Fungi have existed roughly 5/6 of the earth’s existance. They have had a lot of time to adapt to every environment on earth. For comparison, the humanoid species has existed 0,01 % of earth’s time (from the first speciments of the genus to Homo sapiens). Fungi are everywhere – they are so widespread, they make up a large proportion of the biomass in any ecosystem. In estimation, we know a little of the physiology and biochemistry of less than 1 % of all possible fungal species on our planet.

Different species live as decomposers, parasites or mutialists. 80 to 90 % of the plants on this planet only grow in symbiosys with fungi.

Morphology

Most common fungi grow as multicellular filaments and single cells. Many species grow as both.

• Yeasts are single cells mostly inhabit moist environments, where there is a ready supply of soluble nutrients, such as sugars and amino acids.
• Bodies of multicellular fungi most often form a network of filaments – hypahe. Hyphae consist of tubular cell walls surrounding the plasma membrane of the cells. Fungal cell walls are built and strenghthened by chitin, a strong and flexible nitrogen-containing polysaccharide.
• Fungi are not motile, but move places by growing, extending the tips of hyphae.
• Mycelium can grow rapidly, as proteins and other materials synthesised by the fungus are channeled through cytoplasmic streaming to the tips of the extending hyphae.

Hyphae

• A hypha consists of one or more cells surrounded by a tubular cell wall. In most fugi hyphae are divided into cells by internal cross-walls – septa.
• Septa have pores large enough for ribosomes, mitochondria and nuclei to flow between cells.
• The mayor structural polsaccharide polymer in fungal cell walls is chitin – whereas plants and oomycetes (a distinct phylogenetic lineage of fungi) have cellulosic cell walls.
• Each fungus has vast numbers of hyphae, all intertwining to make up the tangled web – mycelium.
• Hyphae web is very important as the forest’s information system. Through it plants communicate; they warn eachother of danger, infections, also use it as a mean of reproduction. Without it most of the plants we see as individuals, couldn’t exsist.
• The ability of fungal hyphae is an important physiological trait of fungi that provides them with a mechanism to move materials within their own bodies (thalli) in relation to gradients of supply and demand: translocation of nutrients and water. This movement of materials may occur at very small scales (mm to cm range), or, in differentiated organs, over distances of meters to tens of meters. At the ecosystem scale, this long-distance movement of nutrients and water has a large effect on ecosystems and distributing resources.

Evolution

Based on DNA data phlogeny, animals and fungi are more closely related to each other, than either is to plants.

Fungi were among the earliest colonisers of land, likely as symbionts with early land plants. Fossils of the earliest known vascular plants from 420 million years ago contain evidence of mycorhhizal relationships between plants and fungi.

Phyllogeny

• Microsporidia: unicellular oportunistic parasites of animals and protists.
• Chitrids: one of the earlies groups, single celled or form multicellular hyphae
• Zygomycetes: include molds, are decomposers, parasites or commensal symbionts
• Glomeromycets:
  • not well known to the broad public, but essential for terrestrial ecosystems. 90 % of all plant species form mycorhhizal assotiations with this group;
  • from a a type of mycorhhizae called arbuscular mycorhhizae;
  • host specificity of arbuscular mycorhhizae is very low because many species colonize a wide range of host plants. Plants can be colonized by a mixture of arbuscular mycorhhizae fungal species, often within the same root;
  • the symbiosis is generally beneficial, but under certain conditions the balance between symbionts may be disturbed. A fungal symbiont may decrease plant growth, conversely, certain non-photosynthetic plants may cheat the fungus by obtaining all their nutrients from them, including carbohydrates.
• Ascomiycetes:
  • diverse, common to marine, freshwater and terrestrial habitats;
  • form fruiting bodies – ascocarps, that size from micro to macroscopic;
  • over 40 % grow with green algae or cyanobacteria as lichens;
  • some from mycorrhizae with plants;
  • some release toxic compounds that help protect plants from insects;
  • important as recipients of carbon, carbon storage and as key fungi in terms of changes resulting from human-caused nutrient enrichment and imbalance.
• Basidiomycetes:
  • form fruiting bodies we know as mushrooms;
  • of all the fungi, these are best decomposers of lignin, an abundant component of wood. Many shelf fungi break down the wood of weak or damaged trees and continue to decompose the wood after the tree dies;
  • by concentrating growth in the hyphae of mushroos, a basidiomycete mycelium can quickly erect its fruiting structurtes in just a few hours – a mushroom grows as it absorbs water.

