Introduction

At the beginning of the semester you learned that, nutritionally, fungi can be broadly categorized as saprobes, parasites and symbionts. We have covered a number of examples of saprobes and parasites, but have neglected saying much about symbionts. Although I am sure that all of you feel that you have a definition of a symbiont, it will probably differ from that of the biological definition of what constitutes a symbiosis. Traditionally, a symbiosis may be defined as the living together in more or less intimate association or close union of two dissimilar organisms. However, all of you probably have a narrower definition than the one I just gave you. The definition, which most of you probably learned in an introductory course in biology, is that a symbiosis represents a reciprocal relationship between two dissimilar organisms. This definition while correct is very narrows and specifically refers only to a mutualistic symbiosis, and lichens are usually cited as an example. However, biologists recognize several forms of symbiosis. For example a parasite-host relationship would also be considered by biologists as a symbiotic relationship.

There are a number of different types of symbiosis that fungi form with other organisms. Unfortunately, we will not be able to cover all or even most of these relationships. We will, instead restrict ourselves to two types of symbiosis in which fungi are involved, lichens and mycorrhizae. However, I would like to briefly describe some rather interesting relationships between insects and fungi.

Ants, Termites and Mushrooms

Social insects have always been of interest because of their seemingly, well ordered societies. In some of these social insects, the mount-building termites of Africa and Asia, and the leaf-cutting ants of Central and South America, there has evolved a rather unique strategy in the consumption of cellulolytic plant material. These insects cultivate cellulolytic fungi, in underground gardens, and I'm using the world cultivate in the true sense of the word, because these insects are deliberately growing these fungi. They establish pure cultures of their fungus. That is they grow only one fungus in their garden, which is not easily done, since there are so many sources of contamination that can occur. However, these insects are able to keep their gardens pure by constantly weeding out foreign fungi. They also care for their garden by providing suitable a food source, i.e. plant material, and moisture. So the fungi obviously benefit from this arrangement, but the ants and termites also benefit from this relationship. These insects are exclusively mycophagous, i.e., they eat fungi. The fungi that they cultivate breakdown the wood and leaves brought in by the termites and ants, respectively, and provide them with digestible and nutritious mycelium.

Leaf-Cutting Ants, Leucoagaricus and Lepiota

These gardening ants are from the New World Tropics and are commonly referred to as the Attine Ants. There are hundreds of species of these ants, in approximately fifty genera. Although you probably have heard of these ants because of the tremendous damage that they cause, you probably have not heard of their ability to cultivate fungi. To the people of South America the ants are an all too familiar sight that they dread. In their search for food, these ants will devastate the natural vegetation and crops that are in their path as they search for plant material to feed their fungus. When the Spanish Conquistadors arrived, they easily conquered the Native Americans, but were unable to do the same with the ants. Their efforts in growing cassava and citrus fruits failed because of their inability to control these ants. Of all the known species, Atta sexdens is the most economically important and the one which is most intensely studied, and the species that we will look at in detail as representative of this group of ants.

Colonies of A. sexdens start from a single winged female, the future queen, carrying a small inoculum of the fungus, in a pocket, in the back of her mouth that apparently has evolved specifically for this purpose. The winged female by this time has already been fertilized and potentially can lay as many as 300 million eggs during her lifetime. In a young colony, the queen and the first workers to hatch from eggs establish the first fungus garden by excavating a chamber and filling it with vegetation brought in by the workers and then inoculating it with the fungus. Different species will utilize different substrate material for their fungus gardens.

In studies that have been carried out in excavated nest, it was found that one nest that was four years old contained 1027 subterranean chambers, of which 390 contained fungus gardens. Another, approximately six years old, had 1920 chambers, or which 248 contained fungus gardens. Gardens are usually 20-30 cm (8-12 in) in diameter and weight approximately 300g (10.5 oz). It is estimated that these colonies had consumed 6000 kg (13,200 lbs) of vegetation.

The Attine Ants are commonly called leaf-cutting ants because they forage for leaves and cut them into pieces with their mandibles before carrying them back to their colony. Once they have returned with the leaf cutting, the workers cut the material into smaller pieces, lick it all over and often deposit on it their anal excreta. The excreta serve as additional nutrients for the fungus garden. The plant material is then wedged into the garden and a tuft of mycelium placed on it.

