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BIO 112 Spring 2016

Protozoan Population Ecology and Interactions- Week 1

Modified from the Davidson College 2015 Lab Manual

Laboratory Objective for Week 1

Population Growth Curves and Rates

You and your group will study the growth of a focal population and how it might be affected by an interaction with another species beginning next week. You will relate the growth of your population to the mathematical situations discussed in Bio-Math Explorations 24.1 (week 1) and 24.2 (week 2) on exponential and logistic growth rates. In those BMEs you will also explore growth rates and doubling times, parameters that we can measure in our test populations and compare between control and treatment situations. After you have been introduced to the concept of population ecology, you will design a study to investigate some aspect of population ecology of single-celled organisms called protozoans, such as competition or predation.

Protozoans Available for Study:

Observing the Species

You will have access to five two species of ciliated protozoans (Table 1; see below). With the exception of the predator Didinium. These species feed on organic particles and bacteria, which they filter from the water. Each species will be available in a separate stoppered flask as a single-species culture. Each culture will have its own pipets, all distinctly labelled. Do not get these pipets mixed up . This may lead to cross-contamination of your samples. Observe each species and record your observations in Table 1 (see below).

Paramecium caudatum: has a typical “paramecium” shape and is ~250–300 μm in length.

Spirostomum ambiguum: This is a very long (up to 2–3 mm!), club-shaped ciliate. Its size and shape are unlike any of the other species.

Other Species in Your Cultures

You probably noticed many other inhabitants in the cultures. Most of these are considerably smaller than the key species and include other protozoans and rotifers (tiny animals). These species are a natural part of the community in which the species described above live. In your experiments you will not be collecting data on these smaller organisms. Since the community of these organisms is essentially the same in all the cultures you will set up, we will ignore them.

Techniques

Counting Population Numbers

Microscopes and counting plates will be used for determining the numbers of organisms present in one-drop samples taken from your cultures. Pasteur pipets will be used to remove samples. Carefully use the following procedure in all data collection.

1. Protozoans tend to cluster on the bottom of the vessel, so gently but thoroughly agitate the culture to be sampled to obtain representative samples. Carefully swirl stir the culture vial with the pipete and while vigorously stirring the culture with a pipet, squeeze and release the bulb to fill it.

2. Hold the pipet at a 45° angle and carefully release two or three drops back into the culture. The next drop should be put into a depression on the counting plate. This operation must be done quickly so that organisms don't begin to settle within the pipet.

3. Release the rest of the sample back into the correct culture vial.

4. Repeat steps 1, 2, and 3 until sufficient five samples have been removed from the culture and placed in separate counting plate depressions.

5. Focus on the first depression in the counting plate, with the black stage disk in position, and carefully count all the relevant organisms. If they are moving too fast to count, you can add a drop of Protoslo to the depression to slow them down.

6. Repeat for all samples on the plate. Use at least five drops to estimate the population density of a culture and calculate an average from these numbers.

7. If more than 15 individuals per drop are present in your cultures, you may need to dilute the sample to make counting easier. Use two counting plates to do this.

a) First add the one-drop samples to the depressions on the first plate.

b) Add enough drops of spring water to dilute the samples.

c) While stirring the contents in the depressions of the first plate, remove one drop and put it in the appropriate depression of the second plate.

8. There are ~20 drops in 1 ml, so each drop equals 0.05 ml. Calculate the average per drop and convert to #/ml. Use a dilution factor if you had to dilute your sample prior to counting. For example, if you diluted one drop of culture with three drops of spring water and then counted 16 organisms in one drop of the diluted sample the number per ml in the original culture would be: (number in diluted sample) × (dilution factor) × 20 drops/ml or 16 organisms/drop × 4 × 20 drops/ml

· 1280 organisms/ml. The dilution factor equals the total number of drops in the diluted sample.

9. Caution: Use only intact Pasteur pipets for sampling. If the end is broken it will release a larger drop and should be discarded. Carefully attach tape labels to your pipets so you don't contaminate cultures with other species.

Copy the following tables into your notebook, or complete them here and paste them in later.

Table 1. Distinguishing features of five species of ciliated protozoans.

Species	Size	Shape	Color	Behavior
Paramecium caudatum
Spirostomum Ambiguum

Table 2. Population density estimates for each species of ciliated protozoans.

Sample	P. caudatum	S. ambiguum
1
2
3
4
5
Average
Concentration (per ml)

Name: _____________________

Prelab 7-

BME 24.1- Exponential Population Growth

Practice Problems to help you determine the number of cells found in a number of generations :

· 1: Write the # of cells after 1, 2, 3, 4 and 5 generations by multiplying 8 (initial # of cells) by 2 the appropriate times.

· How many times did you multiply by 2 to find the number of cells after 3 generations?

· After 5 generations?

· How many times would you need to multiply by 2 to get the number of cells after n generations?

· 2: Another way to represent repeated multiplication by 2 is to multiply the 2’s first, and a convenient way to write this in a formula is with an exponent. Express 2 x 2 x 2 x 2 x 2 as a power of 2 (i.e., 2 raised to a power).

· Express # of cells after 5 generations as the initial # of cells times the appropriate power of 2.

· Repeat for the number of cells after n generations.

· 3: Use the formula you developed in BME IQ #2 to estimate the number of cells after 10 generations, and after 20 generations.

· 4: Following the logic above, find a formula for the number of cells in a population that starts with 8 cells and divides every 20 minutes. Write the formula as a function of time, t. Repeat for a population that divides every 45 minutes.

· 5: Where does the doubling time appear in the formulas you found in Bio-Math Exploration Integrating Question #4?

· What is the doubling time for a population in which the number of cells after t hours of growth is given by x = 8 x 25t?

· By x = 8 x 20.2t?