A team of marine biologists conducted an investigation to determine how varying concentrat… — ACT

A team of marine biologists conducted an investigation to determine how varying concentrations of dissolved iron ( $\text{Fe}^{2+}$ ) in seawater affect the growth rate of the phytoplankton species *Emiliania huxleyi*. Four 10 L culture vessels were filled with filtered seawater and kept at a constant temperature of $18^\circ\text{C}$ under identical 12-hour light/14-hour dark photoperiods. Each vessel received a different concentration of $\text{Fe}^{2+}$ ( $0.1\ \mu\text{M}$ , $0.5\ \mu\text{M}$ , $1.0\ \mu\text{M}$ , and $2.0\ \mu\text{M}$ ). After 14 days, the final cell density ( $\text{cells/mL}$ ) of *Emiliania huxleyi* in each vessel was recorded.

Which of the following represents the independent variable in this experiment?

The concentration of dissolved iron ( $\text{Fe}^{2+}$ ) in the seawaterAnswer
B
The final cell density of *Emiliania huxleyi* recorded after 14 days
C
The temperature of the seawater maintained in the culture vessels
D
The volume of seawater contained in each culture vessel

Answer

The concentration of dissolved iron (

\text{Fe}^{2+}

) in the seawater is the independent variable.

The independent variable is the factor directly manipulated by the experimenter. In this scenario, the researchers intentionally varied the concentration of dissolved iron (

\text{Fe}^{2+}

) across the four culture vessels to observe its effect.

Step-by-Step Solution

Identify the variable deliberately manipulated by the experimenters.

The experimenters set four distinct concentrations of

\text{Fe}^{2+}

(

0.1\ \mu\text{M}

0.5\ \mu\text{M}

1.0\ \mu\text{M}

, and

2.0\ \mu\text{M}

) across the vessels.

The independent variable is the parameter intentionally changed or tested by the researchers.

Distinguish the independent variable from the dependent variable and controlled variables.

Cell density is the measured response (dependent variable), while seawater volume and temperature are kept constant (controlled variables).

Isolating the manipulated factor confirms which experimental component functions as the independent variable.

Key Concept

Identifying Independent, Dependent, and Controlled Variables

Estimated Time:1m 0s

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