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Safety in Clumps: Pond Algae and Hungry Paramecia

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Safety in Clumps: Pond Algae and Hungry Paramecia

Use microscope video and a jar test to explain which algae survive and reproduce.

Part 1: Under the microscope

Read the passage, then answer questions 1 to 4.

Green Water From the School Pond

1In September, Tamsin Okwuosa's eighth grade class at Willow Bend Middle School took water samples from the pond behind the school. Water from the shallow edge looked pale green. Under a microscope at 400 times magnification, the class saw that the color came from a one-celled green alga, a tiny plantlike organism that makes its own food from sunlight. Some algae cells floated alone, while others sat in tight clumps of 4 to 16 cells. Swimming among them were paramecia, one-celled organisms that move with tiny hairs and sweep bits of food into a groove on one side of the body.

2Their teacher, Mr. Lorenzo Haugland, works with a university lab that raised two strains of this alga from the same pond. Each strain grew from a single cell, so all the cells in one strain carry the same genes. In both strains, a cell reproduces by dividing into 4 or 8 new cells inside its wall. In the single strain, the new cells break free and drift apart. In the clump strain, they stay stuck together. The lab grew both strains for six months in the same water, light and temperature, with no paramecia, and each strain kept its own form, generation after generation. A single cell is about 10 µm (micrometers) wide, and a clump is 25 to 40 µm across.

3To see how paramecia feed, the class mixed both strains in one drop with a few paramecia and filmed the drop for 10 minutes. In the video, four paramecia swallowed 46 single cells and 2 small clumps of 4 cells each. Larger clumps bumped into a paramecium's groove and were swept away again.

4Next, the class ran a jar test. They set up four jars with the same filtered pond water, kept at 22 °C under a lamp for 14 hours a day. Each jar started with 1,000 single-strain cells and 1,000 clump-strain cells in every milliliter (mL); cells inside clumps were counted one by one. Two jars also got 50 paramecia per mL, and two jars got none. After 7 days, the students counted the algae cells in a small sample from each jar with a counting slide and worked out the number of cells per mL. Table 1 shows the average for each pair of jars.

5Mr. Haugland pointed out one more difference between the strains. A clump is heavier than a single cell, so in still water clumps sink faster, and the jars were never stirred. Light from the lamp was dimmer near the bottom of each jar, just as light is dimmer in deeper pond water.

1.
Use paragraph 3. Which observation is the best evidence that paramecia have a harder time eating clumps than single cells?
Question 1 options
2.
Use paragraph 3. Count every algae cell the paramecia swallowed in the video, including the cells inside the 2 clumps.

In all, the paramecia swallowed algae cells. Rounded to the nearest whole percent, % of the cells they swallowed were single cells.

3.
Use paragraph 2. Why does it matter that the lab grew both strains for six months in the same conditions with no paramecia?
Question 3 options
4.
Sort each difference between algae cells by whether it is inherited variation or a difference caused by the cell's surroundings.

Part 2: The jar test

Use Table 1 and paragraphs 4 and 5 to answer questions 5 to 11.

Table 1: Algae Cells per mL (average of 2 jars)
When and which jarsSingle-strain cellsClump-strain cells
Day 0, all jars1,0001,000
Day 7, no paramecia9,0006,000
Day 7, with paramecia1,2004,800
5.
Use Table 1. Look only at the jars with paramecia. Do not round.

From day 0 to day 7, the single-strain cells grew to times their starting number, and the clump-strain cells grew to times their starting number.

6.
Use Table 1. On day 0, half of the algae cells in every jar were clump-strain cells. Find the share of clump-strain cells on day 7.

In the jars with paramecia, % of the algae cells were clump-strain cells. In the jars with no paramecia, % were clump-strain cells.

7.
In the jars with no paramecia, the single strain reached 9,000 cells per mL but the clump strain reached 6,000. Which explanation fits paragraph 5 best?
Question 7 options
8.
Circle the two results that, taken together, best show that clumping is an advantage when paramecia are present but a drawback when they are absent.

9.
Each summer, paramecia become very common in the school pond for several weeks. Over those weeks, how will the mix of algae in the pond most likely change?
Question 9 options
10.
A classmate says, "The paramecia made the algae form clumps to protect themselves." Which evidence from the page best shows a problem with this idea?
Question 10 options
11.
Explain how variation in clumping affected which algae cells in the jars survived and reproduced. Write a claim, use at least two numbers from the page as evidence, and explain your reasoning. Include the jars with no paramecia in your answer.