What Are Lichens?
What is a lichen? • Lichens are two or more different organisms (fungi, and algae or cyanobacteria), living together in a relationship where each partner derives. Yeast emerges as hidden third partner in lichen symbiosis. Date: July The fungus, in turn, forms the main structure of the lichen and offers its. A lichen is a composite organism consisting of a fungus and an alga functioning in a symbiotic relationship.
In fact, chloroplasts, which are the site of photosynthesis in land plants, are adapted forms of cyanobacteria. These early cyanobacteria were engulfed by primitive plants cells sometime in the late Proterozoic, or in the early Cambrian periodaccording to the University of California Museum of Paleontology. So when a fungus, which is the dominant partner in this relationship, associates with an alga usually from the green algae or cyanobacterium to form a lichen, it is providing itself with constant access to a source of nourishment.
He described it as the controlled growth of a carbon-providing organism, just like we grow wheat, rice or potatoes.
He added that cyanobacteria also provide fungi with the additional benefit of nitrogen fixation. This is the biochemical reaction wherein atmospheric nitrogen is converted to ammonia, a more usable form of the element. In return, algae and cyanobacteria secure a protected environment, especially from damaging ultraviolet rays.
Finally, as lichens, fungi, algae and cyanobacteria are able to live in environments that they could not live in otherwise.
Weird World of Lichen: On his website dedicated to lichen, Alan Silverside, now retired from the University of the West of Scotland, gives the example of the fungus Sticta canariensis. This fungus is capable of forming two different lichen associations with an alga and cyanobacterium, yet both lichens are referred to as Sticta canariensis.
This is how early lichens might have looked like million years ago.
It is the thallus that gives lichens their characteristic outer appearance. Lichen thalli come in many different forms. Examples on Silverside's pages include foliose lichen, which look flat and leafy; fruticose lichen, which have a wiry, tufted appearance; squamulose lichen, which have flat, overlapping scales; and crustose lichen, which as the name suggests, form a tightly attached crust over the surface it inhabits.
In general, the inside of the lichen thallus appears stratified, with the mycobiont and photobiont cells arranged in layers. According to the U. Forest Servicethe outer layer or cortex is made up of thick, tightly packed fungal cells. This is followed by a segment with the photobiont either green algae or cyanobacteria. If a lichen has both an algal and a cyanobacterial partner, the cyanobacteria can be seen within little compartments above the upper cortex. The only common feature that they all share is that the mycelium of the fungal symbiont will gain entry into the host, root cells by cellulolytic enzymes.
Unlike the ectomycorrhizae, roots which are infected with mycorrhizal fungi do not differ morphologically from those that are not infected, i. However, the type of association that is formed between the host and fungus vary a great deal in the different categories of endomycorrhizae.
When fungi and algae marry | Science News for Students
Arbuscular Mycorrhizae This category of mycorrhiza can be found throughout the world, but more abundant in the tropics than in temperate regions, and is associated with more plants than any of the other categories of mycorrhizae. The name of this type of mycorrhizae comes from the distinct structures called arbuscules that can be seen inside the cells of infected roots.
These structures can be recognized by their branched tree-like appearance. Another structure that can be frequently observed are the rounded vesicles. The vesicles and arbuscules contain the stored minerals that are needed by the plant.25 Lichens Mycorrhiza Viruses Mycorrhiza
These structures lyse in the root cells and in this way the minerals become available to the plant. There is also extensive mycelium in the soil, but do not appear to be organized in any fashion.
Vesicles in roots cells of Sesbania sp. Note some vesicles have been displaced from cells due to preparation of slide. Arbuscule in root cell. Arbuscules are characterized by their tree-like appearance.
The group of fungi involved is always a member of the Zygomycota. There are only a few genera of fungi involved, but because of the lack of specificity of these genera to specific host plants, they have been found to have largest host range of any mycorrhizal group. The VAM fungi normally produce assorted types of spores which can be used in the identification of these fungi, i. It was once thought that these fungi were nothing more than a rare curiosity.
However, this was only because a technique was needed, which could more efficiently find VAM spores, than by simply sifting through the soil. Once this technique was found, this type of mycorrhiza was found to be the most common in nature.
It is because VAM have a broad host range they were once considered to be a future tool in agriculture, i. However, because these fungi cannot be grown in the absence of a host plant, individual inoculations would have to be done for each plant. This would be impractical for any grains grown as well as for most crops, but have been utilized in planting of fruit trees which are planted individually. There are a number of native plants which are endangered, in which attempts at growing them from seeds and cuttings at NTBG have not been very good.
