### Passage A team of geobiologists investigated the role of the soil bacterium *Bacillus … — ACT

### Passage
A team of geobiologists investigated the role of the soil bacterium *Bacillus subtilis* and soil humic acids on the dissolution rate of calcite ( $CaCO_3$ ), a common carbonate mineral. Five experimental trials were conducted at $25^\circ\text{C}$ for 14 days. In each trial, 10.0 grams of calcite was placed in a reaction vessel containing 500 mL of an aqueous solution. The dissolution rate was determined by measuring the cumulative concentration of calcium ions ( $Ca^{2+}$ ) released into the solution. The experimental setups are detailed below:

* Trial 1: Sterile deionized water (pH $7.0$ ), no bacteria.
* Trial 2: Aqueous solution containing $10\text{ mg/L}$ of humic acids (pH $5.5$ ), no bacteria.
* Trial 3: Sterile deionized water (pH $7.0$ ) inoculated with $10^6\text{ cells/mL}$ of living *B. subtilis*.
* Trial 4: Aqueous solution containing $10\text{ mg/L}$ of humic acids (pH $5.5$ ) inoculated with $10^6\text{ cells/mL}$ of living *B. subtilis*.
* Trial 5: Aqueous solution containing $10\text{ mg/L}$ of humic acids (pH $5.5$ ) inoculated with $10^6\text{ cells/mL}$ of heat-killed *B. subtilis*.

To determine if the enhancement of calcite dissolution in the presence of humic acids is specifically driven by the active metabolic processes of living *B. subtilis* rather than the passive physical presence of bacterial cell structures, researchers must compare Trial 4 to a control group. Which trial serves as the most appropriate control group for this comparison, and why?

A
Trial 2, because it contains humic acids but lacks the bacterial cells entirely, isolating the chemical effect of the acid.
B
Trial 3, because it contains living bacteria in deionized water, isolating the effect of bacterial viability in a neutral environment.
Trial 5, because it contains heat-killed bacteria in humic acids, isolating the effect of active metabolic processes from the physical presence of bacterial cell structures.Answer
D
Trial 1, because it contains only sterile water, providing a baseline for calcite dissolution under completely abiotic and neutral conditions.

Answer

The trial with heat-killed bacteria in humic acids serves as the most appropriate control group because it keeps the physical presence of bacterial cells and the humic acid medium constant, leaving the viability of the bacteria as the only variable that differs.

The correct option is the one stating that Trial 5 serves as the control group because it contains heat-killed bacteria in humic acids, isolating the effect of active metabolic processes from the physical presence of bacterial cell structures. By keeping the chemical environment (humic acids at pH 5.5) and the physical presence of the cells constant, any difference in calcite dissolution between the two trials can be attributed solely to the active metabolic processes of the living bacteria.

Step-by-Step Solution

Identify the experimental group under investigation.

The experimental group contains living *B. subtilis* in a humic acid solution (pH 5.5).

This defines the conditions containing the active variable of interest: metabolic processes in an acidic medium.

Determine the specific factor to be isolated.

The active metabolic processes of the bacteria must be isolated from the passive physical presence of the bacterial cell structures in the humic acid environment.

To determine if metabolic processes are the cause, the control must include the physical cells and the chemical medium but exclude metabolic activity.

Select the trial that holds the medium and presence of cells constant while removing cell viability.

The trial containing heat-killed bacteria (non-viable cells) in a humic acid solution is chosen.

Comparing these two setups isolates the variable of cell viability (living vs. dead), showing whether active metabolism is responsible for the difference in dissolution rates.

Key Concept

Determining Control Groups and Baseline Conditions

Estimated Time:2m 0s

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