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Difficulty: Very hardCommand of Evidence: Textual

In marine biology, the phenotypic response of diatoms to ocean acidification is a subject of ongoing study. Biologist Dr. Sandra Lopez and her team investigated the phenotypic plasticity of the marine diatom *Thalassiosira pseudonana* under fluctuating pH conditions. Traditional models suggest that highly variable environmental pH selects for generalist genotypes with broad, static tolerance ranges. Lopez, however, hypothesized that fluctuating pH environments do not select for static tolerance; instead, they favor lineages that dynamically adjust their metabolic processes—specifically by downregulating energy-intensive silica deposition only during transient high-acidity events to conserve resources, while returning to normal deposition rates when pH stabilizes.

Which choice, if true, would most directly support Lopez's hypothesis?

  1. A
    *Thalassiosira pseudonana* cells exposed to variable pH conditions showed a higher overall survival rate than cells acclimated to stable pH conditions when both groups were transferred to a highly acidic environment.
  2. B
    *Thalassiosira pseudonana* cells from variable pH habitats maintained a constant, reduced level of silica deposition across all pH exposures to ensure structural integrity.
  3. *Thalassiosira pseudonana* cells from variable pH habitats exhibited decreased silica cell-wall thickness only during transient high-acidity intervals, returning to standard thickness when pH returned to baseline.Answer
  4. D
    Genetic sequencing of *Thalassiosira pseudonana* lineages from fluctuating environments revealed a high frequency of genes associated with general stress response pathways, which researchers believe are critical for surviving chemical fluctuations.

Answer

The statement describing that *Thalassiosira pseudonana* cells from variable pH habitats exhibited decreased silica cell-wall thickness only during transient high-acidity intervals, returning to standard thickness when pH returned to baseline.
The correct answer directly supports the hypothesis by providing empirical evidence of the dynamic adjustment of silica deposition. Specifically, it shows that the cells decrease their cell-wall thickness (indicating downregulated silica deposition) exclusively during high-acidity events and return to normal baseline thickness when conditions stabilize, directly matching the proposed physiological response.

Step-by-Step Solution

1
Identify the core claim or hypothesis in the passage.
Lopez hypothesized that fluctuating pH environments favor diatoms that dynamically adjust metabolic processes by downregulating silica deposition (reducing cell-wall thickness) only during transient high-acidity events, and returning to normal rates when pH stabilizes.
This establishes the precise target that the correct evidence must support.
2
Evaluate the choices to see which one demonstrates a dynamic, reversible change in silica deposition in response to transient pH shifts.
The choice stating that *Thalassiosira pseudonana* cells show decreased silica cell-wall thickness only during high-acidity intervals and return to standard thickness when pH returns to baseline directly matches the hypothesized mechanism.
It shows both the downregulation of silica during peak acidity and the return to baseline rates when conditions stabilize.
3
Differentiate the correct choice from distractors that support unrelated adaptation claims or describe static traits.
Other choices describe general survival, static (non-dynamic) downregulation, or general stress genes, none of which directly substantiate the specific claim of dynamic silica regulation.
This confirms that only the correct choice provides direct evidence for the hypothesized metabolic plasticity.

Key Concept

Identifying textual evidence that directly supports a specific scientific hypothesis regarding dynamic phenotypic plasticity versus static environmental tolerance.
Estimated Time:2m 0s
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Topics

Command of Evidence: Textual

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