Lichens and the limits of the five kingdoms

Two organisms, one body

On an old wall or on the bark of a tree you may see a grey, green or orange crust. It is a lichen. It looks like one organism, but it is two.

A lichen is a symbiotic association between an alga and a fungus. Symbiotic means that both partners gain.

• The algal partner is the phycobiont. It is autotrophic, and it makes food for both.
• The fungal partner is the mycobiont. It is heterotrophic. It gives shelter, and takes in water and mineral nutrients for the alga.

The partners are so close that, in nature, you cannot separate them.

Lichens grow slowly

You should find that a patch $60\ \text{mm}$ wide, growing $1.5\ \text{mm}$ a year, is about $40$ years old. Slow growth is why a lichen-covered wall is usually an old one.

Lichens and pollution

Lichens are very good pollution indicators. They do not grow in polluted areas. Many are very sensitive to certain gases in the air. Rich lichen growth on trees in a town means clean air. None may mean polluted air.

What the five kingdoms leave out

Whittaker's system covers organisms made of cells, with one kingdom for each main kind of cell and body. It does not include:

• viruses, viroids and prions, which are not cells;
• lichens, which are a partnership of two kingdoms, not one organism.

So a system of classification is a tool, made by people to put order in knowledge. When we learn new facts, the system is improved. Other systems exist, and new methods, such as the study of DNA, still change our ideas.

Try it in Python

Here is a small set of data, made up for this lesson: the number of lichen species on trees at six distances from a busy road. The program finds how closely the two go together, and draws a graph.

Practise with new numbers

Common mistakes

• Calling a lichen a plant. It is an alga and a fungus living together.
• Swapping the partners' names. Phycobiont: the alga. Mycobiont: the fungus.
• Thinking lichens like polluted air. They are missing where the air is polluted.
• Thinking the five kingdoms cover everything. Viruses, viroids, prions and lichens are left out.

Remember these

• A lichen is an alga (phycobiont) and a fungus (mycobiont) living together, to the benefit of both.
• The alga makes food. The fungus gives shelter and takes in water and minerals.
• Lichens grow slowly, and they are pollution indicators: they do not grow where the air is polluted.
• Whittaker's five kingdoms do not include viruses, viroids, prions and lichens.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which partner of a lichen is the phycobiont?

2. Why does the alga need the fungus?

3. What does the absence of lichens on the trees of a town suggest?

4. Name two kinds of things that Whittaker's five kingdoms leave out.