Regardless of the species of ants, the colony only contains one species of fungus. This is a difficult condition to maintain since, as you now know from experience, fungi and bacteria are everywhere ready to take advantage of whatever organic material that becomes available. The worker ants in probing the mycelium with their antennae are able to distinguish their fungus from alien fungi. When foreign fungi are detected, they are removed by the workers. Some foreign fungi, undoubtedly, are present, but with the far more prevalent, cultured fungus present, they are unable to compete and do not make up an appreciable part of the garden. When colonies are abandoned because of disturbance or migration, the fungus garden left behind deteriorates and becomes contaminated with other fungi and bacteria. Before abandoning their colony, the Attine Ants always take some of the fungus garden with them as an inoculum to start their new fungus garden.

As the mycelium grows, hyphal tips are formed, which is the part of the hyphae that the ants consume. Although a great deal of plant material is brought into the colony, apparently none of it is consumed by the ants. It is used entirely to feed the fungus and the ants only feed upon the fungus.

The discovery of the identity of the fungal species involved in these ants were determined by taking pure cultures into the lab and in some cases fruiting bodies have formed. In most instances, they have been determined to be species of the mushroom genera, Leucoagaricus and Lepiota. Other fungi that have been fruited include species of Auricularia, and Xylaria (Ascomycota). For reasons unknown, these species do not form their fruiting bodies around or near the colony.

Termites and Termitomyces

Where the ants that cultivate fungi are in the new-world tropics, the termites that cultivate fungi are native to the old-world tropic. These termites belong to the subfamily Macrotermitinae which include approximately twelve genera genera that are distributed in Africa, Madagascar, India and much of south-east Asia. The subfamily Macrotermitinae differ from other termites in that they do not have the cellulolytic protozoan symbiont in their gut that allows them to digest wood.

The start of new colonies is similar to that of the Attine Ants. The colony begins with a winged male and female rather than a winged female that has already been impregnated. The two termites wall themselves into an underground "royal chamber" from which they will never leave. After the queen begins laying eggs, the workers, which are the first produced, bring food to the couple, take new eggs away for incubation and add further to the nest. The workers continue to build the fungus garden around the royal chamber. Above the royal chamber, the workers build mounds which may be as much as six meters tall and three meters across at the base. The mound have air shafts that leads to the fungus garden, which by this time, may be a large central structure that is 50 cm (20 in) in diameter and weighing as much as 25 kg (55 lbs) or may be made up of a number of smaller chambers.

Colonies may contain as many as a million termites that forage for plant debris, mostly in the form of wood. Unlike the Attine Ants, the termites will consume the woody plant material while foraging and upon returning to the colony will deposit fecal droppings in the fungus garden. While tending the garden, the workers will nibble on the fungus. The cellulolytic enzymes, that are in the mycelium, remain active in the gut of the workers. The king, queen, soldiers and nymphs do not eat the fungus directly, and live on the salivary secretion of the workers.

The fungi that are in these termite mounts, unlike those in the Attine Ant colonies, are well known since they fruit readily in nature. When there is rainfall of more than 2 cm/day, the fungus gardens will produce mushrooms with long stalks that will grow through the soil surface and produce the mushroom cap. These mushrooms have been identified as Termitomyces, a genus known only from the termite mounds. There are thirty species in this genus. In some species of termites, Xylaria, may also be found growing with the mushrooms. Termitomyces species have become highly prized, edible mushrooms, in the tropics, and attempts are now underway to cultivate these species.

Mycorrhizae

We have already mentioned mycorrhizae on several occasions so that you at least know what a mycorrhiza is, but let’s define it one more time. A mycorrhiza is a symbiotic relationship between the roots of plants and fungi. The term mycorrhiza literally means root fungus, but in the broad sense of the term, the interaction does not always occur only with the roots of plants. A mycorrhizal relationship also includes plants that do not have roots, such as Psilotum and bryophytes (mosses and liverworts).