A few years ago, while Drs. While inoculation of VAM fungi did greatly improve the survival of the young plants, it would not be the whole answer to their problems. Some species of native Hawaiian plants that were given inoculated with and without VAM fungi are shown on Figs. Left plant with and right without mycorrhiza. Left plant without and right with mycorrhiza, respectively.
Orchid Mycorrhizae Orchid mycorrhiza is endomycorrhizal and have fungal partners that are saprotrophic or pathogenic species of Basidiomycota, but a some are ectomycorrhizae, e. All orchids must form mycorrhizae. In most plants, the seed contains a food supply that will feed the embryo, until germination occurs, at which time the plant becomes photosynthetic and can produce its own food.
However, orchid seeds are very minute and contain a very small food reserve for the embryo. This food supply is usually depleted by the time that the first few cell divisions of the embryo has occurred.
During this critical period, the fungal symbiont colonizes the plant shortly after seed germination and form characteristic, coiled hyphae within the cortical cells of the root.
The hyphae in the host cells collapse or are digested by the host that will supply the embryo with its carbon source and vitamins until it is able to photosynthesize. Unlike other mycorrhizal fungi, orchid mycorrhizal fungi can also digest organic materials, from the surrounding environment of the orchid, into glucose, ribose and other simple carbohydrate and these nutrients are translocated into the orchid to support their growth. The relationships that orchid species have with the mycorrhizal fungi are variable and is dependent on their nutritional needs.
Those orchids that are photosynthetic still retain their fungal partners, but it is not clear as to what role it is playing. However, the achlorophyllous orchids will require it even as adult plants. In these species the associate fungus forms a tripartate relationship, where the fungus also forms a relationship with a photosynthetic plant and channel its nutrient to the orchid.
The fungus will also supply both plants with inorganic nutrients. Ericaceous Mycorrhizae The mycorrhiza formed in this group is between fungi in the Ascomycota, and more rarely in the Deuteromycota, and species in the families Epacridaceae, Ericaceae and Pyrolaceae. Three subcategories are recognized, arbutoid, ericoid and monotropoid.
We will briefly cover the latter two groups. Arbutoid Mycorrhiza This group forms associations with plants that are trees and shrubs that belong to the genera Arbutus madroneArctostaphylos manzanita and Arctous alpinus mountain bearberry. They have characteristics that are both ecto- and endo-mycorrhizae: There is a formation of an external mantle of mycelium that forms a hartig's net, as in ectomycorrhiza, but intracelllar penetration of cortical cells occurs as in endomycorrhiza.
Fungi forming this association are members of the Basidiomycota. Ericoid Mycorrhizae Plants having this group of mycorrhiza are commonly found in acidic, peatland soils and include members of genera Calluna heatherRhododendron, Azaleas and Vaccinium blueberriesof the family Ericaceae.
Ericoid mycorrhizae have evolved in association with plants that are continually stressed by factors within the soil. The soil is typically extremely acid, peatland soil, low in available minerals because mineralization is inhibited.
Plants with ericoid mycorrhizae seem to have a high tolerance to these stresses and there is good reason to believe that this is related to the presence of the mycorrhizal fungus and that the survival of the host is dependent upon the fungus. The mycorrhizal association is most similar to that of an endomycorrhiza because fungus growth is extensive in the root cortex. The fungus penetrates the cell wall and invaginates plasmalemma and is filled with coiled hyphae, like those in orchid mycorrhizae.
No mantle is formed. Infected cells are fully packed with fungal hyphae. Fungus species are mostly members of the Ascomycota, in the genus Hymenoscyphus. Cross section of ericoid root, showing coiled hyphae. The host cell dies as the association disintegrates, thereby restricting the functional life, i. Monotropoid Mycorrhizae One of the characteristics that we normally attribute to plants is that they have chlorophyll and can produce their own food through the process of photosynthesis.
However, this is not true of all plants. The Monotropaceae and Pyrolaceae are two families of plants that are achlorophyllous. Thus, plants in these families are more dependent upon their mycorrhizal partners than plants which can carry out photosynthesis.
Monotropa uniflora left from http: The means by which food is obtained by these plants is the same as in achlorophyllous orchids. However, morphologically, they are very different. The achlorophyllous host has mycorrhizae roots that appear to be formed by an ectomycorrhizal fungus, but the epidermal and outer cortical cells are penetrated by the fungus, as in endomycorrhizal plants. The fungus also forms an ectomycorrhizal relationship with a tree which is capable of photosynthesis. So, as in the case of the epiphytic orchids, the photosynthetic tree indirectly provides carbohydrates to these achlorophyllous plants, as well as to the fungus.
