Lichen is simply a symbiotic association of slow-growing microorganisms that is composed mainly of a fungus and cyanobacteria or green algae. They are a composition of twin or double organisms, and are very rich in chemical compounds as expressed by the varying colours that they produce on the surfaces where they form. Lichens produce important secondary metabolites that are of pharmaceutical, industrial and medical importance. Lichens grow on a wide variety of surfaces including tree trunks, roof tops of houses, bare soils and rock surfaces where they form colourful growths that may include yellow, red, black, white, oranges, green and brown [Figure 1]. Though an association between cyanobacteria (a prokaryote) and fungi (a eukaryote) are often the most versatile lichen association known; green algae (a eukaryote) can also go into association with a fungus (a eukaryote) to form a lichen relationship. Lichens are widespread and can be found in both terrestrial and aquatic environments. They can reproduce sexually and asexually.
The morphology of lichens is usually made up of a lichen thallus that comprises of fungal hyphae (used for attachment to surfaces) and photobiont or phycobiont cells (used for photosynthetic activity) from the partnering cyanobacteria or green algae. The fungal hyphae of the lichen are the main determinants of lichen structure or morphology, and it is generally referred to as mycobiont. Cyanobacteria are assemblages of bacteria that have the innate ability to carry out oxygenic photosynthesis. The two partners in a lichen association are usually a fungus and a cyanobacteria or green algae. The fungal partner is called a mycobiont while the cyanobacteria partner is known as a phycobiont. Notably, it is not all species or genera of fungi and cyanobacteria that can form a mutualistic lichen association. Ascomycetes (commonly known as sac fungi) are the class of fungi that go into symbiotic association with a cyanobacterium to form lichens. Both organisms in a lichen association interact amongst each other on only a mutual basis. They both derive a reciprocated type of benefit from each other such as nutrient sharing and protection from untoward effects in their surrounding environment.
Fig. 1. Lichen growth on a tree trunk
While the fungus mainly provides structure, absorb water and provide protection for the cyanobacterium; the cyanobacteria on the other hand is highly photosynthetic and thus acts as the primary food producer by providing carbohydrates for both itself and its fungal partner in the lichenized association. Their ability to colonize any surface is based on this mutual relationship between the two organisms in the lichen association. Most lichen association use cyanobacteria as their photosynthetic partner. Due to their ability to tolerate extreme conditions in their environment, cyanobacteria are widespread and ubiquitous in the soil, water and on rock surfaces. On the surfaces of lakes, streams, rivers and ponds, cyanobacteria (for example, Nostoc spp. and Anabaena spp.) are notorious in forming blooms (community of microorganisms comprising mainly of algae and cyanobacteria) that cover water surfaces and render it almost unhygienic and dirty-looking.
CATEGORY OF LICHENS
Lichens are amongst one of the least forms of life known to mankind. Their very nature which comprises of a symbiosis of two different organisms (i.e., a fungus and a cyanobacterium) is unique; and this gives them the expertise to inhabit environmental areas which are usually harsh to other forms of microbial life. Lichens unlike other forms of microbial life, come in various colours and structures as shown in Figure 1; and they can inhabit a wide variety of surfaces such as tree barks, rocks, streams, wood, and leave surfaces. Lichens are usually classified based on their shape/morphology or by their growth form on their substrates. They provide microhabitats for other forms of life such as insects and other small animals. Lichens exist in three different forms including crustose lichens, fruticose lichens and foliose lichens.
- Crustose lichens: Crustose lichens are crust-like lichens which usually inhabit the bark of trees, soils and rock surfaces. They have a flat structure (thallus) that is securely adhered to the lower surfaces of its substrate. The algae partner in crustose lichens are usually dispersed from their fungal partner. Crustose lichens are usually hard to remove from their attached surfaces; and the substrate on which they are growing can be damaged in the process of removal. The structure of the crustose lichen can also be damaged during removal from attached surfaces due to their strong attachment to their substrates. Crustose lichens are pressed against their substrates. They form a variety of colours that includes yellow, green and red. Crustose lichens can also be called crust or crusty, because they are known to form a crust or covering over their substrate or surface upon which they are growing. Examples of crustose lichens include Acarospora species and Lecanora
- Fruticose lichens: Fruticose lichens are the most highly developed forms of lichens. They can also be referred to as stalked lichens due to their structure and ability to form fruiting bodies. Fruticose lichens have different top and bottom layer; and their algae partner is usually within the fungal partner which forms a round branching fungal layer on the outside. Structurally, fruticose lichens are hair-like, upright and shrubby; and they can also be called shrubby due to their characteristic morphology that resembles the beard of an old man. Usnea species, the source of usnic acids used as potential lead compounds for antiviral drug development belong to this category of lichens. Fruticose lichens form on trees and on soils; and they generally have a vertical growth pattern even though some may assume flat branches that tangle or coil with each other.
- Foliose lichens: Foliose lichens are also known as leafy lichens due to their characteristic morphology. Lichens in this category are leafy-like in nature; and they are made up of several bumps and ridges that also define their morphology. They are leaf-like lichens which also form on tree trunks and rock surfaces. Foliose lichens have their algae partner in the middle of the surrounding fungal layer; and they usually have different top and bottom layers. They have a characteristic puffed body with a black undersurface. Examples of foliose lichens include Pseudocyphellaria species and Hypogymnia
SOME SPECIFIC FUNCTIONS OF LICHEN ACIDS (SECONDARY METABOLITES)
Lichens produce two types of metabolites: primary metabolites (for example, carbohydrates and amino acids) and secondary metabolites (for example, alkaloids and lichen acids). Primary metabolites (which are intracellularly secreted) are critical to the survival of the lichens while secondary metabolites are rarely involved in the metabolism or growth of the lichenized fungi. Secondary metabolites of lichens are generally called lichen acids, and they are usually deposited on the surface of the lichen hyphae. Thus, secondary metabolites of lichens are extracellular secretions of lichenized fungi, and they are primarily produced by the mycobiont (i.e., the fungi partner) of the lichen formation. Lichen acids exhibit numerous amounts of biological and non-biological activities; and these have been exploited by mankind to solve many health related and non-health problems. The secondary metabolites produced by lichens (which are numerous) are unique, and they are rarely sourced from other natural sources. It is noteworthy that lichens only produce secondary metabolites when a suitable fungus is in association with a functional and compatible cyanobacterium and/or green algae. Thus, a suitable fungal partner and an appropriate phycobiont are essential for the secretion of biologically active compounds of lichens. However, some of the biological functions or activities of lichen secondary metabolites are elucidated in this section.
- Lichen acids protect the cyanobacteria (phycobiont) from drying.
- They serve as anti-herbivores, thus preventing herbivorous animals from feeding on them.
- They protect the lichenized formation from possible microbial assault.
- They are used in the systematic and phylogeny of lichen classification.
- Lichen acids are used as lead compounds in drug development.
- They are used in the formulation of dyes, perfumes and even food.
- Lichen secondary metabolites possesses anti-tumor, antiviral and cytotoxic activities.
- They help make the lichenized association to be pollution tolerant.
- Lichen secondary metabolites are used for the development of antibiotics, analgesics, anti-pyretics, and antioxidants.
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