The layperson probably has the impression that if plants are in an area with rich soil and have enough water and sunshine that they will grow well. However, that is often not the case. Often plants are dependent upon some form of symbiotic relationship to survive in nature. We will restrict ourselves to mycorrhizae symbiotic relationships. In the case of mycorrhizal, we are actually talking about a number of different types of relationships. Another words, there are different categories of mycorrhizae. In the most common types, the fungus will usually receive carbohydrates of some sort from the plant and the fungus reciprocates by enhancing mineral transport to the plant. You should recall that in order for plants to grow normally, they require certain essential elements, and I will not review those elements at this time since knowing what they are is really not essential in understanding the concept of mycorrhizae. Generally, in nature, the soil composition is often deficient in one to several essential elements that are required by plants, and it is thought that because the mycelium of the fungus is more extensive than even the roots of the host plant, the fungus is able to enhance nutrient uptake for the plant. In addition to the enhanced nutrient uptake, different categories of mycorrhizae may protect roots against pathogens, produce plant hormones and translocate carbohydrates between plants. However, there are some generalizations that can be made, concerning most mycorrhizae:

• Mycorrhiza infection area occurs only on the smallest order of secondary roots. These are the root tips that are still growing, elongating and increasing in girth. So, infection occurs only in a very small part of the root system of a plant. This makes a great deal of sense since this is the only part of the root system that will be capable of water and mineral absorption.

• When infection has occurred, only the cortical cells of the root are invaded by the fungus. This is the area between the epidermis and the vascular tissue of the root.

• Occurs in practically all plants with the exception of the Brassicaceae; The Crucifer Family; Chenopodiaceae, The Goosefoot Family; Cyperaceae; The Sedge Family and in aquatic plants. All other families form mycorrhizae.

• It is believed that for plants that form mycorrhizae, they would be unable to survive in their natural habitat without this symbiotic relationship. This has been demonstrated to be true for numerous plants.

Mycorrhizae is relevant, not only to the world’s present flora and its practical applications in agriculture and forestry, but it has been claimed by some mycorrhizal researchers that without mycorrhizae, terrestrial plants would never have been able to establish themselves on land. These researchers believe that long before plants became established in the terrestrial environment, fungi, as mycorrhizal partners, were already associated with plants, in the aquatic environment, and that it was the mycorrhizal fungi that made possible the plant’s conquest of the terrestrial environment.

Categories of Mycorrhizae Recognized

• Ectomycorrhizae: Characterized by forming an external sheath of mycelium around the root tips. Hyphal cells do not penetrate the cell walls although they may go between cells in the cortex. Root hairs are also absent in this type of mycorrhiza.

• Endomycorrhizae: characterized by the lack of an external sheath around root tip and the penetration of cortical cells by the fungus mycelium. A very variable group of mycorrhizae.

• Ectendomycorrhizae: mycorrhizal type that seems to be intermediate between ecto- and endomycorrhizae. Mycelium sheath around root is reduced, or may even be absent. The hyphal cells may penetrate the cortical cells as in endomycorrhizae. Similar to ectomycorrhizae. We will not consider this group further.

Description of mycorrhizae types:

Ectomycorrhizae

This category of mycorrhiza is very uniform in appearance, and biologically identical despite having literally thousands of different species fungi, in the Ascomycota and Basidiomycota. For this reason, it is not subdivided into further subcategories as in endomycorrhizae. It is referred to as "ecto-" because the fungal symbiont does not invade the cell protoplasm. However, the fungus does form a thick sheath around the root tip and mycelium also grows between the cells of the cortex.

While there are a large number of fungi that are ectomycorrhizae, plants that have ectomycorrhizae are restricted to only a few families of plants, and these plants are always trees. They are also more common in temperate regions than in the tropics. This is one reason why there are far fewer mushrooms here in Hawai‘i than on the mainland. This type of mycorrhiza is very important in forestry because its association with trees.

In this type of mycorrhiza, the fungal sheath, that forms around the secondary root tips, accumulate minerals from the decomposing litter, before they are able to pass into the deeper mineral layers of the soil where they are unavailable to the roots. The fungus obtains simple carbohydrates that are produced, but not used by the plant. So it appears that these carbohydrates may be produced by the plant specifically for the fungus since they are not utilized by the plant. Fungi involved are members of the Basidiomycota and the Ascomycota. Also, they are usually species that form large fruitbodies, such as mushrooms, puffballs, truffles, etc.

Endomycorrhizae

Although far less conspicuous because they do not produce large fruiting bodies, such as mushrooms, this category of mycorrhiza is far more common than the ectomycorrhizal type. Generally, it can be said that plants that do not form ectomycorrhizae will be the ones that form endomycorrhizae. However, because of the absence of a macroscopic of macroscopic fruitbodies, the presence of endomycorrhizae is more difficult to demonstrate.

