- 27 Sep, 2020
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Life Sciences/Microbiology
Question 2. (18 points)
Chlamydomonas reinhardtii is a single cell green alga that can grow photoautotrophically in
mineral medium or chemoheterotrophically in mineral medium supplemented with acetate.
These low nutritional requirements and versatile energy metabolism equip it to grow in diverse
environments, such as soil, freshwater, and seawater.
a. Design an experiment to compare the chemotrophic and phototrophic growth of C.
reinhardtii using mineral medium. What culturing conditions (i.e. chemicals and light) will be
needed for each? (8 points)
b. Identify any controls you will need. (2 points)
c. How many replicates would you need for each experimental group (treatment and controls)?
(2 points)
d. Draw a hypothetical graph of your results, illustrating growth in the photoautotrophic and
chemoautotrophic cultures and the controls. The y-axis should be the OD-reading and the x-axis
should be time. (6 points)
Question 3. (17 points)
To determine the number of viable cells in a stationary culture of E. coli, you serially dilute the
culture (1:10, 1:100, 1:1,000, 1:10,000, and 1:100,000) and spread three 10-μl samples from
each dilution on LB (“lysogeny broth”) agar plates. After overnight incubation at 37°C, you
quantify the number of colonies on each plate:
• 1:100,000 dilution: 0, 0, and 0 colonies
• 1:10,000 dilution: 48, 121, and 64 colonies
• 1:1,000 dilution: 552, 1,324, and 845
• 1:100 dilution: colonies too crowded to count
• 1:10 dilution: lawns of cells
a. What plates would you use to determine the total viable counts in the culture? (5 points)
b. Why do you plate each dilution in triplicate? (2 points)
c. How many CFUs (colony-forming units) per milliliter were in your culture? (5 points)
d. Do you expect the total viable count to match the total number of cell particles estimated by
OD measurement? Why? (5 points)