Fungi in food absorption and production

• Fungi are heterotrophic organisms, meaning they use energy stored in plant and animal biomass to create their own material for maintenance, growth and reproduction.
• They are mostly decomposers, this diverse communitiy also includes bacteria and invertebrates (nematodes, earthworms, snails, beetles, i.e.).
• Almost any carbon-containing substrate can be consumed by at least one species of fungi. Fungi and bacteria are responsible for keeping ecosystems stocked with the inorganic nutrients essential for plant growth. Without these processes, carbon, nitrogen and other elements would remain bonded in organic matter.
• They decompose proteins, simple and complex carbohydrates, such as cellulose nad lignin (this one molecule bacteria can’t decompose).
• Fungi excrete digestive enzymes which break down complex organic compounds into soluble nutrients – simple sugars, nitrates and phophates. Fungi then absorb the nutrients into their cells. Which means they have, what could be called, extracellular digestion.
• The absorption of products is never 100 % efficient and the mineralised nutrients that are not absorbed by the fungus are left in the environment, accessible to other organisms. This activity provides soil fertility for plant growth or increased nutrient content for the growth of algae in water ecosystems.
• Circulation of nitrogen: in the process od decomposition of organic matter fungi incorporate nitrogen into mycelium that is othewise locked into proteins. Fungi metabolise these proteins and release organic forms of nitrogen, such as nitrate, that can be be absorbed by plant roots. When these cells decay, nitrogen becomes accessible to plants.
• Yeasts carry out alcohol fermentation: pyruvate is converted to ethanol and ethyl alcohol.
• Cells of the fungus also convert energy stored in certain organic molecules to light, a process of bioluminiscence. The glow may attract insects that benefit the fungus by dispersing their spores (more on this topic in one of the following posts, stay tuned!).
• Fungi are rich in important nutrients, particularly nitrogen, phosphorus, minerals, and vitamins. All elements other than calcium are more concentrated in fungal tissue than the forest floor material.
• The fruiting bodies are an ideal food source for grazing animals because the production of spores demands energy and additional nutrients, so these fruiting structures contain carbon and nutrients.
• Many animals rely partially, or wholly, on fungi as a food source. Herbivorous mammals are opportunistic fungus feeders, eating fungi if they come across it while browsing in the forest. For some animals fungi makes up a large part of their diets. Many invertebrates also eat fungi, both opportunistically and actively.
• Some invertebrate animals are entirely fungiverous, whereas others ingest fungi along with plant remains or soil. For a number of vertebrates, fungi or fungal-based food serves as a primary or temporary food source for times of the year when little other food is available.

While on the subject of food, we need to know that fungi and their mushrooms also draw heavy metals, especially cadmium and mercury, also lead from the soil as they grow. The cumulation factor can range from 30 to 500 times to the amount in the soil. The amounts accumulated vary by species, environment and the substrate they grow on.

Mycorrhizae

• Mycorrhiza comes from the Greek words “myco” meaning fungus and “rhizo” meaning root, it’s an association between vascular plant roots and their symbiotic fungi. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant, thus increasing a plant’s nutrient uptake. In exchange, the plant supplies the products of photosynthesis for the metabolism of the fungus.
• Evoluton of mutualstic assotiations between roots as fungi was a vital step in the successful colonisation by plants, given the poorly developed soils at that time. Mycorrhizal fungi are responsible for most nutrient uptake by the majority of land plants.
• Mycorrrhizal hyphae provide plant roots with larger suface for absorbing water and minerals, particularly phosphate. Up to 3 m of hyphae can extend for each cm of root’s lenght, enabling access to a much greater volume of soil. Fungi secrete growth factors that stimulate roots to grow and produce antibiotics that help protect plant from pathogents in the soil.
• Mycorrhizae is vital for orchids as without fungi they cannot reproduce. Orchids are epiphytes that form small seeds without much supply to sustain germination and growth. Thus these seeds will not germinate without a mycorrhizal partner. After nutrients in the seed are used, fungal symbionts support the growth by providing necessary carbohydrates and minerals. Some orchids continue to be mycorrhizal throughout their lifecycle.

Terrestrial ecosystems

Fungi are a key group of organisms in the regulation of ecosystem processes due to the vast array of interactions among fungi and other living and dead organisms. They are important in driving the mineral and energy cycling within ecosystems.

Fungi constitute an important component of the ecosystem, they are found in all the major ecosystems of the world and play a large variety of roles. Fungi are important in soil formation, soil fertility, decomposition, primary production, secondary productio, population regulation and influence plant community composition.