There are several categories of endomycorrhizae. 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.e. root hairs are present and sheath is not formed around the root tip. However, the type of association that is formed between the host and fungus vary a great deal in the different categories of endomycorrhizae.

Vesicular-Arbuscular Mycorrhizae (VAM)

We have previously covered this category of mycorrhiza during the lecture on the Zygomycota (Glomales). This group of fungi 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 group of mycorrhiza is based on the two distinct structures that can be seen inside the cells of the infected roots: The rounded vesicles and the branched tree-like arbuscules. There is also extensive mycelium in the soil, but none of it is organized in any fashion. The vesicles and arbuscules contain stored minerals that are utilized by the plant. These structures lyse in the root cells and the mineral then becomes available to the plant.

There are only a few genera of mycorrhizal fungi in this group, but because of their lack of specificity to specific host plants, they have the 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.e. zygospores, chlamydospores and azygospores. It was once thought that these fungi were nothing more than a rare curiosity. However, this was prior to an efficent technique for collecting VAM spores. Now they are known to be common in nature.

It is because VAM have a broad host range they were once considered to be a future tool in agriculture, i.e. fertilizer substitute. However, because these fungi cannot be grown in the absence of a host plant, individual inoculations would have to be carried out in order to infect each plant. This would be impractical for for most crops, but have been utilized in fruit tree orchards, which are planted individually.

Recently, VAM fungi have been utilized as at The National Tropical Botanical Garden (NTBG) in an effort to save endangered species of native Hawaiian plants. There are a number of native Hawaiian plants, which are presently near extinction. Efforts to perpetuate these species by growing them from seeds and cuttings at NTBG have largely been unsuccessful. However, afew years ago, Richard Koske and Jane Gemma, two University of Rhode Island mycologists, who are normally given work space at NTBG in order to carry out their research on the ecology of Hawaiian VAM fungi, suggested that perhaps the absence of VAM fungi was the reason why the plants were not growing. Although inoculation of VAM fungi did greatly improve the survival of the young plants, it proved not be the whole answer to their problems. Nevertheless, this indicates that VAM fungi may be important in the conservation of native Hawaiian plants.

Orchid Mycorrhizae

This category of endomycorrhizae are mostly members of the Basidiomycota. All orchids are infected with this type of mycorrhizal fungus. Orchid mycorrhizae are functionally different than in the above two types because of the unique nutritional needs of orchid plants. 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 of time between the end of their stored food supply until they become photosynthetic (if they are photosynthetic orchids, many are not), they are dependent upon the mycorrhizae for survival. Most orchid seeds will not even germinate until the fungal symbiont penetrates seed coat of the seed. Because of the lack of food in the embryo of the orchid, the fungus not only supplies minerals, but also supplies to the orchid carbohydrates and possibly other metabolites such as vitamins. Thus, it is the orchid that is deriving the carbohydrate from the fungus rather than the other way around. Unlike the other mycorrhizal fungi, these fungi 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 own growth. The relationships that orchid species have with the mycorrhizal fungi are variable and is dependent on their nutritional needs. Some orchids become photosynthetic when their leaves develop while others remain achlorophyllous. Thus, those that are photosynthetic generally do not require the mycorrhizae at that time, but often still retains the fungal symbiont as a partner. However, the achlorophyllous species will require it even as adult plants.

Some relationship are unique and very interesting. Many orchids are epiphytes, i.e. they live on other plants rather than in soil, and achlorophyllous. It has been demonstrated that in some species of mycorrhizal fungi, associated with the epiphytic orchid roots, a parasitic relationship also develops with the plant, on which the orchid is growing. In this type of relationship, food is being obtained by the fungus, from the tree on which the orchid is growing, and the food is utilized by the orchid and the fungus.

Monotropoid Mycorrhizae

One of the characteristics that we always 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, which are achlorophyllous. Thus, plants in these families are more dependent upon their mycorrhizal partners than plants that can carry out photosynthesis. The means by which food is obtained by these plants is similar to that of the epiphytic orchids described above. However, morphologically, they are very different. The achlorophyllous host has mycorrhizal roots that appear to be ectomycorrhizae, but the epidermal and outer cortical cells are penetrated by the fungus. At the same time the fungus also forms an ectomycorrhizal relationship with a another plant, which is capable of photosynthesis. So, as in the case of the epiphytic orchids, the photosynthetic plant indirectly provides carbohydrates to the achlorophyllous plant, as well as to the fungus. Both hosts obtain their mineral requirements through the fungus.

