than 60% whereas green algae showed little response to high water content. Photosynthesis rates are also dependent on temperature, with rates increasing up to approximately 28 °C (82 °F). Estimates for annual carbon inputs range from 0.4 to 37 g/cm*year depending on successional state. Estimates of total net carbon uptake by crusts globally are ~3.9 Pg/year (2.1–7.4 Pg/year). Biological soil crust contributions to

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