• All the mushrooms we can see above ground are just a part of a single large fungus. Its subterranean network of filaments spreads through whole forests. Their diversity has enabled fungi to colonise all terrestrial habitat. Airborne spores have been found 160 km above ground.
• Reasons for their ecological success are the versatile enzymes and body structure which increases the efficiency of nutrient absorbtion.
• Fungi are major contributors to the fertility of soil by their action of decomposing organic residues derived from dead plant and animal remains.
• Saprobiontic fungi are very active in the processes of humification and mineralisation. Their functioning is important in the humus horizon (bacteria are active prodominantly in the organic horizon, where they participate in the first stages of degradation of organic matter).

Most trees live in symbiosis with tens or hundreds species of fungi. Saplings who have full access to the fungi network, have higher options of surviving. The amounts of carbon plants exchange are sufficient to serve old plants as well as saplings. This exchange occurs among all plants, not just closely related. The forest functions as a single organism. Cooperation develops between species which enable each other to survive. There are no individuals, no separate species. Every part of the forest is forest.

Fungi strategically distribute nutrients and minerals, more elements are sent to the plants that are in higher need for certain substances. Plants can exchange carbon and nutrients via mycorrhizal bridges connecting the root systems of adjacent individual plants. Thus there is a greater likelihood that the community structure and existence will be supplied during periods of stress or disturbance—ecosystem homeostasis. This proves the fact that evolution is not just competition, it’s also collaboration.

As perennial organisms, fungi are able to connect patches of different resources in the ecosystem and effect translocation of nutrients and energy among ecosystem components.

The risk of erosion is greatest where bare mineral soils are exposed. In these conditions the primary colonizers are an important component of stabilizing the mineral particles. The role of fungi in these communities of bacteria, algae in lichen symbioses, is to physically hold the soil particles together. The fungal hyphae grow between the soil particles and act as a web to physically retain thel particles. Polysaccharide secretions of both fungi and bacteria aid this process, acting as a glue to bind mineral particles together. In these highly exposed conditions, the longevity of fungi, compared with the rapid turnover of bacterial cells, is also beneficial, as this enables a more permanent soil stabilizing function.

Aquatic environments

There are more than 600 species of aquatic fungi, many of which have specific morphological and physiological adaptation to enable them to live in aquatic ecosystems.

The success of fungal species adapted for aquatic habitats is that, in comparison, terrestrial fungi are unable to break down the resources when submerged and also colonization of plants by tetraradiate and sigmoid spores of aquatic fungal species is more efficient than by rounded spores of terrestrial fungi, which are adapted for wind dispersal.

Aquatic fungi are involved in similar processes of decomposition of organic resources. Interactions between fungi and their environment leads to similar patterns of resource obtaning. Their decompostition impacts the environment upon their activity in the same manner as in the terrestrial environment.

Mycorrhizae are also known to exist in salt marshes.

Lichens

Lichens are important pioneers on newly cleared rock and soil surfaces, such as burnt forests and volcanic flows. They grow on bare rocks, tree bark and various surfaces. They break down the the surface by overgrowing and chemically disintergrating it. Lichens can survive extended periods of drought: they become completely desiccated and then rapidly become active once water is available again.

Lichens are an example of a mutualism in which a fungus (usually a member of the Ascomycota or Basidiomycota phylum) lives in close contact with a photosynthetic organism (an eukaryotic algae or a prokaryotic cyanobacterium). Neither of the photosynthetic organism can survive alone outside of the symbiotic relationship. The body of a lichen, the thallus, is formed of hyphae wrapped around the photosynthetic partner. Tissues are formed by hyphae and form most of the lichen’s mass. The algae or cyanobacteria occupy the inner layer. Algae provide carbon compounds, cyanobactera also fix and provide organic nitrogen. In return, the fungus provides minerals and protection from dryness and excessive light by encasing the algae in its mycelium. The fungus also attaches the organism to the substrate.

The physical arrangement of hypahe allows for gas exchange and retains water and minerals, most of which are absorbed either from airborne dust or rain. Fungi also secrete acids, which aid in the uptake of minerals. Fungal pigments shade the algae or cyanobacteria from sunlight, some fungal compounds are toxic and prevent lichens from being eaten by animals. Both partners reproduce independently. The thallus of lichens grows very slowly, expanding its diameter a few millimeters per year. Lichens produce soredia, clusters of algal cells surrounded by mycelia. Soredia are dispersed by wind and water and form new lichens as a way of expanding, but is not reproduction.

As optimised as liches are, many do not survive air pollution. Their passive mineral intake from rain and air makes them sensitive to sulfur dioxide and other aibourne poisons. Thus lichens fulfill many ecological roles, including as an indicator species, which allow scientists to track the health of a habitat because of their sensitivity to air pollution.