Lichens

The concept of what constitutes a lichen has broaden significantly in the last 25 years to include species of mushrooms (Basidiomycota), Deuteromycetes, Zygomycota and even slime molds (Myxomycetes). However, we will discuss lichens in the traditional sense, as an association between a fungus that is a member of the Ascomycota 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 was credited with the discovery of their dual nature in 1868. 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.e., mosses and liverworts. 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.

Although the lichen thallus is composed of an algal and fungal component, they 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. Of these most are taxonomist. 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. Lichens have often been used as the classical representation of a mutualistic symbiosis. The algal component is interpreted as contributing the food supply through photosynthesis, and the fungus reciprocates in protecting the alga from dessication, harmful solar radiation and provision of water and inorganic nutrients. Beatrix Potter, the writer and illustrator of Peter Rabbit was actually the first to propose this idea.

The Lichen Thallus

In the traditional sense of lichens, their thallus can be artificially divided into four forms: foliose, crustose, fruticose and squamulose.

• Foliose Lichens: This thallus type is generally "leaf-like", in appearance and attached, by its lower surface, to the substrate at various points. 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:

• Upper Cortex: This layer is usually composed of tightly interwoven mycelium, which gives it a cellular appearance. This cellular appearance is similar in appearance to parenchyma cells in plants and is referred to as pseudoparenchymatous.

• Algal Layer: This layer is composed of interwoven hyphae with the host algal cells. This is the ideal location for the algal cells. Beneath the upper cortex so that it receives the optimal amount of solar radiation, for photosynthesis, but not direct solar radiation which would be harmful.

• Medullary Layer: This layer is composed of loosely interwoven mycelium and is entirely fungal.

• Lower Cortex: This layer is similar to the upper cortex, but has root-like structures called rhizines throughout the lower surface that attach the thallus to the substrate. Thus, the foliose lichens also have what is referred to as a dorsiventral thallus, i.e. a distinct upper and lower surface.

• Crustose Lichens: This lichen thallus is very thin and flattened against the substrate. The reason for the thinner thallus is the absence of a lower cortex. This thallus type is attached by the medullary and upper cortex. These lichens are so thin that they often appear to be part of the substrate on which they are growing. Species that are brightly colored often give the substrate a "spray-painted" appearance. The upper cortex, algal and medullary layers is similar in appearance to the foliose lichens.

• Fruticose Lichens: The thallus is often composed of pendulous ("hair-like") or less commonly upright, finger-like branches. The thallus is attached to the substrate at a single point by a holdfast. In cross section, the thallus can usually be seen to be radially symmetrical, i.e. it does not have a top and bottom. The layers that can be recognized are the cortex, algal layer, medullary layer, and in some species the center has a "cord" which is composed of tightly interwoven mycelium. Other species have a hollow center and lack this central cord.

• Squamulose Lichens: This thallus type begins its development as either a small crustose or foliose lichen, but then forms podetia, which are vertical, branched or unbranched extensions of the thallus. The podetia are typically the prominent part of the thallus and the flattened portion is usually much reduced.

Biology of Lichens

In looking at the anatomy of the lichen, it is obvious that there is interaction between the phycobiont and mycobiont, but "what kind of interaction is occurring?" Do they represent a mutualistic symbiosis or a host-parasite relationship? In studies utilizing the transmission electron microscope, it can be clearly seen that haustoria from the fungus penetrate the alga cells. This is the reason that lichenologist have expanded their definition of lichens to include any host-parasite relationship between a fungus and an alga. However, this is rather simplistic. There is evidence to the contrary.

If we think about fungi and algae in general, we know that they are normally going to be found in a moist to wet environment where they are not receiving direct solar radiation. Conditions outside these parameters will usually be fatal for most species of fungi and algae. Thus, their habitats are usually very restricted. Lichens, on the other hand, occur all over the world. They may be found in habitats from the cold Arctics to the hot, dry desert where few organisms can live or even survive. Thus, not only is the lichen thallus able to exploit habitats that its individual components cannot, but few other organisms are able to utilize such habitats. This ability to endure such a wide range of environments must be due to the mutualistic, symbiotic relationship.

