### Passage An agricultural biologist investigated the physiological stress responses of t… — ACT

### Passage
An agricultural biologist investigated the physiological stress responses of the green alga *Chlorella vulgaris* exposed to common components of agricultural runoff. The study focused on four environmental variables: nitrate ( $NO_3^-$ ) enrichment, phosphate ( $PO_4^{3-}$ ) enrichment, atrazine (a widely used herbicide) exposure, and elevated temperature.

The biologist set up 5 culture flasks with identical initial densities of *C. vulgaris*. Each flask was subjected to a specific combination of nutrient concentrations, atrazine concentration, and temperature for 7 days. The experimental conditions for each flask are detailed in Table 1.

### Table 1

Flask	Temperature ( $^\circ\text{C}$ )	Added $NO_3^-$ ( $\text{mg/L}$ )	Added $PO_4^{3-}$ ( $\text{mg/L}$ )	Atrazine ( $\text{mg/L}$ )
1	20	0.0	0.0	0.0
2	20	5.0	0.0	0.0
3	20	5.0	1.0	0.0
4	20	5.0	1.0	0.1
5	25	5.0	1.0	0.1

To evaluate the specific, independent contribution of each variable or combination of variables to algal stress, the biologist must compare the growth rates of algae in the experimental flasks against their appropriate control groups or baseline conditions.

### Matching Task
Match each of the following experimental objectives with the specific Flask that serves as its primary control group or baseline condition.

Determining the baseline growth rate of *Chlorella vulgaris* under standard conditions without any chemical additions or thermal stress.Flask 1
Isolating the independent effect of adding phosphate to an environment already containing elevated nitrate.Flask 2
Isolating the independent effect of atrazine exposure in a nutrient-enriched environment.Flask 3
Isolating the independent effect of a $5^\circ\text{C}$ temperature increase under nutrient-enriched and herbicide-exposed conditions.Flask 4

Answer

Matching pairs: (1) Baseline growth under standard conditions matches Flask 1; (2) Isolating the effect of phosphate in a nitrate-containing environment matches Flask 2; (3) Isolating the effect of atrazine matches Flask 3; (4) Isolating the effect of a temperature increase matches Flask 4.

Each experimental objective requires a control group that differs by exactly one variable to establish a proper comparison. The baseline growth rate is measured by Flask 1 because it has no experimental manipulations. The independent effect of phosphate in a nitrate environment (Flask 3) is isolated by Flask 2, which contains nitrate but no phosphate. The independent effect of atrazine (Flask 4) is isolated by Flask 3, which has the same nutrient levels but no atrazine. The independent effect of elevated temperature (Flask 5) is isolated by Flask 4, which has the same nutrients and atrazine but at the baseline temperature.

Step-by-Step Solution

Identify the baseline or reference state of the experiment.

Flask 1 has no added nitrate, no added phosphate, no atrazine, and is at the baseline temperature of

20^\circ\text{C}

This serves as the negative control or baseline condition for the entire experiment.

Determine the control needed to isolate the effect of added phosphate when nitrate is present.

Flask 3 introduces phosphate (

1.0\text{ mg/L}

) to a medium already containing nitrate (

5.0\text{ mg/L}

) at

20^\circ\text{C}

. Its control must keep temperature and nitrate constant but lack phosphate, which corresponds to Flask 2.

By comparing Flask 3 to Flask 2, the only difference is the presence of phosphate, thereby isolating its independent effect.

Determine the control needed to isolate the effect of atrazine in a nutrient-enriched medium.

Flask 4 contains nutrients (

5.0\text{ mg/L}

nitrate,

1.0\text{ mg/L}

phosphate) and introduces

0.1\text{ mg/L}

atrazine at

20^\circ\text{C}

. Its control must have the same nutrients but no atrazine, which corresponds to Flask 3.

Comparing Flask 4 to Flask 3 isolates the biological impact of the herbicide atrazine.

Determine the control needed to isolate the effect of temperature under nutrient-enriched and herbicide-exposed conditions.

Flask 5 is at

25^\circ\text{C}

with nutrients and atrazine. Its control must have the exact same chemical additions but be held at the standard temperature of

20^\circ\text{C}

, which corresponds to Flask 4.

Comparing Flask 5 to Flask 4 isolates the independent effect of the

5^\circ\text{C}

temperature increase.

Key Concept

A control group must be identical to the experimental group in every factor except the single independent variable being tested. This isolates the independent variable's effect on the dependent variable.

Estimated Time:3m 0s

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More questions from this topic

Flask	Temperature ( $^\circ\text{C}$ )	Added $NO_3^-$ ( $\text{mg/L}$ )	Added $PO_4^{3-}$ ( $\text{mg/L}$ )	Atrazine ( $\text{mg/L}$ )
1	20	0.0	0.0	0.0
2	20	5.0	0.0	0.0
3	20	5.0	1.0	0.0
4	20	5.0	1.0	0.1
5	25	5.0	1.0	0.1

Flask	Temperature ( $^\circ\text{C}$ )	Added $NO_3^-$ ( $\text{mg/L}$ )	Added $PO_4^{3-}$ ( $\text{mg/L}$ )	Atrazine ( $\text{mg/L}$ )
1	20	0.0	0.0	0.0
2	20	5.0	0.0	0.0
3	20	5.0	1.0	0.0
4	20	5.0	1.0	0.1
5	25	5.0	1.0	0.1

Flask	Temperature ( $^\circ\text{C}$ )	Added $NO_3^-$ ( $\text{mg/L}$ )	Added $PO_4^{3-}$ ( $\text{mg/L}$ )	Atrazine ( $\text{mg/L}$ )
1	20	0.0	0.0	0.0
2	20	5.0	0.0	0.0
3	20	5.0	1.0	0.0
4	20	5.0	1.0	0.1
5	25	5.0	1.0	0.1