In the five-kingdom classification of Whittaker, there is no mention of some acellular organisms like viruses and viroids and lichens.

A lichen is a long-living composite organism that arises from algae or cyanobacteria (or both) living among filaments of a fungus in a mutually beneficial relationship (symbiotic relationship). The algal component of a lichen is known as phycobiont and the fungal component is called mycobiont.

The fungus benefits from the symbiotic relation because algae or cyanobacteria produce food by photosynthesis, which is utilized by fungus. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus, which absorb moisture and nutrients from the environment, and (usually) provide an anchor to it. Such relation in which both the organisms are benefited from one another is called symbiosis.

There about 400 genera and 15,000 species of lichens. They are found worldwide. They usually grow on the barks of trees, dry logs of wood, bare rocks. They are xerophytes in nature and can withstand a long period of drought.

Types of Lichens: 

On the basis of fungal components:

• Ascolychens: The fungal partner belongs to Ascomycetes
• Basidiolichens: The fungal partner belongs to Basidiomycetes
• Deuterolichens: The fungal partner belongs to Deuteromycetes

On the basis of Thallus:

Crustose Lichens:

• In this case, the thallus form crust-like structure.
• It closely adheres to the substrate.
• They are found on bark or rock.
• Examples: Graphis, Lecanora, Haematomma

Foliose Lichens:

• In this case, thallus has leaf-like lobes.
• They are fixed from the substrate by hairy rhizoids like structure called rhizines.
• They are attached only at central points.
• Example: Parmellia, Collema, Peltigera

Fruticose Lichens:

• Their thalli are cylindrical ribbon-like and branched.
• It is attached only at the base by basal mucilagenous disc.
• They are commonly called as shrubby lichens.
• Examples: Cladonia, Usnea, Alectoria

Economic Importance of Lichens:

Use as Food:

• In Iceland Cetraria islandica commonly known as “Iceland moss” is grounded up and mixed with wheat flour.
• A leathery lichen Umbillicaria, commonly known as “root tripe” has been eaten by travellers in arctic regions in danger of starvation.
• Two species of Lecanora have been used as food in the barren plains and mountains of Western Asia and Northern Africa.
• Certain classes of East Siberian inhabitants use lichens as vegetable diet.
• In Tamil Nadu, a species of Parmelia known in Telugu as “rathapu” or rock flower has been used in curry preparation and is famous for its delicacy. It is also used for the preparation of chocolates and pastries.

Use as Fodder:

• Cladonia rangifera, commonly known as “reindeer moss” grows in dense tufts of up to 12 inches height in extremely cold regions. It is food for reindeer and cattle.
• Species of Cladonia, Citraria, Evernia, Parmelia are used as fodder.

Medicinal Use:

• Usnic acid obtained from the Usnea and Cladonia species is used as an antibiotic against Gram-positive bacteria.
• Peltigera canina, the dog lichen is used as medicine for hydrophobia in ancient days.
• Lobaria pulmonaria, lungwort are used for the diseases of lungs (respiratory diseases) and T.B.
• Parmelia is useful against epilepsy.
• Usnea species are good against urinary diseases. Xanthoria sp is used in jaundice etc
• Some lichens possess anticarcinogenic properties.

Industrial Use:

• The lungwort lichen is used in tanning, in perfumery.
• The cell walls of the fungi of certain lichens contain colouring matters. Species of Rocella and Lecanora yield a most important colouring matter known as Orchil or Cudbear. This is used in colouring woolen and silk fabrics. It is also used for manufacturing litmus papers which are used in laboratories for acid and base identification. It is obtained from Roccella montagnei.
• In Russia and Sweden, lichens are used for the production of alcohol.
• Some lichens are used to manufacture Hawan Samagri like Dhoop, agar batties and other perfumes due to their pleasant smell.
• Some lichens are harmful to industrial products because they cause considerable damage to glass surfaces and marble stones due to etching.

Ecological Importance of Lichens:

Lichens Contribute to Soil Formation:

Lichens thrive in undisturbed sites where nothing else will grow. They grow on rocks, barren soil and the bark of dead or live trees. They are not parasitic when they grow on trees, they just use the tree bark as a home.