A laboratory experiment carried out by Vernon Ahmadjian also demonstrated the mutualistic, symbiotic nature of the lichen thallus. Although, it is not difficult to separate the myco- and phycobiont components of the lichen thallus and grow them separately in an agar medium, putting the components back together is another story. For many years it was not possible to reform the lichen thallus. The reason for this was the method that was used in attempting to reform the lichen thallus. The growth media used in attempts to grow lichens contained nutrients that were required by both the myco- and phycobiont. These media were not sucessful in reforming the lichen thallus. Ahmadjian reasoned that if the lichen represents a symbiosis, the reason that the relationship formed was because, in nature, neither one could obtain all the nutrients necessary for survival and that it was only after the two organisms interacted was this possible. Thus, Ahmadjian created a "minimal medium", which would not support the growth of either the myco- or phycobiont, alone, and inoculated them into that medium. This method successfully reformed the lichen thallus, in the laboratory, for the first time.

A Few Words on The Lichen Component

Although there are approximately 13,500 species of lichens recognized, the number of taxonomic groups of fungi and algae that produce the lichen thallus are few:

Mycobionts

In the traditional sense of lichens, which is how we are defining lichens, the fungal components are always in the Ascomycota. Specifically, these groups that form their asci and ascospores in apothecia (=Discomycetes), perithecia (=Pyrenomycetes) and ascostroma (Loculoascomycetes). These fungi are never found to be free-living in nature.

Phycobiont

Regardless of whether we are using the traditional or expanded definition of lichens, the algae involved in the association are the same. Of all the different species of algae that are known, only the phyla Chlorophyta ("green algae") and Cyanophyta ("blue-green" algae or Cyanobacteria ) are involved in lichen formation. The latter are really more closely related to bacteria than to the algae. Furthermore, within these divisions, only a few genera are involved in the lichen symbiosis. Some genera, such as Trebouxia, are known to only occur in lichens and are not free-living, but there are also genera that are free-living. However, the species of these genera, which are a component of a lichen to not appear to be free living.

Economic Relevance

Economically, lichens are of little significance. Perhaps this is why there is so little interest in this group of organisms. One way that they have been utilized is in the extraction of blue, red, brown or yellow dyes in the garment industry. The extracted dyes have also been used as indicator pigments used in litmus paper. Lichens have also been exploited as a source of pharmaceutical compounds. You can include some "folk" remedies in this category as well. They are used in the cosmetic industry, in the making of perfumes and some species have even been used as food. One species, Lecanora esculenta, is a species that grows in the mountains near Israel and are typically blown free from their substrate. Desert tribes grind up the lichen, dry it and mix it with dry meal to form a flour. It is postulated that this is the species of lichen that is referred to as the "Manna from Heaven" when Moses led the Hebrews across the desert during biblical time. Another species Cladonia rangiferina (reindeer moss) has been utilized as fodder for reindeers and cattle. This led to the discovery that lichens readily absorb radioactive fallout material from open-air, atomic testing. Both Alaskan Eskimos and Scandinavian Laplanders have been found to have high levels of radioactive radiation in their bodies, which resulted from eating reindeer and/ or cattle, which had consumed radioactive lichens.

Other Uses for Lichens

Lichens are conspicuously absent in and surrounding cities because many species are sensitive to pollution, especially to sulfur dioxide and fluorine, which are common pollutants. For this reason, they have been commonly used as indicators of pollutants. In urban areas, where lichen surveys have been carried out, the absence of certain indicator species is used as early warnings of decrease in air quality.

Lichens also play a very significant role in nature. They are the pioneers in rocky substrates, where there is no soil. Lichens break down the rocky substrate into soil and their decomposing thallus fertilize the newly produced soil, making it possible for the plant habitation.

Reproduction

Reproduction of the lichen is entirely asexual by two types of asexual propagules. Soredia, spore-like structures which are composed of one to a few algal cells and attached hyphae that are dispersed from the upper surface of the thallus. The soredia originate from the algal layer, which becomes exposed as a result of the rupturing of the upper cortex. Isidia are columnar fragment of the lichen thallus that are produced on the upper cortex, and break off, is dispersed and gives rise to another thallus.

Ascospores and conidia also form, but these will only reproduce the fungus. It has been assumed that these fungal spores may come in contact with a suitable algal host and resynthesis the lichen, but this seems unlikely.