Misplaced Pages

#112887

78-436: Alexandre Noël Charles Acloque (1871–1941) was a French botanist who was an expert in lichens . Acloque was broadly interested in natural history and wrote books on the flora and fauna (insects) of France. This article about a French botanist 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 )

156-436: 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

234-625: 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

312-406: 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

390-413: 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

468-437: 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

546-407: 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

624-488: 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

702-448: 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

780-459: 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

858-526: 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,

SECTION 10

#1732776049113

936-462: 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

1014-440: 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

1092-409: A pool of genes with new functions. Some key factors that can preferentially select for the host or the symbionts to adapt to stressors are: (1) the features of the stressor, such as its frequency or amplitude, but also its combination with another stressor that can lead to an additive, synergistic or antagonist interaction; (2) the transmission mode of the symbionts; (3) the specificity and

1170-428: 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

1248-415: A role. Evidence of this process have been recently proven showing that the majority, up to 95%, of the seed microbiome is mistranslated across generations. The plant holobiont is relatively well-studied, with particular focus on agricultural species such as legumes and grains . Bacteria, fungi, archaea, protists, and viruses are all members of the plant holobiont. The bacteria phyla known to be part of

1326-403: 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

1404-473: A single origin for eukaryotic cells through the symbiotic assimilation of prokaryotes to form first mitochondria and later plastids (the latter through several independent symbiotic events) via phagocytosis (reviewed in Archibald, 2015). These ancestral and founding symbiotic events, which prompted the metabolic and cellular complexity of eukaryotic life, most likely occurred in the ocean. Despite

1482-476: 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

1560-764: 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

1638-431: A unit of selection sensu Margulis for corals, where the holobiont comprised the cnidarian polyp (host), Zooxanthellae algae, various ectosymbionts ( endolithic algae, prokaryotes, fungi, other unicellular eukaryotes), and viruses. Although initially driven by studies of marine organisms, much of the research on the emerging properties and significance of holobionts has since been carried out in other fields of research:

SECTION 20

#1732776049113

1716-422: 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

1794-557: Is a collection of closely associated species that have complex interactions, such as a plant species and the members of its microbiome . Each species present in a holobiont is a biont, and the genomes of all bionts taken together are the hologenome, or the "comprehensive gene system" of the holobiont. A holobiont typically includes a eukaryote host and all of the symbiotic viruses , bacteria , fungi , etc. that live on or inside it. Holobionts are distinct from superorganisms ; superorganisms consist of many individuals, sometimes of

1872-841: 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 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

1950-399: Is a substantial body of literature on plant holobionts. Plant-associated microbial communities impact both key components of the fitness of plants, growth and survival, and are shaped by nutrient availability and plant defense mechanisms. Several habitats have been described to harbor plant-associated microbes, including the rhizoplane (surface of root tissue), the rhizosphere (periphery of

2028-442: 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

2106-546: 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

2184-456: Is higher in aquatic realms compared to land, with much of the aquatic diversity yet to be uncovered, especially marine viruses. Host: The host member of a holobiont is typically a multicellular eukaryote , such as a plant or human. Notable hosts that are well-studied include humans, corals, and poplar trees. Microbiome: The microbiome includes bacteria, archaea , microscopic fungi, and microscopic protists .   Virome: All of

2262-400: 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,

2340-412: Is unhelpful to the study of host interactions with resident microorganisms because it focuses on one level of selection (the holobiont), and as a result it is concerned with cooperative and integrative features of host-microbe systems to the exclusion of other kinds of interactions, including antagonism among microorganisms and conflicts between host and microbial partners." The holobiont and by extension

2418-452: 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

Alexandre Acloque - Misplaced Pages Continue

2496-403: 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

2574-436: 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

2652-415: The meadow fescue holobiont and provide herbivore resistance by producing ergot alkaloids , which cause ergotism in mammals. Protist members of the plant holobiont are less well-studied, with most knowledge oriented towards pathogens. However, there are examples of commensalistic plant-protist associations, such as Phytomonas ( Trypanosomatidae ). Reef-building corals are holobionts that include

