Part: III - Lichens

Introduction to Lichens Lichens are a small group of curious plants. They are made up of algae and fungal components, living together in a symbiotic relationship. The algal component is known as phycobiont (phykos - algae, bios - life), and the fungal component is known as mycobiont (mykes - fungus, bios - life). The plant body of lichens neither resembles algae nor fungi. Thus, lichens are an association of a fungus and an algal photosynthetic symbiont residing in a stable thallus of specific structure. Lichens are biology's extreme survivors - they live in the driest deserts and wettest forests, on the world's highest summits, and in the coldest regions. General Characteristics of Lichens Lichens have a composite thallus structure formed by the association of algae and fungi. The algal component of lichens forms carbohydrates by photosynthesis, which are utilized both by the algae and the fungus. The fungal component is responsible for the absorption and retention of water.   On the basis of the structure of the thallus, lichens have been classified into three broad types: crustose, foliose, and fruticose. They reproduce asexually by the formation of various types of spores, such as soredia, pycnidia, and isidia. Only sexual reproduction is known in certain genera of lichen. The female sex organ is known as carpogonium, while the male sex organ is known as spermatium. The fruiting bodies of lichens are disc-shaped apothecia or flask-shaped perithecia. The ascus produces eight ascospores. Upon germination, the ascospore comes in contact with a suitable substrate and forms a lichen thallus. The growth of lichens is extremely slow. They can survive high temperatures and desiccation for a long time. Hence, they can grow easily in dry and barren areas. Lichens produce a specific acid known as lichen acid. Introduction to Lichens Occurrence Lichens are found in diverse climates and also grow on diverse substrates. They are abundant on rocks, trees, leaves, soil, and other lichens. They can survive in harsh environments like deserts, polar regions, and rain forests. They are found in various habitats like swamps, heaths, pine forests, branchless trees, arctic and alpine zones, and mangrove forests. They are able to form long-lived, stable communities. Nature of Association Lichens are organisms formed by the symbiotic association between algae and fungi. The algal partner is called phycobiont, and the fungal partner is called mycobiont. There are two views regarding the nature of association in lichens: Symbiosis: De Bary (1879) termed this association as symbiosis where both partners are benefitted. Algae gets mechanical protection by fungal hyphae network. Fungus absorbs more water and nutrients from the substrate, thus promoting algal growth. Fungus gets organic food from algae as algae is photosynthetic and produces organic food. Helotium: Some researchers defined this association as helotism, a master-slave relationship where algae act as a slave for the fungus master. Algae acts as a slave and prepares food for the fungus master, which some researchers believe. Algae produces haustoria-like projections when associated with fungi. The fungal influence inhibits the sexual reproduction and adoption of a slow growth order in algae. Introduction to Lichens Thallus Organization The plant body of lichens is thalloid and irregular in shape. It is usually grey or brown in color, but some species are yellow, red, orange, and green. The thallus shows diversity in morphology of lichens. There are also different growth forms of lichens. Based on thallus morphology, lichens are divided into three major groups: (i) Crustose (crustaceous) lichen (ii) Foliose lichen (iii) Fruticose lichen (i) Crustose Lichens Crustose lichens do not have well-organized thallus. These are encrusting lichens, firmly adhered to the substratum and do not have a distinct thallus. They are commonly embedded in the substratum and the fine thallus is barely visible above the surface of the substratum. Crustose lichens are considered as the most primitive type of thallus organization in lichens. Examples of crustose lichens: Graphis, Lecanora, Lecidia, Strigula, etc.   (ii) Foliose Lichens Foliose lichens have flat, lobed, leaf-like thallus. They look like the thallus of liverworts (bryophytes). They are attached to the substrate with the help of rhizoid-like structures called rhizines. Foliose lichens are more advanced than crustose lichens. Examples: Collema, Physcia, Xanthoria, Parmelia, Cladonia, etc.   (iii) Fruticose Lichens Fruticose lichens have well-developed, shrub-like, cylindrical, and branched thallus. They grow on or hang down from the substrate, such as tree trunks or rock surfaces. Fruticose lichens have well-organized internal structure and are considered as the most advanced thallus organization in lichens. Examples: Usnea, Alectoria, Cladonia, Letharia, etc. Growth Forms of Lichens Lichens are found in a wide variety of shapes and forms. The vegetative body of lichen is known as the thallus. Thallus is the most prominent and morphologically visible portion of lichens, and the grouping of lichens is mainly based on their morphological nature of thallus. The different growth forms of lichen thalli are: Leprose: These are minute scales, attached superficially to the substratum. Squamulose: Lichens where thallus contains minute scale-like squamules. Some workers group them under crustose lichens. When a crustose lichen grows old, it develops cracks which are called areoles. When the edges of the areoles are lifted, the thallus is called squamulose. Placodioid: When the crustose lichen grows from the center and appears to radiate out, it's called crustose placodioid type. Gelatinous Form: The thallus is gelatinous but when dried, it looks like a leafy one. Filamentous Form: These are stringy or hair-like masses and look like teased wool. Byssoid Forms: Gelatinous and stringy, but they look like teased wool. Introduction to Lichens Growth Forms There are variations in lichen growth forms, even in a single lichen species. Foliose lichens may branch and look like fruticose lichens, and fruticose lichens may become leafy or foliose. Some lichens may grow inside the rocks with their fruiting bodies only visible outside the rocks. Such lichens are called endolithic lichens. Reproduction Lichens reproduce by vegetative, asexual, and sexual methods. Vegetative Reproduction Vegetative reproduction occurs by fragmentation, soredia, and cephalodions. Fragmentation: Small fragments of the thallus break off and develop into a new thallus. Soredia: These are small, bud-like outgrowths containing both algal and fungal components. They are easily detached from the thallus by wind or rain. Cephalodia: These are pustule-like structures formed on the thallus. They contain algal cells different from those of the thallus. They may originate from the thallus or may be derived from other lichens. Sexual Reproduction Sexual reproduction in lichens is rare and occurs only under special conditions. The reproductive organs of lichens are not formed separately. The ascogonium and antheridium develop from ordinary vegetative cells. The ascogonium is a coiled, sac-like structure that contains the female nucleus. The antheridium is a small, uninucleate hypha that grows towards the ascogonium. The antheridium fuses with the ascogonium, resulting in karyogamy. The diploid zygote undergoes meiosis to form eight haploid ascospores. The ascospores are arranged in a linear or sac-like structure called the ascus. The ascus is surrounded by sterile hyphae, which form the peridium or the wall of the fruiting body. Ascospores are released by the rupture of the peridium and germinate to form new thalli. Sexual Reproduction The sexual reproduction in lichens is initiated by the formation of a specialized structure called the carpogonium. The carpogonium is a coiled, sac-like structure, usually differentiated into a basal portion and a trichogyne. The trichogyne emerges from the surface of the thallus and receives the spermatia. The spermatia are flagellated male gametes produced by the male sex organ, the spermogonium. Spermogonia usually develop on the lower surface of the thallus. When a spermatium comes in contact with the trichogyne, the nucleus of the spermatium migrates into the carpogonium. This results in plasmogamy, where the male and female nuclei fuse. The fertilized carpogonium develops into ascogenous hyphae. These hyphae are branched and multinucleate. The terminal or penultimate binucleate cell of the ascogenous hypha develops into an ascus. The two nuclei within the ascus fuse to form a diploid nucleus. The diploid nucleus undergoes meiosis and then mitosis to form eight haploid nuclei.   These haploid nuclei develop into ascospores. The ascospores are arranged in a linear or sac-like structure called the ascus. The ascus is surrounded by sterile hyphae, which form the peridium or the wall of the fruiting body. The fruiting body is called the apothecium in lichens. Ascospores are released by the rupture of the peridium. After liberation, the ascospores germinate to form new thalli. Mycorrhizae The term "mycorrhiza" was first introduced by Frank in 1885. It means "fungus roots." It refers to the symbiotic relationship between certain fungi and the roots of plants. It is a symbiotic mutualistic association where both organisms benefit from the association. The association between the host plant, fungi, and other soil factors is a three-way interaction. The mycorrhizae have been divided into two groups: endomycorrhizae and ectomycorrhizae. Types of Mycorrhizae There are several types of mycorrhizae associations, but the two major types are endomycorrhizae and ectomycorrhizae. Endomycorrhizae In an endomycorrhizal association, the hyphae invade the cortical region of the host root without penetrating the vascular bundles. The main distinguishing features of this type of mycorrhizae are: The formation of a hyphal network called Hartig net around the cortical cells of the root. The infection of plant roots with ectomycorrhizal fungi leads to changes in the feeder roots. The feeder roots become swollen, knobby, branched, and often unicolored. These roots become thicker and more branched due to the