Researchers investigated the effects of ocean acidification and microplastic pollution on … — ACT

Researchers investigated the effects of ocean acidification and microplastic pollution on the calcification rate (measured in $\text{mg}$ of $\text{CaCO}_3$ per $\text{g}$ of dry weight per day) of the reef-building coral *Porites lutea*.

Four trials were conducted for 30 days in separate tanks containing $10\text{ L}$ of artificial seawater maintained at $26^\circ\text{C}$ . The salinity, light intensity, and calcium concentration were identical across all tanks. In each trial, the partial pressure of carbon dioxide ( $p\text{CO}_2$ ), measured in microatmospheres ( $\mu\text{atm}$ ), and the concentration of polyethylene microplastic beads, measured in milligrams per liter ( $\text{mg/L}$ ), were varied as shown in Table 1.

Table 1

Trial	$p\text{CO}_2$ ( $\mu\text{atm}$ )	Microplastic concentration ( $\text{mg/L}$ )
1	400	0
2	400	10
3	800	0
4	800	10

To isolate the specific impact of microplastic contamination on the calcification rate under projected future ocean acidification conditions (elevated $p\text{CO}_2$ ), researchers should compare the results of which two trials?

A
Trial 2 and Trial 1, because Trial 1 serves as the control group representing ambient $p\text{CO}_2$ without microplastics.
B
Trial 4 and Trial 1, because Trial 1 serves as the control group representing baseline conditions without either stressor.
Trial 4 and Trial 3, because Trial 3 serves as the control group representing elevated $p\text{CO}_2$ conditions without microplastics.Answer
D
Trial 4 and Trial 2, because Trial 2 serves as the control group representing microplastic contamination at ambient $p\text{CO}_2$ .

Answer

Trial 4 and Trial 3, because Trial 3 serves as the control group representing elevated

p\text{CO}_2

conditions without microplastics.

To isolate the effect of microplastics under projected future ocean acidification conditions, the carbon dioxide level must be held constant at the elevated value (

800\ \mu\text{atm}

) while only the microplastic concentration is varied. According to Table 1, Trial 3 and Trial 4 both feature a

p\text{CO}_2

800\ \mu\text{atm}

. Trial 4 contains microplastics (

10\ \text{mg/L}

) and represents the experimental group, while Trial 3 has no microplastics (

0\ \text{mg/L}

) and serves as the baseline control. Comparing Trial 4 and Trial 3 successfully isolates the impact of microplastics under ocean acidification.

Step-by-Step Solution

Identify the independent variable to be isolated and the constant background condition.

The independent variable is microplastic contamination (varying from

0

10\ \text{mg/L}

), and the background condition is projected future ocean acidification (constant elevated

p\text{CO}_2

800\ \mu\text{atm}

To isolate the effect of a single factor, all other potential independent variables (such as

p\text{CO}_2

) must be held constant at the desired levels.

Select the trials that maintain the background condition at the elevated level.

Trial 3 and Trial 4 both have a

p\text{CO}_2

800\ \mu\text{atm}

This ensures that the impact of microplastics is evaluated specifically under the conditions representing future ocean acidification.

Determine which of the selected trials serves as the experimental group and which serves as the control/baseline group.

Trial 4 is the experimental group (contains

10\ \text{mg/L}

microplastics) and Trial 3 is the control group (contains

0\ \text{mg/L}

microplastics).

A control group must represent the baseline condition (absence of the treatment variable being tested) under the same environmental constraints.

Key Concept

Identifying the correct control group in a multi-variable experiment requires keeping all non-target independent variables constant while comparing a treatment group to a baseline group that lacks the target variable.

Estimated Time:1m 30s

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