Kingdom Protista: the single-celled eukaryotes

Between the kingdoms

Look at a drop of pond water under a microscope. You may see a green cell with a whipping tail, a clear cell that changes its shape, and a cell with a hard glassy shell. These are protists.

Kingdom Protista has all the single-celled eukaryotes. A protist has a true nucleus, with a nuclear membrane. Its boundaries are not sharp: some protists make food like plants, some hunt like animals, and some feed on dead matter like fungi. So a protist is a connecting link between the kingdoms. Many have flagella or cilia for movement. They reproduce by cell division, and also by the fusion of cells and the formation of a zygote.

Protists include the chrysophytes, the dinoflagellates, the euglenoids, the slime moulds and the protozoans. We study the first four here, and the protozoans in the next lesson.

Chrysophytes: diatoms and golden algae

• They live in fresh water and in the sea. They are microscopic, and they float with the currents, so they are plankton.
• The diatoms have a cell wall of two thin shells, one fitting over the other, like a soap box. The walls hold silica, so they are indestructible.
• When diatoms die, the walls pile up as diatomaceous earth, a gritty deposit used for polishing and for filtering oils and syrups.
• In the oceans, diatoms are the chief producers of food.

Dinoflagellates

• Most are marine and photosynthetic.
• They look yellow, green, brown, blue or red, from the pigments they have.
• The cell wall has stiff plates of cellulose on the outside.
• Most have two flagella: one along the cell, the other across it, in a furrow between the plates.
• Some red dinoflagellates, such as Gonyaulax, multiply very fast. The sea turns red: a red tide. Their toxins can kill fish and other marine animals.

You should find that $1000$ cells in a litre need $10$ divisions to pass a million. At one division every $1.5$ days, the sea turns red in about $15$ days.

Euglenoids

• Most live in fresh, still water.
• They have no cell wall. A protein-rich layer, the pellicle, makes the body flexible.
• They have two flagella, one short and one long.
• They are photosynthetic in light. In the dark, they feed on other small organisms, like heterotrophs.
• Their pigments are the same as those of higher plants. The example is Euglena.

Slime moulds

• They are saprophytes, feeding on dead matter.
• The body moves along decaying twigs and leaves, and engulfs the organic material.
• In good conditions, the cells join into a plasmodium, which may spread over several feet.
• In bad conditions, the plasmodium makes fruiting bodies with spores at their tips. The spores have true walls, survive for many years, and are carried by the air.

Try it in Python

This program is a table of the groups of protists, the covering of their cells and their food.

Practise with new numbers

Common mistakes

• Calling protists many-celled. Kingdom Protista holds the single-celled eukaryotes.
• Giving euglenoids a cell wall. They have a flexible pellicle instead.
• Mixing up diatom and dinoflagellate walls. Diatoms: silica shells. Dinoflagellates: cellulose plates.
• Calling slime moulds fungi. In the five kingdoms they are protists.

Remember these

• Kingdom Protista: single-celled eukaryotes, a link between the kingdoms.
• Diatoms have silica walls and make diatomaceous earth. Dinoflagellates have cellulose plates, two flagella, and cause red tides.
• Euglenoids have a pellicle and no cell wall. They make food in light and eat in the dark.
• Slime moulds are saprophytes that form a plasmodium and spores.

Check yourself

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

1. What is the wall of a diatom made of?

2. What does a euglenoid do in the dark?

3. What is a red tide?

4. What does a slime mould make when conditions turn bad?