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Monterey Bay Aquarium Research Institute
Marine Botany

Caloplaca coralloides

Symbiosis

scanning electron micrograph of algal layer in apothecium showing algae embedded in fungal hyphae

This is a scanning electron micrograph
of the green algal cells entangled in
the fungal hyphae in the algal layer of an
apothecium of Caloplaca coralloides.
The algae are dehydrated and therefore
collapsed. Photograph taken by Megan
Kelso with the help of Chris Patton.
The symbiosis in lichens is between a fungus and a green alga or cyanobacterium (blue-green alga). The fungal component is called the mycobiont and is an ascomycete or a basidiomycete fungus. Lichens are defined by their symbiosis with algae, which is a method of nutrition, not due to a common ancestry. All kinds of unrelated fungi form lichens. Therefore, lichens cannot be grouped easily onto one branch of a phylogenetic tree.

cross section of apothecium showing spores in asci, pencil sketchThe fungal component of C. coralloides is an ascomycete fungus, meaning that its spores develop and are released from sacs called asci. To the right is a drawing of a cross section of the fruiting body of Caloplaca coralloides showing the eight ascospores in asci within the fertile layer of the fruiting body. There are 15,000 species of ascomycete fungi that form lichens. The fungal components of some other lichens are basidiomycetes, which are mushroom forming fungi.

The composite thallus of a lichen does not reproduce sexually; only the fungus produces sexual spores. Therefore, for these fungal spores to successfully grow into a lichen they must encounter a suitable alga on the substratum once they have germinated.

The lichen symbiosis is different than other kinds of symbioses because the lichen takes on a new body shape that neither the fungus nor the alga had independently. Without the alga, the fungus would not develop the organized tissue layers present in the lichen. The alga is able somehow to “turn on” the fungal genes that control morphogenesis.

The algal and fungal components of the lichen need to grow at similar rates for the lichen to survive. If one grows faster than the other, then the lichen breaks down. This requirement explains why lichens grow slowly.

The fungal component of a lichen is very specific about which alga it forms a symbiosis with, and usually a species of lichen has the same species of alga. However, some species of lichens have different photobionts in different parts of their geographic ranges. Often lichens maintain the same morphology even when they have different photobionts. It is also possible, although more rare, for lichens to have more than one photobiont. For example, some lichens have both a green algal component and a blue-green component (C. coralloides does not). Having cyanobacteria is particularly useful for lichens living in nitrogen-poor environments such as on rock or bare soil because the cyanobacteria can fix atmospheric nitrogen into a form useable by the lichen. In a lichen that is harboring both green and blue-green algae, the blue-greens are often contained within gall-like growths called cephalodia.

light microscope picture of green alga zoom out to pencil sketch of green algal structure
Many different lichens have the same photobiont. The most common alga is Trebouxia, a green alga that is rarely found free-living.  An alga common in crusts is Trentepohlia. Blue-green algae are mostly found in dark brown or black gelatinous and filamentous lichens. The type of alga contributes to the color, overall shape and gelatinous consistency of the thallus of a lichen. C. coralloides has a green alga symbiont.

There are 25 genera of green algae, a few golden algae, one brown alga and 12 genera of cyanobacteria that are known to become lichenized.

The fungus recognizes its alga by lectins, a kind of protein on the cell wall of the alga.

It is often difficult for people to identify the algal component of a lichen because the alga is modified by the fungus. To identify the alga in a lichen, it must be separated out and cultured. Only 2-3% of lichens have photobionts identified to the species level (Brodo). On the other hand, cyanobacteria don’t change much in symbiosis—they’re still identifiable.

The fungus provides the alga with a more steady supply of moisture, structure (the lichen thallus can allow more surface area from which to absorb light for photosynthesis) and sun protection (by way of sunscreen pigments).  Algae are damaged by too much light. By living symbiotically with the fungus, the alga gets to live places that they wouldn’t otherwise have been able to live in.

In return the alga provides the fungus with food.  Being associated with the fungus makes the cell walls of the alga become somewhat permeable to carbohydrates, so that some of the carbohydrates that it produces leak out and can be absorbed by the fungal tissue. The carbohydrates that leak out of the alga are sugar alcohols such as ribitol and sorbitol (from green algae) or glucose (from cyanobacteria). These substances are stored in the fungus as mannitol.

The fungus has special food-absorbing hyphae called haustoria. Different growth forms of lichen have different types of haustoria. Fruticose and foliose lichens have haustoria that don’t penetrate the algal cell, while crustose lichens have haustoria that do penetrate the algal cell. Therefore, because C.coralloides is a fruticose lichen, its haustoria are the kind that do not penetrate the photobiont cell.

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2005 Megan Kelso

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