Researchers investigated the effects of ocean warming and acidification on the calcificati… — ACT

Researchers investigated the effects of ocean warming and acidification on the calcification rates of the coccolithophore *Emiliania huxleyi*. They grew cultures of the marine microalgae under various combinations of temperature, partial pressure of carbon dioxide ( $pCO_2$ ), and salinity for 14 days. All other growth parameters, including light-dark cycles and nutrient concentrations, were held constant. The conditions for Trials 1–5 are shown in the table below:

Trial	Temperature (°C)	$pCO_2$ (µatm)	Salinity (psu)
1	15	400	35
2	15	800	35
3	19	400	35
4	19	800	35
5	19	800	30

In Trial 5, salinity was lowered to 30 psu to simulate the influx of fresh water from glacier melting, which is associated with warming oceans. To isolate and determine the specific effect of this reduced salinity on the calcification rate under projected future conditions of ocean warming and acidification, which of the other trials should be used as the control?

A
Trial 1, because it represents the ambient baseline conditions for temperature, $pCO_2$ , and salinity.
B
Trial 3, because it maintains the same elevated temperature of 19°C but keeps $pCO_2$ and salinity at their baseline levels.
Trial 4, because it maintains the same elevated temperature and elevated $pCO_2$ levels as Trial 5, with salinity being the only differing variable.Answer
D
Trial 2, because it isolates the effect of elevated $pCO_2$ at the baseline temperature of 15°C and standard salinity of 35 psu.

Answer

Trial 4, because it maintains the same elevated temperature and elevated

pCO_2

levels as Trial 5, with salinity being the only differing variable.

The correct answer identifies Trial 4 as the appropriate control group because it shares the same elevated temperature (19°C) and elevated

pCO_2

(800 µatm) as Trial 5. By keeping these two variables constant and only varying salinity (35 psu in Trial 4 vs. 30 psu in Trial 5), researchers can isolate the specific biological impact of reduced salinity under future warming and acidification conditions.

Step-by-Step Solution

Identify the variable that needs to be isolated in the experimental setup.

The target variable to isolate is salinity (30 psu in Trial 5 vs. 35 psu in standard conditions).

To determine the specific biological effect of reduced salinity, salinity must be the only independent variable that changes between the test group and its control.

Determine the background environmental conditions that must be held constant.

The background conditions represent the projected future scenario: elevated temperature (19°C) and elevated

pCO_2

(800 µatm).

Since the effect of salinity needs to be evaluated under this specific scenario, these other two variables must remain identical in both the experimental and control trials.

Match these requirements to one of the other trials in the experiment.

Trial 4 has a temperature of 19°C,

pCO_2

of 800 µatm, and a salinity of 35 psu.

Comparing Trial 5 (19°C, 800 µatm, 30 psu) to Trial 4 (19°C, 800 µatm, 35 psu) isolates salinity as the single independent variable, making Trial 4 the appropriate control group.

Key Concept

In multi-variable experiments, isolating the effect of a single variable requires a control group that is identical in all other background variables but maintains the baseline level of the variable of interest.

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