Chapter 2. Population Ecology

Learning Objectives

Pre-Lab—Chlamydomonas reinhardtii
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General Purpose

Conceptual

  • Gain an understanding of the basic characteristics of the model organism Chlamydomonas reinhardtii

Procedural

  • Gain experience setting up controlled population ecology experiments.
  • Gain experience graphing.

Background

To examine population growth, a model organism is frequently used. The unicellular green alga Chlamydomonas reinhardtii (see Figure 4-3) is the model organism you will use.

Figure 4-3. A schematic of Chlamydomonas reinhardtii.

C. reinhardtii is a freshwater flagellate cell. This aquatic plant is unusual in that it is motile and can therefore move toward the light. The following link leads to a YouTube video showing Chlamydomonas cells swimming.

Since C. reinhardtii is easy to grow large numbers of in a test tube or flask, this organism has been used widely as a model for the study of various cellular and organismal characteristics. C. reinhardtii reproduces both sexually and asexually. In the light C. reinhardtii grows as a photoautotroph (using light for energy). The population growth of C. reinhardtii is dependent on light, nutrients, temperature, absence of toxins, and space for its growth. The population density (number of cells per mL) of a C. reinhardtii culture can be monitored using a spectrophotometer set to a wavelength of 750 nm.

If you have not reviewed the information about using a spectrophotometer, you should look at that pre-lab material and refer to Appendix A for the general function of the spectrophotometer. During the growth of your C. reinhardtii cultures you will gather absorbance data for the replicates of the different treatments in your experiment. Using the standard curve shown below (see Figure 4-4), or the equation, you can convert your absorbance readings to number of cells per mL.

Figure 4-4. Standard curve relating absorbance at 750 nm to cell number per mL.

This standard curve shows a linear relationship between population density and absorbance at 750 nm. The equation for the best-fit line for this data is Absorbance = 1.058 (millions of cells/mL) − 0.0022 and the correlation coefficient (R2) is 0.9979.

Click to view Chlamydomonas data.

In this lab exercise you will be examining the impact of various factors on the growth of a C. reinhardtii population. The potential growth influencing factors for a population can be separated into abiotic and biotic factors. The abiotic factors are factors that are physical and chemical components of the environment. The abiotic factors you have to choose from for this experiment are listed in Table 4-1.

Table 4-1. Abiotic factors.

In addition to abiotic factors there are also biotic factors which could impact the growth of a C. reinhardtii population. Biotic factors are those factors in an environment which are directly related to the organisms in that environment. This could be an impact from organisms of the same species as the population you are interested in or from a different species than the population you are monitoring. One example of this type of biotic factor is competition. The biotic factors you have to choose from for this experiment are listed in Table 4-2.

Table 4-2. Biotic factors.

In lab, your group will choose one of the factors (either an abiotic or a biotic) and set up an experiment for growing C. reinhardtii populations under various conditions to examine the impact of the factor you selected on the population growth. This population growth will occur over many days and your group will need to make routine measurements to track the growth of your populations of C. reinhardtii.