Biology 2
Lichen Observation Lab
What is a Lichen?
Structurally, lichens are among the most bizarre of all forms of life. That's because every lichen species is actually composed of two, possibly even three, distinct species of organisms. One species is a kind of fungus. Usually the other species is an alga, but sometimes it can be a photosynthesizing bacterium known as a cyanobacterium. Sometimes all three organisms are found in one lichen.
The drawing below gives an idea of what fungal hyphae wrapping around alga cells might look like at the microscopic level.
In this amazing association the fungus benefits from the algae because fungi, having no chlorophyll, can't photosynthesize their own food. A lichen's fungal part is thus "fed" by its photosynthesizing algal part. The algae benefit from the association because the fungus is better able to find, soak up, and retain water and nutrients than the algae. Also, the fungus gives the resulting lichen shape, and provides the reproductive structures. This kind of relationship between two or more organisms, where both organisms benefit, is known as mutualism. The photobiont is the algae or green bacteria part of the lichen. Its role is to undergo photosynthesis and provide food for all partners in the lichen relationship. The mycobiont is the fungus portion of the lichen. Its role is to envelope and protect the photobiont from drying out and also absorbs the moisture for both partners in the lichen relationship.
The main body of a lichen is called a thallus, while the tiny cup-shaped structures are known as the ascocarps. Inside the ascocarps are the spores that are produced by the mycobiont. Since this is a cup-like structure the type of fungus belongs to Phylum Ascomycota. See the diagram below for more detail.
At
the left you see the British Soldier Lichen, Cladonia cristatella. It's only
about ¼-inch high (6 mm). In this common lichen the red spore-producing
reproductive structures are clearly visible. The lichen's name, Cladonia
cristatella, is actually the name of the fungus. The alga species in the lichen
is known as Trebouxia erici. However, it's customary to name a lichen after its
fungal part, so the whole lichen is known as Cladonia cristatella. British
Soldiers are usually find these on decaying wood, soil, mossy logs, tree bases, and
stumps. They help break down old wood and put nutrients back into the soil where
they can be used by plants. Lichens also take nitrogen from the air and put it
into the soil so plants can use it.
Lichens
are very sensitive to air pollution, so if your town has dirty air your backyard
may not have many lichens to study. Moreover, unless you know what you're
looking for, you can be staring right at a healthy lichen and not even know it.
On the other hand, the picture at the right is a scan of a lichen-covered twig
fallen from a big Pecan tree (only a
little magnified) so obviously the air pollution was not that serious where this
sample was taken. The lichen shown below has absorbed some of the air
pollutants (sulfur dioxide) and the stress of these toxins is evident in the
color change of the lichen. What normally should be green, is now a
yellowish-brown color.
The map below shows the levels of sulfur dioxide in the air in micrometers per liter. Notice where the larger concentration of sulfur dioxide is occurring on the map. The east coast has more due to the industrial factories.
Lichens occur in one of four basic growth forms, as illustrated below:
Despite the wide diversity of the basic growth forms, all lichens have a similar internal morphology. The bulk of the lichen's body is formed from filaments of the fungal partner, and the relative density of these filaments defines the layers within the lichen.
At its outer surface, where it comes in contact with the environment, the filaments are packed tightly together to form the cortex. The dense cortex serves to keep out other organisms, and helps to reduce the intensity of light which may damage the alga cells.
The algal partner cells are distributed just below the cortex in a layer where the fungal filaments are not so dense. This is very similar to the arrangement in a plant leaf, where the photosynthetic cells are loosely packed to allow air circulation.
Below the algal layer is the medulla, a loosely woven layer of fungal filaments. In foliose lichens, there is a second cortex below the medulla, but in crustose and squamulose lichens, the medulla is in direct contact with the underlying substrate, to which the lichen is attached. When mature, the lichen will produce a fruiting body that will then produce the spores.
Lichens reproduce in two main ways:
1. The fungus part produces reproductive structures that further produce spores. If a spore lands and germinates, and the resulting hypha finds the right species of alga in the neighborhood, the hypha will grow through the algal cells and a new lichen will start developing.
2. By asexual (vegetative) techniques. One asexual strategy is that of fragmentation, which simply involves a piece of a lichen breaking off and this fragment then grows into a new lichen. Lichens also produce on their surfaces microscopic, dust-like particles composed of one or several algal cells closely enveloped by fungus hyphae. These are known as soredia. Each soredium can produce a new plant. Lichen fragments and soredia can be transported great distances by wind and water.
Lichens absorb water and minerals from rainwater and directly from the atmosphere, over their entire surface area. This makes them extremely sensitive to atmospheric pollution. As a result, there are usually very few lichens around industrial centers and towns. One particular pollutant, sulfur dioxide, can be harmful to the lichen. Sulfur dioxide is a key component to factory and car emmissions.
Different lichen species vary in their tolerance to pollution and therefore make very good biological indicators of levels of atmospheric pollution. This means that they are able to provide information about the quality of the air or water in an environment. The reactions of the lichens to the pollutants can vary depending on the pollutant. Some reactions may include: changing colors from a greenish-gray to a bright yellow or orange. This is due to the absorption of the sulfur dioxide particles. In other cases it may just turn a dark grey color, to indicate that their is not enough moisture in the air.
In worst case scenarios, the lichen may actually die.
A walk around your local cemetary can often reveal a lot about air quality in your area. Graveyards are usually relatively undisturbed areas, with stone headstones which provide a good substrate for lichens. A good look at these lichens will give an indication of how good the air quality is locally.
Lichens have had a wide variety of uses over the ages. Before the advent of modern dyes they were extremely important sources of dyes for clothing. Different lichens yielded different dye colours and they could be mixed to produce a wide variety of colours.
Lichens also have an interesting chemistry and produce a large number of acids, many of them found only in lichens. The litmus dye used so widely as an acid/alkaline indicator in chemistry comes from lichens. Some species also have antibiotic properties. Some of the lichen acids are utilized in drugs that can be more effective than penicillin.
One of the more bizarre uses of lichens from the past is as packing material for ancient Egyptian mummies
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