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Difficulty: MediumDetermining Control Groups and Baseline Conditions

A team of biogeochemists investigated anaerobic oxidation of methane by marine sediment microbes under simulated deep-sea temperature conditions (4∘C4^\circ\text{C}). Four distinct reactor setups were prepared containing equal masses of marine sediment and methane gas, but with varying additions of electron acceptors, heat sterilization, and hydrostatic pressures.

Match each experimental setup to its corresponding role or baseline function in the experiment.

  • Setup 1: Untreated sediment + sulphate (SO42−SO_4^{2-}) at 10 MPa10\text{ MPa} hydrostatic pressurePrimary experimental group testing microbial methane oxidation under simulated in situ conditions
  • Setup 2: Untreated sediment without added electron acceptors at 10 MPa10\text{ MPa} hydrostatic pressureBaseline control group to measure background methane consumption without external electron acceptors
  • Setup 3: Autoclaved (heat-sterilized) sediment + sulphate (SO42−SO_4^{2-}) at 10 MPa10\text{ MPa} hydrostatic pressureAbiotic negative control to quantify non-biological methane loss or chemical leakage
  • Setup 4: Untreated sediment + sulphate (SO42−SO_4^{2-}) at 0.1 MPa0.1\text{ MPa} (atmospheric) hydrostatic pressureVariable comparison group to evaluate the specific influence of hydrostatic pressure on metabolic rates

Answer

Setup 1 matches the primary experimental group under simulated in situ conditions; Setup 2 matches the baseline control measuring activity without added electron acceptors; Setup 3 matches the abiotic negative control using heat sterilization; Setup 4 matches the variable comparison group evaluating the effect of hydrostatic pressure.
In experimental design, control groups isolate variables and establish baseline measurements. Heat-sterilized setups serve as abiotic controls to confirm biological necessity. Setups lacking specific reactants (such as sulphate) provide a baseline for background activity without that variable. Setups altering a single physical condition (such as hydrostatic pressure) allow direct comparison of that specific variable against the primary experimental group.

Step-by-Step Solution

1
Identify the setup testing the target hypothesis under full simulated conditions.
Setup 1 contains active sediment, the electron acceptor sulphate, and deep-sea pressure (10 MPa10\text{ MPa}), making it the primary experimental group.
This setup establishes the baseline rate of sulphate-dependent methane oxidation under realistic environmental conditions.
2
Identify the setup that isolates non-biological background effects.
Setup 3 uses autoclaved (heat-sterilized) sediment.
Sterilization eliminates living micro-organisms, serving as a negative control to prove that methane depletion is driven by biological processes rather than physical leakage or abiotic reactions.
3
Determine the baseline control for chemical additions and the variable comparison group for physical factors.
Setup 2 omits sulphate to serve as a baseline control without added electron acceptors, while Setup 4 alters pressure to 0.1 MPa0.1\text{ MPa} to measure the specific impact of hydrostatic pressure.
Comparing outcomes against setups missing specific factors isolates the individual impact of each independent variable.

Key Concept

Determining Control Groups and Baseline Conditions
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Topics

Determining Control Groups and Baseline Conditions

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