Students of lichenology will probably not be surprised to read that lichen fungi can be difficult to identify, partly due to the paucity of morphological characters to go on, but also due to the repeated and independent evolution of such characters. For example, the fruticose habit has evolved repeatedly within the Lecanora-group, but also within the distantly related Arthonia -group. Unrelated fungi repeatedly evolve similar morphologies to succeed under similar conditions, making morphological identification especially difficult in some groups.
Fungi are heterotrophic, meaning that, like animals, they require a carbon source to survive. The lichen fungi share a common ecological strategy of hosting an internal population of photosynthetic cells, from which they obtain their carbon source in the form of simple sugars. These photosynthetic cells can either be green algae Chlorophyta or cyanobacteria or sometimes both, in which case the cyanobacteria are localised in distinct areas of the thallus. As the photosynthetic partners come from divergent parts of the tree of life green plants vs bacteria , the term photobiont is used as collective term for any of them.
The role of the photobiont in lichens is clear — to provide carbon in the form of simple sugars. These sugars are used by the fungi to maintain physiological functions, to grow, and reproduce. However, in the case of lichens with both green algae and cyanobacteria, the lichen gets an added nutrient input from the cyanobacteria in the form of fixed nitrogen.
Although lichens can probably access inorganic nitrogen from the atmosphere directly, it can be a limiting nutrient, so having an internal source can be an advantage especially in heavily leached environments. Only about species of photobionts are commonly found across all known lichens, representing 4 main genera. The vast majority of photobionts are from the genus Trebouxia , followed by Trentopohlia both Chlorophyta , Nostoc and Scytonema both Cyanobacteria.
Most green-algal photobionts are unicellular green forms, but small colonial types and filamentous algae occur as well. Within the lichen thallus, most photobionts have a different morphology than they would when grown in isolation, so few photobionts can be reliably identified using traditional microscopic methods.
Instead, it is best to rely on culturing studies, and more often, on molecular methods, as many different strains have very similar morphology. The jelly-lichens are one exception, where the chain-of-pearls structure of Nostoc is very clear under the microscope.
For example, the same fungal species will use different photobionts in different ecological settings, even within similar geographic areas. Some individual lichens even contain more than a single photobiont strain, a situation that has been better explored in other symbiotic systems. In corals and other marine reef organisms, the animals can regulate the populations of their varying photobionts to maximize photosynthetic output according to environmental variation.
Skip to main content Skip to search. The British Lichen Society toggle. Main menu. You are here. This species is rare in Colorado.
Photo by Gay Austin, U. Look closely at this moss. Notice how leaf-like it is. The structures on top produce spores. They are the main mode of moss reproduction. Photo by Charles Peirce, Michigan Wildflowers. Although mosses are very primitive, they still have plant-like structures that look like and function like leaves, stems and roots. They have chloroplasts throughout their entire bodies and can photosynthesize from all sides of their structures.
Lichens, on the other hand, are completely different. They do not have any roots, stems or leaves and their chloroplasts are contained only in the algae on the top surface of the lichen. Remember not to confuse lichens with moss. Photo courtesy of the U.
Xanthoparmelia sp. Can you tell which is the lichen and which is the moss? Photo by Doug Ladd. What lichens and mosses do have in common are size and habitats. In fact, mosses retain water, which is what lichens use to prolong their growth cycle. So, the next time you see a bunch of "mossy" stuff hanging from a tree or sitting on a rock, ask yourself, "Is that a lichen or a moss?
Lichens are important for several reasons. One of the most obvious is that they are beautiful to look at. How colorful would the rocks and cliffs be in the Rocky Mountains without the reds, yellows, and greens of the crust lichens?
Without these living creatures hanging off of trees or clinging to rocks, our natural areas would look pretty boring and a little more lifeless. Alectoria sarmentosa. Photo by Karen Dillman. Lichen-covered boulders. Another important function of lichens is that they provide a mode of survival in harsh environments where algae cannot normally survive. Since the fungus can protect its algae, these normally water-requiring organisms can live in dry, sunny climates without dying, as long as there are occasional rain showers or flooding to let them recharge and store food for the next drought period.
This is an example of a fruticose lichen with two-sided flat branches. Since these branches are not distinctive no top or bottom , this is still considered fruticose. Crustose lichens are just that, crusts. They form a crust over a surface, like a boulder, the soil, a car, or your roof shingles. They can come in many bright, vibrant colors like sunny yellow, orange, and red, as well as grays and greens. Crustose lichens are pressed against their substrate.
Many colorful crusts on a rock. Photo by Larry St. Lecanora garovaglii , gray crust, with other crusts, on a rock. Pleopsidium chlorophanum , yellow crust, with other crusts, on a rock. Can you imagine how two different species living in the same body reproduce? We know that to reproduce, we need to have two of the same species to produce fertile offspring.
Lichens are unique in that they are composed of two or more different species, or in fact, kingdoms. Lichens are different. Unlike plants that can produce seeds that grow into new plants, lichens do not have a straightforward way to grow more lichen. Since the fungus is the dominant partner in the relationship, it gets to develop its fruiting bodies and produce spores. These spores can produce another fungus, but unfortunately, for the alga, it does not get the opportunity to reproduce at all.
Either the new fungus has to find an algal partner or it perishes. Acarospora badiofusca , a crustose lichen, with several fruiting bodies. Many spores are produce from one fruiting body. Peltigera didactyla , dog-pelt lichen. This young specimen is covered in soredia, which are vegetative granules of algae and fungi intertwined and ready for dispersal.
Of course, evolution must go on, and lichens have adapted to their bi-specific struggle. These organisms can vegetatively reproduce; that is, they have structures specifically developed for the spread of fragments of their thalli, with both partners.
Not only are there specific vegetative structures, but simple fragments of lichen can grow into full-size thalli. Breadcrumb Home Celebrating Wildflowers. Lichen Biology Structure Lichens do not have a waxy cuticle like plants have on their leaves, nor do they have vascular tissue such as xylem and phloem to move nutrients and water around their thalli as a plant does. The general structure of a lichen is composed of layers of fungus and alga. Algal Layer You can usually tell what kind of alga a lichen has just by color alone.
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