Fungi & Animal Mutualism

Fungi have evolved mutualisms with numerous insects. Arthropods depend on the fungus for protection from predators and pathogens, while the fungus obtains nutrients and a way to spread spores into new environments.

Leaf-cutting ants of Central and South America farm fungi. They cut leaves from plants and pile them up in gardens. Fungi are cultivated in these gardens, digesting the cellulose in the leaves that the ants cannot break down. Once smaller sugar molecules are produced and consumed by the fungi, the fungi in turn become a meal for the ants. Both ants and fungi benefit from the association. The fungus receives a steady supply of leaves and freedom from competition, while the ants feed on the fungi they cultivate. Such links have existed for over 50 million years. The species have become so intertwined, they can only exist together, one without the other can not survive.

Some fungi share their digestive services with animals, helping break down plant material in the guts of cattle and other grazing mammals.

Pathogens

• 30 % of fungi are pathogens, mostly of plants;
• parasitic fungi absorb nutrients from cells of living hosts. Some are pathogenic, they are responsible for 80 % of plant deseases;
• fungi are major pathogens of marine animals;
• mitosporic fungi have been shown to cause disease of crustaceans and to cause damage to corals and juvenile clams. Many oomycetes are highly destructive pathogens in finfish, mollusks, and shellfish;
• fungal pathogens of frogs and fish cause major declines in populations. From the perspective of conserving biodiversity in the tropics, the decline in anuran populations due to fungal pathogens is a highly pressing problem.

Bioremediation

• Much effort has been invested in the potential and actual use of fungal mycelia for accumulation of radionuclides and plants to accumulate pollutants (mainly heavy metals, in the process of phytoremediation). This could be a useful means to attempt a radionuclide cleanup from industrial processes and from contamination of natural environments.
• The ability of fungal species to accumulate radionuclides could be used in the restoration of radionuclide contaminated terrestrial ecosystems. The formation of large, harvestable fruiting structures (mushrooms) provides a potential means of removal of radionuclides that have been accumulated within. Mycorrhizal fungi have been shown to be a major component of the radionuclide accumulation (radiocesium concentration) in a boreal coniferous ecosystems. Mushrooms would thus be harvested and removed from the site.
• The presence of a methylation process in fungi could have important implications in the movement and toxicity of arsenic and mercury in the environment and its removal from food chains.
• Cadmium was also detected in the vacuoles of fungal hyphae.
• Fungi appear to be very resistant to radionuclides in the environment. It is possible that the presence of melanin pigment in the hyphae may provide some protection against ionizing radiation in the same way that it has been shown to protect against ultraviolet light. Due to the long-lived and extensive hyphal network, fungi appear to be very efficient in absorbing radionuclides from the environment. Internal translocation of radionuclides between sources and physiological sinks occurs in the same way as essential nutrients. This attribute has been used in industrial effluent cleanup.

We still have limited knowledge of the taxonomic diversity of fungi in ecosystems and even less understanding of their physiology. To give an idea of the magnitude, is it estimated that there may be over 3 million species of fungi on planet Earth, yet 120.000 species have been described so far.

Fun facts

• did you know fungi produce vitamin D?
• did you know that about 80 known species are bioluminescent?
• 14.000 species of mushroom-forming fungi have been described so far
• 216 known species are researched to be hallucinogenic
• composition of cultivated mushrooms: 1 % mineral salts and vitamins, 1 % fats, 3 % protein, 5 % carbohydrates, 90 % (and more) water

Latest news

There is a global fungi network research starting, mapping different ecosystems for presence of fungi in order to protect both – the ecosystems and the fungi in them:

• https://www.theguardian.com/science/2021/nov/30/worlds-vast-networks-of-underground-fungi-to-be-mapped-for-first-time
• https://spun.earth/about/

Sources:

• http://www.countrysideinfo.co.uk/fungi/struct.htm
• http://sciencing.com/fungi-contribute-ecosystem-21989.html
• http://sciencing.com/role-fungi-play-food-chains-13253.html
• http://tolweb.org/Glomeromycota
• Campbell Biology 10th edition
• https://courses.lumenlearning.com/boundless-biology/chapter/ecology-of-fungi/https://sciencing.com/fungi-contribute-ecosystem-21989.html
• Powell, J. R. Biodiversity of arbuscular mycorrhizal fungi and ecosystem function. 2018
• Koide, R. T. Determining place and process: functional traits of ectomycorrhizal fungi that affect both community structure and ecosystem function. 2014
• Hart M. M. Diversity of Arbuscular Mycorrhizal Fungi and Ecosystem Functioning. 2003
• https://www.lanl.gov/museum/news/newsletter/2018/01/fungi.php