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Difficulty: MediumDetermining Control Groups and Baseline Conditions

A team of environmental scientists conducted an experiment to evaluate the degradation of polyethylene microplastic films by the marine benthic fungus *Aspergillus flavus* over a 30-day period. Four experimental vessels were prepared with identical initial masses of microplastic film (50 mg50\text{ mg}) and incubated at 25∘C25^\circ\text{C} under identical light conditions:

- Vessel 1: Sterilized seawater + active *A. flavus* spores + microplastic film
- Vessel 2: Sterilized seawater + microplastic film (no spores added)
- Vessel 3: Sterilized seawater + autoclaved (heat-killed) *A. flavus* spores + microplastic film
- Vessel 4: Distilled water (no salts) + active *A. flavus* spores + microplastic film

Match each experimental vessel to its primary function or baseline condition role in the study.

  • Vessel 1Primary experimental group used to measure active fungal degradation rate of polyethylene.
  • Vessel 2Abiotic control group used to establish baseline plastic degradation in seawater in the absence of biological activity.
  • Vessel 3Inert biological control group used to distinguish active metabolic breakdown from passive physical surface adsorption.
  • Vessel 4Environmental baseline control group used to evaluate whether marine salinity conditions are required for active fungal degradation.

Answer

Vessel 1 matches with the primary experimental group measuring active degradation. Vessel 2 matches with the abiotic baseline control in seawater without biological activity. Vessel 3 matches with the inert biological control distinguishing active metabolism from passive adsorption. Vessel 4 matches with the environmental baseline control testing salinity requirements.
Each vessel is paired with its specific experimental role by examining which variable is isolated: Vessel 1 is the primary treatment group with active fungus in natural seawater; Vessel 2 isolates non-biological seawater degradation; Vessel 3 isolates passive biomass binding using dead spores; and Vessel 4 isolates environmental salinity requirements.

Step-by-Step Solution

1
Analyze the components of each vessel to identify the independent variables manipulated and controlled.
Vessel 1 has live organism + target substrate + standard medium (seawater). Vessel 2 omits the live organism. Vessel 3 uses dead organism biomass. Vessel 4 alters the liquid medium.
Control groups isolate specific confounding factors by holding all variables constant except the one being tested.
2
Determine the baseline purpose of Vessel 2.
Omitting the biological agent (spores) establishes the baseline rate of plastic degradation caused purely by chemical or physical factors in seawater (abiotic control).
Without Vessel 2, any observed mass loss could not be definitively attributed to the fungal activity.
3
Determine the baseline purpose of Vessel 3.
Using heat-killed spores controls for passive surface adherence or adsorption of plastic to organic biomass.
This confirms that mass loss or structural breakdown requires active fungal enzymatic metabolism, not just physical contact with fungal tissue.
4
Determine the baseline purpose of Vessel 4.
Removing salts (using distilled water) tests the environmental baseline requirement of salinity.
Comparing Vessel 1 and Vessel 4 reveals whether marine ionic strength is necessary for fungal breakdown.

Key Concept

Identifying control groups and baseline conditions to isolate experimental variables
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Topics

Determining Control Groups and Baseline Conditions

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