2730-416: 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

2808-420: 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

2886-645: The behavior of a system that is "larger than the sum of its parts". However, a major shift away from holism occurred during the Age of Enlightenment when the dominant thought summarized as "dissection science" was to focus on the smallest component of a system as a means of understanding it. The idea of holism started to regain popularity in biology when the endosymbiosis theory was first proposed by Konstantin Mereschkowski in 1905 and further developed by Ivan Wallin in 1925. Still accepted today, this theory posits

2964-446: 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. Holobiont A holobiont

3042-632: The characterisation and quantification of the entirety of similar individual elements in order to draw conclusions about the structure, function, and dynamics of a system. To infer the properties and interactions of the symbiotic partners, techniques in molecular biology, ecology, and modelling are combined. Recent years have seen the development of powerful but relatively inexpensive tools for characterising microbial communities, including high throughput sequencing technologies such as whole genome shotgun sequencing . These technological advances have led to an explosion of interest in microbial ecology and in

3120-445: 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

3198-402: The concept of holism . Holobiomics aims to study the holobionts of a system, their properties, and their interactions in their entirety. The term "holobiomics" is composed of Greek elements όλος ( hólos ), "all, whole, total", and βίος ( bíos ), "life", ending on -ome ( biome ); and the suffix - omics (-ομική, feminine), which identifies subfields of modern biology that aim at

Alexandre Acloque - Misplaced Pages Continue

3276-428: The concept was not widely used until it was co-opted by coral biologists over a decade later. Corals and the dinoflagellate algae called Zooxanthellae are one of the most iconic examples of symbioses found in nature; most corals are incapable of long-term survival without the products of photosynthesis provided by their endosymbiotic algae. Rohwer et al. (2002) were the first to use the word holobiont to describe

3354-427: The coral itself (a eukaryotic invertebrate within class Anthozoa ), photosynthetic dinoflagellates called zooxanthellae ( Symbiodinium ), and associated bacteria and viruses. Co-evolutionary patterns exist for coral microbial communities and coral phylogeny. Stressors can alter directly the host physiology and immunity, and the symbiotic community composition and density. Stressors may also indirectly affect

3432-435: 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

3510-640: 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

3588-402: The efficiency of the given buffering mechanism, and the net balance between its cost and its benefit. Holobiomics is the scientific analysis of a community of holobionts, which focuses on the interconnections between its components in the context of the prevailing environmental conditions rather than on the individual parts. The scientific approach for this emerging research field is based on

3666-709: The environment, dubbed infochemicals . Nevertheless, we can identify two major differences between terrestrial and aquatic systems. First, the physicochemical properties of water result in higher chemical connectivity and signaling between macro- and micro-organisms in aquatic or moist environments. In marine ecosystems, carbon fluxes also appear to be swifter and trophic modes more flexible, leading to higher plasticity of functional interactions across holobionts. Moreover, dispersal barriers are usually lower, allowing for faster microbial community shifts in marine holobionts. Secondly, phylogenetic diversity at broad taxonomic scales (i.e., supra-kingdom, kingdom and phylum levels),

3744-510: The evolution of microbe-host relationships. Some researchers question whether the holobiont concept is needed, and whether it does justice to the intricacies of host-symbiont relationships. In 2016, Douglas and Werren took issue with the concept that "the holobiont (host plus its microbiome) and its constituent hologenome (the totality of genomes in the holobiont) are a unit of selection, and therefore this unit has properties similar to an individual organism". They argue that "the hologenome concept

3822-462: The evolution of the host. There is still some controversy surrounding these terms, and they have been used interchangeably in some publications. Holism is a philosophical notion first proposed by Aristotle in the 4th century BC. It states that systems should be studied in their entirety, with a focus on the interconnections between their various components rather than on the individual parts. Such systems have emergent properties that result from

3900-441: 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

3978-421: 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

SECTION 50

#1732776049113

4056-501: 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

4134-460: 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

4212-535: The general acceptance of the endosymbiosis theory, the term holobiosis or holobiont did not immediately enter the scientific vernacular. It was coined independently by the German Adolf Meyer-Abich in 1943, and by Lynn Margulis in 1990, who proposed that evolution has worked mainly through symbiosis-driven leaps that merged organisms into new forms, referred to as "holobionts", and only secondarily through gradual mutational changes. However,