They enrich the soil by trapping water, dust, and silt. When lichens die they contribute organic matter to the soil, improving the soil so that other plants can grow there. They release carbonic and oxalic acid which leads to weathering of the rock and it leads to the formation of soil. This process is called pedogenesis.

Lichens Fix Nitrogen:

Due to their association with algae, they are able to convert nitrogen in the air into nitrates, which they need for their growth. Conversion of atmospheric nitrogen impacts the ecosystem because when it rains, nitrates are leached from lichens for use by nearby soil-based plants. Thus they make the soil fertile.

Lichens Need Clean Air:

Although they are tough and can survive in extreme climates, including extreme heat, cold and drought, they are sensitive to air pollution. Because they are so pollution-sensitive, some scientists use them to assess the air pollution coming from industrial plants and urban areas.

They absorb everything from the air, including carbon dioxide and heavy metals. Scientists can extract toxic compounds from lichens and determine the level of air pollution in a given area. The dying of lichens at a site is an early warning sign of harmful pollution.

Science > Biology > Classification of Microbes, Protists, and Fungi > Lichens

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Kingdom Fungi:

The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat.  e.g., yeast is used to make bread and beer. Other fungi cause diseases in plants and animals; wheat rust-causing Puccinia is an important example. Some are the source of antibiotics. e.g., Penicillium.

Characteristics of Kingdom Fungi:

• They are cosmopolitan and occur in air, water, soil and on animals and plants.
• They prefer to grow in warm and humid places.
• With exception of yeasts which are unicellular, fungi are filamentous and multicellular.
• Their bodies consist of long, slender thread-like structures called hyphae.  The network of hyphae is known as mycelium. Some hyphae are continuous tubes filled with multinucleated cytoplasm – these are called coenocytic hyphae. Others have septae or cross walls in their hyphae.

• The cell walls of fungi are composed of chitin and polysaccharides
• Most of them are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes.  Those that depend on living plants and animals are called parasites.  They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.
• Reproduction in them can take place by vegetative means – fragmentation, fission and budding.

Reproduction in Fungi:

Asexual reproduction:

It is by spores called conidia or sporangiospores or zoospores, and sexual reproduction is by oospores, ascospores and basidiospores. The various spores are produced in distinct structures called fruiting bodies.

Sexual Reproduction:

The sexual cycle involves the following three steps:

• Fusion of protoplasms between two motile or non-motile gametes called plasmogamy.
• Fusion of two nuclei called karyogamy.
• Meiosis in zygote resulting in haploid spores.

When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse. In some fungi the fusion of two haploid cells immediately results in diploid cells (2n). However, in other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n), i.e., two nuclei per cell occurs; such a condition is called a dikaryon and the phase is called dikaryophase of fungus. Later, the parental nuclei fuse and the cells become diploid.

The fungi, form fruiting bodies in which reduction division occurs, leading to formation of haploid spores.

Classes of Fungi:

Phycomycetes:

• Members of Phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants. They are called algal fungi.
• The mycelium is aseptate and coenocytic.
• Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile).
• These spores are endogenously produced in the sporangium.
• A zygospore is formed by the fusion of two gametes. These gametes are similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).
• Some common examples are Mucor, Rhizopus (the bread mold) and Albugo (the parasitic fungi on mustard). They are parasitic.

Ascomycetes:

• Commonly known as sac-fungi, the ascomycetes are mostly multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast (Saccharomyces).
• They are saprophytic, decomposers, parasitic or coprophilous (growing on dung).
• Mycelium is branched and septate.
• The asexual spores are conidia produced exogenously on the special mycelium called conidiophores. Conidia on germination produce mycelium.
• Sexual spores are called ascospores which are produced endogenously in sac-like asci (singular ascus). These asci are arranged in different types of fruiting bodies called ascocarps.
• Some examples are Aspergillus, Claviceps and Neurospora. Neurospora is used extensively in biochemical and genetic work.
• Many members like morels and buffles are edible and are considered delicacies.