increased number of cells. Significance of Ectomycorrhizae It is a mutualistic association between the fungus and the plant. Both organisms are benefited. The fungus provides the source of carbon (food) and energy. It is estimated that up to 10% of the photosynthetic products of the trees are passed to the mycorrhizae and other rhizosphere organisms. The plant provides the organic matter and mineral nutrients required by the fungus. The fungus helps in the absorption of water and mineral nutrients from the soil. It improves the growth and development of the plant. It increases the resistance of the plant to diseases and pathogens. It helps in the formation of aggregates in the soil, which improves soil structure and water retention. Mycorrhizae Benefits to the Tree Numerous studies have shown that the best plant growth occurs when mycorrhizae are present. Enhanced Growth: Increased supply of nutrients to the trees Increased volume of soil roots The extra mycelial hyphae extend over a long distance in the soil, thus increasing the availability of phosphorus by solubilizing. Several hormones produced by the fungus change the physiological status of roots, leading to an increase in the absorption area of roots. The plant becomes tolerant to drought, high temperature, pH extremes, and heavy metals. Disease Resistance: The presence of a physical shield formed by the network of fungal hyphae around the root protects the roots from pathogens. Ectomycorrhizal fungi produce certain antibiotics that reduce the incidence of root pathogens. Ecological Significance Ectomycorrhizal fungi play a significant role in forest ecosystems. They help in soil aggregation and soil structure. They detoxify soils contaminated with heavy metals. They increase the organic matter content in the soil. They form a major link between producers and soil food webs. They provide organic matter and fixed carbon to soil food webs, including bacteria, protozoa, arthropods, and micro-fungi. They produce organic acids that increase the availability of nutrients in the soil. They play a crucial role in forest ecosystem restoration and conservation. Mycorrhizae Endomycorrhizae Endomycorrhizal associations are more common than ectomycorrhizal ones and are found in almost all groups of plants, higher plants. The best known endomycorrhizae are those of orchids. Orchids are known to show stunted growth and poor development unless associated with mycorrhizal fungi. In endomycorrhizal associations, the fungi penetrate the cortical cells and form clusters of finely divided hyphae known as arbuscules. The hyphae may also form vesicles. There have been further classified into five different types based on their structure and mycorrhizal associations: ericoid, orchidoid, arbuscular, Paris type, and monotropid. The arbuscular mycorrhizae were previously known as vesicular-arbuscular mycorrhizae (VAM). In this type, the infecting hyphae of the fungus enter the cortical cells, branch profusely, and give rise to balloon-like structures called vesicles. These vesicles may store nutrients. The hyphae penetrate the cortical cells, producing intracellular structures known as arbuscules. These arbuscules eventually degenerate, thus releasing nutrients to the host plant. Significance of Endomycorrhizae (AM - Arbuscular Mycorrhizae) Arbuscular mycorrhizal associations are also very significant with respect to the ecology and utilization of soil nutrients. In an AM association, both the symbionts, the fungus and the plant, benefit. The fungus obtains carbon compounds and energy from the plant. The plant receives phosphorus and other nutrients from the fungus. The fine, thread-like hyphae extend from the root into the soil, increasing the surface area of the root and access to soil nutrients. The fungus exudes acids to break down soil minerals and bring these minerals to the plant in a usable form. Phosphorus is a limiting factor for plant growth, and it becomes available by the action of AM fungi. Mycorrhizae Benefits to the Tree Nitrogen-fixing bacteria: Mycorrhizal fungi can enhance the activity of nitrogen-fixing bacteria. Phosphorus availability: Mycorrhizal fungi can improve phosphorus uptake from the soil. Glomalin: A sticky substance produced by AMF, which binds soil particles together, improves soil structure, and increases water infiltration. Ecological Significance Mycorrhizal associations can alter the microbial community in the rhizosphere. They can encourage beneficial organisms and discourage harmful ones. Mycorrhizal fungi produce antibiotics that suppress pathogens or compete with pathogens for nutrients. They can activate plant defense mechanisms. Glomalin A super glue produced by AMF. It is a sticky substance that binds soil particles together, increasing the soil's aggregation and stability. It makes the soil more porous and allows for better water infiltration and reduces water erosion. It gives the soil its exclusive quality known as "krumella." Glomalin coats the surface of mycorrhizal spores and hyphae. It helps in the transport of nutrients by the hyphae. Glomalin is 30-40% carbon and makes up to 27% of soil carbon stock.