Both hosts probably obtain their mineral requirements through the fungus. Lichens The most well known example of a symbiosis between fungi and plants is the lichen, if you will allow me to include algae as plants. The concept of what constitutes a lichen has broaden significantly in the last 25 years to include some species of mushrooms, slime molds, and some members of the Zygomycota. However, we will discuss lichens in the traditional sense, as an association between a fungus and an alga that develops into a unique morphological form that is distinct from either partner.
The fungus component of the lichen is referred to as the mycobiont and the alga is the phycobiont. Because the morphology of lichen species was so distinct, they were once thought to be genetically autonomous until the Swiss Botanist Simon Schwendener described their dual nature in Prior to that time, because of the morphology of many of the "leafy" species of lichens, they were considered to be related to bryophytes, i. Although, lichens are now known to be composite organisms, they are still named for the fungus part of the association since that is the prominent part of the lichen thallus.
A thallus is an old botanical term used to describe "plants" that do not have leaves, stems and roots, and its origin goes back to a time when only two kingdoms were recognized in classifying organisms, i.
Prior toorganisms such as algae, bacteria and fungi, were included in the plant kingdom. InWhitaker, proposed a five kingdom system that was used for many years, but may soon also become outdated. Although, this term is antiquated, it is still used to describe the "bodies" of algae, fungi and of course lichens. The only group of plants, in which we still use the term thallus, to refer to the plant body, are the bryophytes.
Although the lichen thallus is composed of an algal and fungal component, lichens are not studied in mycology or phycology that part of botany that studies algae. Instead, they are studied in their own discipline, lichenology. There are relatively few lichen researchers.
When fungi and algae marry
Of these most are systematists. As a result, there are still some basic questions concerning this symbiosis that are unanswered or at least up for debate. One of the most basic questions, that has been asked since the discovery of the lichen symbiosis, concerns whether lichens represent a true mutualistic symbiosis or nothing more than a variation of a host-parasite relationship.
There is evidence supporting both sides. That it represented a mutualistic symbiosis, in which the alga was believed to contribute the food supply through photosynthesis, and the fungus protected the alga from desiccation, harmful solar radiation and provided the alga with water and inorganic nutrients, was postulated by Beatrix Potter, the writer and illustrator of Peter Rabbit, soon after Schwendener had determined the true nature of the lichen thallus.
In order to understand both sides of the issue, lets look at the morphology and anatomy of lichens. The Lichen Thallus In the traditional sense of lichens, their thallus can be artificially divided into four forms: Foliose Lichens Lichen thallus which is generally "leaf-like", in appearance and attached to the substrate at various points by root-like structures called rhizines.
What Are Lichens?
Because of their loose attachment, they can easily be removed. These are the lichens which can generally be mistaken for bryophytes, specifically liverworts. It is possible, or even probable, that herbaria still contain lichens that have been mistakenly identified as liverworts. If we look at these a foliose lichen in longitudinal section, from top to bottom, we would be able to distinguished the following layers: Often composed of tightly interwoven mycelium, which gives it a cellular appearance.
This cellular appearance is referred to as pseudoparenchymatous. Different species of lichens can look very different from each other. Thorsten Lumbsch, a lichenologist at The Field Museum in Chicago, took photographs of two varieties of lichens—a rock lichen above and a spot lichen below —during a recent Thorsten Lumbsch Thorsten Lumbsch Through photosynthesis, the alga harvests the sun's energy to make food for the fungus, which provides a place for the alga to live.
But the relationship is lopsided, Schmitt says, with algae caged like prisoners—even slaves—inside their fungal hosts. Around the world, scientists have identified tens of thousands of types of lichens. At least as many probably still await discovery, says Thorsten Lumbsch, a lichenologist at the Field Museum in Chicago. By putting together a lichen family tree, they hope to understand why so many different types of lichens have evolved in so many places around the world.
Most research involves attempts to understand basic facts about the organisms and their interrelationships. But researchers are also teaming up with lichens to monitor the health of the environment, among other applications.
Tough work Studying lichens is rarely easy. Most species depend on very specific conditions, and scientists can rarely get them to grow in laboratories. This provides lichenologists a great excuse to travel around the world, scouting new specimens and insights. Lumbsch, for one, makes several trips to Australia and South America each year. In the field, he searches for a group of crusty lichens that tends to be quite tiny—usually less than a few millimeters long. Finding samples takes patience and a trained eye.
So, I go there and crawl on my knees on the forest floor with a hand lens. This photograph was taken on a research trip to India in January Thorsten Lumbsch Spotting lichens is challenging enough. Identifying them is even harder. Many species look exactly alike, even when they are only distant cousins.
Closely related species, meanwhile, can live in totally different environments, or on opposite ends of the Earth.