Basidiomycetes:

• They are called club fungi. Commonly known forms of basidiomycetes are mushrooms, bracket fungi or puffballs.
• They grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.
• The mycelium is branched and septate.
• The asexual spores are generally not found, but vegetative reproduction by fragmentation is common.
• The sex organs are absent, but plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains or genotypes.
• e,.g. Agaricus (mushroom), Ustilago (smut) and Puccinia (rust fungus).

Deuteromycetes:

• It is a temporary class. Commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known.
• When the sexual forms of these fungi were discovered they were moved into classes they rightly belong to.
• It is also possible that the asexual and vegetative stage have been given one name (and placed under Deuteromycetes) and the sexual stage another (and placed under another class).
• Later when the linkages were established, the fungi were correctly identified and moved out of Deuteromycetes.
• Once perfect (sexual) stages of members of Dueteromycetes were discovered they were often moved to ascomycetes and basidiomycetes.
• The Deuteromycetes reproduce only by asexual spores known as conidia.
• The mycelium is septate and branched.
• Some members are saprophytes or parasites while a large number of them are decomposers of litter and help in mineral cycling.
• Some examples are Alternaria, Colletotrichum and Trichoderma.

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In this article, we shall get a brief idea of the classification of lower-level organisms i.e we shall study bird eye view of Kingdom Monera, Kingdom Protista, and Kingdom Fungi. In subsequent articles, we shall discuss the classification in detail. In lower-level organisms there is no tissue-level organization is present. Their one cell is capable of performing all the functions of life.

Kingdom Monera:

• Organisms of this kingdom are called lower-level organisms because no tissue-level organization is present in them. Their one cell is capable of performing all the functions of life.
• These organisms are single-celled prokaryotes and do not have a defined nucleus or organelles. Some of them have cell walls.
• Membrane-bound organelles such as the nucleus, endoplasmic reticulum, mitochondria, Golgi apparatus are absent.
• The mode of nutrition is either by synthesizing their own food (autotrophic) or getting it from the environment (heterotrophic).
• Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms. Bacteria occur almost everywhere.
• Bacteria are grouped into four categories based on their shape: The spherical Coccus (pl.: cocci), the rod-shaped Bacillus (pl.: bacilli), the comma-shaped Vibrium (pl.: vibrio) and the spiral Spirillum (pl.: spirilla).
• Examples: Bacteria -Vibrio cholerae, Salmonella typhi,  and Blue-green algae – Anabaena, Nostoc

Kingdom Protista (Greek – protistos – First of all):

• Organisms of this kingdom are called lower-level organisms because no tissue-level organization is present in them. Their one cell is capable of performing all the functions of life.
• They are unicellular prokaryotic organisms. They have a well-defined membrane-bound nucleus.
• Membrane-bound organelles such as a nucleus, endoplasmic reticulum, mitochondria, Golgi apparatus are present.
• The cell wall may or may not be present.
• These organisms use appendages, such as hair-like cilia or whip-like flagella for moving around.
• Their mode of nutrition can be autotrophic or heterotrophic. Amoeba and Paramecium ingest food like animals. Euglena has chlorophyll in them hence can synthesize their food in the presence of sunlight. In the absence of light, it shows the heterotrophic mode of nutrition by ingesting protists like protozoa. Thus Euglena has a dual mode of nutrition. Due to this dual mode of nutrition, they are placed in both plant and animal kingdom.
• Some examples are unicellular Amoeba, Paramecium, Euglena

Kingdom Fungi:

• Organisms of this kingdom are called lower-level organisms because no tissue-level organization is present in them. Their one cell is capable of performing all the functions of life.
• These are heterotrophic, eukaryotic, multicellular organisms. Exception yeast which is unicellular.
• Their bodies consist of long, slender thread-like structures called hyphae.  The network of hyphae is known as mycelium.
• They have cell walls made of a tough complex sugar called chitin.
• Most of them are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes.  Those that depend on living plants and animals are called parasites.  They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.
• Some examples are yeast, mushrooms, Penicillium.

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