Experiment with Protozoa

Materials:

• Pond water or a protozoa culture kit
• Microscope slides and coverslips
• Medicine dropper or pipet
• Compound microscope (with at least 400x)
• Methyl cellulose or a cotton ball (Optional)

What to do:

If you are using a protozoa culture kit, protists will usually begin to appear after 24 hours with the most variety after about 3 days. Different creatures will grow at different depths of your cup of water, so take samples from different parts of the cup.

1. Use a pipet to take a sample of the water and place 1-2 drops on a plain microscope slide. Cover the drops with a coverslip.
2. Examine the slide with your microscope starting at 40x. Most protists have little color and are difficult to see in bright light, so turn your microscope diaphragm to the lowest light setting. It will take patience to adjust the lighting and focus the microscope.
3. Initially you will see very tiny dots moving around on the slide. Some move very rapidly, others more slowly. You can slow them down for observation by adding a drop of methyl cellulose, or you can place a few fibers from a cotton ball on the slide. The fibers will act as obstacles to prevent the protists from moving out of the field of view too quickly.
4. Once you find an area of protist activity on the slide, turn the magnification up to 100x or even 400x to see them better.
5. If no animals are visible, try again each following day. Many conditions, such as water hardness, temperature, and water acidity, can affect the growth and development rate of these organisms. Each succeeding day you will typically find more and different varieties of protozoa in your culture. Initially, smaller species will be prevalent. As the days pass larger species will appear. You will also see different algae forms appear. Certain species will be more common from the top of the cup and others from near the bottom. Gradually, food and water conditions will change, affecting the growth and development rates of the different protozoa.

What to look for:

Type of movement: Protozoa use different methods of locomotion and are usually categorized based on how they move. An amoeba uses slow amoeboid movement, flowing along with pseudopods, or temporary foot-like extensions. One part of its cell wall flows out, looking like a foot, and then pulls the rest of the amoeba after it. (This is also the way the white blood cells in our bodies move.) Creatures like a euglena move with fast flagellate movement. They propel themselves with one or two whip-like flagella. Other protists, like paramecium, use ciliate movement. They are covered with tiny hair-like threads called cilia that beat back and forth rhythmically, propelling them through water. Flagella and cilia can be hard to see—try reducing the light entering the microscope and increasing the magnification.

Eating method: Eating habits amongst protozoans vary, too. Some protists, such as euglena or volvox (a type of algae), use chloroplasts to generate energy through photosynthesis, similar to plants. Euglena also serve as decomposers, by feeding off dead organisms. The amoeba, on the other hand, engulfs its prey with its pseudopodia and brings the food into its food vacuole (a sac where food is stored until digested). A paramecium sweeps its food down an oral groove lined with cilia into a gullet that closes off when full and becomes a food vacuole.

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