4290-472: 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

4368-400: The holobiont's phenotype but have not coevolved with the host are coloured red. Those that do not affect the holobiont's phenotype at all are coloured gray. Microbes may be transmitted vertically or horizontally, may be acquired from the environment, and can be constant or inconstant in the host. It follows that holobiont phenotypes can change in time and space as microbes come into and out of

4446-542: The holobiont. Microbes in the environment are not part of the holobiont (white). Hologenomes then encompass the genomes of the host and all of its microbes at any given time point, with individual genomes and genes falling into the same three functional categories of blue, red, and gray. Holobionts and hologenomes are entities, whereas coevolution or the evolution of host-symbiont interactions are processes. Although most work on host-microbe interactions has been focused on animal systems such as corals, sponges, or humans, there

4524-409: The hologenome concept remain controversial, particularly in regard to the host and its microbiome as a single evolutionary unit. In order to validate the holobiont concept from an evolutionary perspective, new theoretical approaches are needed that acknowledge the different levels at which natural selection can operate in the context of microbiome-host interactions. For example, selection could occur at

4602-441: 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

4680-576: The level of the holobiont when a transgenerational association among specific host and symbiont genotypes can be maintained. Nevertheless, the holobiont concept has resulted in a shift from the focus on symbioses involving one microbial partner and a single host (squids and luminescent Aliivibrio , legumes and Rhizobium , aphids and Buchnera ) toward a greater interest in symbioses in complex multi-partner consortia (animal gut systems, marine invertebrates, plant and seaweed epiphytes, microbe-microbe interactions in soil, aquatic biomes). Moreover, there

4758-409: 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

SECTION 60

#1732776049113

4836-495: The microbiota of the rhizosphere of plants or the animal gut became predominant models and have led to an ongoing paradigm shift in agronomy and medical sciences. Holobionts occur in terrestrial and aquatic habitats alike, and several analogies between these ecosystems can be made. For example, in all of these habitats, interactions within and across holobionts such as induction of chemical defenses, nutrient acquisition, or biofilm formation are mediated by chemical cues and signals in

4914-420: 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

4992-543: 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

5070-414: 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

5148-434: 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

5226-645: The plant holobiont are Actinomycetota , Bacteroidota , Bacillota , and Pseudomonadota . For example, nitrogen-fixers such as Azotobacter (Pseudomonadota) and Bacillus (Bacillota) greatly improve plant performance. Fungi of the phyla Ascomycota , Basidiomycota , Glomeromycota , and Mucoromycotina colonize plant tissues and provide a variety of functions for the plant host. Arbuscular mycorrhizal fungi (Glomeromycota), for instance, are common across plant groups and provide improved nutrient acquisition, temperature and drought resistance, and reduced pathogen load. Epichloë species (Ascomycota) are part of

5304-442: 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

5382-402: The roots), the endosphere (inside plant tissue), and the phyllosphere (total above-ground surface area). The holobiont concept originally suggested that a significant fraction of the microbiome genome together with the host genome is transmitted from one generation to the next and thus can propagate unique properties of the holobiont". In this regard, studies have shown that seeds can play such

5460-430: The same species, and the term is commonly applied to eusocial insects. An ant colony can be described as a superorganism, whereas an individual ant and its associated bacteria, fungi, etc. are a holobiont. There is no doubt that symbiotic microorganisms are pivotal for the biology and ecology of the host by providing vitamins, energy and inorganic or organic nutrients, participating in defense mechanisms, or by driving

5538-506: 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

5616-412: 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

5694-438: 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

5772-410: 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

5850-492: 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

5928-477: 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

6006-481: The symbiotic community by altering the host physiology (which represents the symbiotic niche), and the immune state of the host. Conversely, symbionts can buffer stressors via nutrient provision, physiological tolerance, and defense against host natural enemies. The holobiont response to stressors is difficult to predict, as many factors can be under selection. This includes host resistance genes and plastic mechanisms, but also acquisition of symbionts that can constitute

6084-449: The viruses included in a holobiont are collectively referred to as the virome Fungi: Multicellular fungi can be included in holobionts, such as arbuscular mycorrhizal fungi (AMF) in the roots of plants. Holobionts are entities composed of a host and all of its symbiotic microbes. In the diagram, the symbiotic microbes that affect a holobiont's phenotype and have coevolved with the host are coloured blue, while those which affect

#112887