A marine biologist conducted an experiment to investigate how varying water salinity affec… — ACT

A marine biologist conducted an experiment to investigate how varying water salinity affects the bioluminescence of the dinoflagellate *Pyrocystis fusiformis*. Four identical $500\text{ mL}$ cultures were maintained at $22^\circ\text{C}$ with a cell density of $1,000\text{ cells/mL}$ . Each culture was exposed to a different salinity level ( $20\text{ PSU}$ , $25\text{ PSU}$ , $30\text{ PSU}$ , and $35\text{ PSU}$ ) and subjected to identical mechanical agitation to induce bioluminescence, after which the peak light intensity was measured.

Match each experimental component listed on the left to its corresponding variable classification on the right.

Water salinity level ( $20\text{ PSU}$ to $35\text{ PSU}$ )Independent Variable
Peak bioluminescence light intensity measuredDependent Variable
Water temperature, culture volume, cell density, and agitation rateControlled Variables

Answer

Water salinity level is the independent variable; peak bioluminescence intensity is the dependent variable; and water temperature, culture volume, cell density, and agitation rate are controlled variables.

In experimental design, the independent variable is the condition purposefully altered by the experimenter (here, water salinity). The dependent variable is the metric measured to evaluate the effect of that alteration (here, peak bioluminescence intensity). Controlled variables are kept uniform across all treatments to ensure that changes in the dependent variable are solely attributable to the independent variable (here, temperature, volume, cell density, and agitation rate).

Step-by-Step Solution

Identify the factor systematically changed by the researcher.

Water salinity level (

20\text{ PSU}

35\text{ PSU}

) is changed across treatments, which identifies it as the independent variable.

The independent variable is the single parameter manipulated to test its effect.

Identify the observed outcome or measurement collected.

Peak bioluminescence light intensity is measured following agitation, identifying it as the dependent variable.

The dependent variable responds to changes made to the independent variable.

Identify the parameters held constant across all test groups.

Water temperature (

22^\circ\text{C}

), culture volume (

500\text{ mL}

), cell density (

1,000\text{ cells/mL}

), and mechanical agitation rate are kept uniform, identifying them as controlled variables.

Controlled variables prevent confounding factors from influencing the measured dependent variable.

Key Concept

Identifying Independent, Dependent, and Controlled Variables

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