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Difficulty: MediumArgument Analysis and Evaluating Claims

Marine ecologists studying coccolithophores—microscopic plankton that construct calcium carbonate shells—have long held that ocean acidification, driven by rising atmospheric carbon dioxide, uniformly impairs shell calcification. However, a recent longitudinal study in the North Atlantic revealed that *Emiliania huxleyi*, the most prevalent coccolithophore species, exhibited increased calcification rates and shell mass under moderately elevated carbon dioxide levels, provided that ambient nutrient concentrations remained above a critical threshold. The researchers hypothesized that elevated carbon dioxide enhances photosynthetic efficiency, generating surplus metabolic energy that offsets the heightened energetic cost of calcification in acidic waters.

Based on the passage, which of the following statements, if true, would undermine the researchers' physiological hypothesis? Select all that apply.

  1. In laboratory cultures with optimal nutrient levels, individual *Emiliania huxleyi* cells exposed to elevated carbon dioxide show no measurable increase in photosynthetic output.Answer
  2. B
    Nutrient-depleted regions of the North Atlantic show a marked decline in overall coccolithophore shell calcification during periods of ocean acidification.
  3. The metabolic energy required for calcification in *Emiliania huxleyi* under elevated carbon dioxide conditions exceeds the additional energy generated by any increase in photosynthetic rate.Answer

Answer

The statements indicating that carbon dioxide does not increase photosynthetic output in laboratory cultures, and that the energy required for calcification exceeds the extra energy generated by photosynthesis, both undermine the researchers' hypothesis.
The researchers' hypothesis depends on elevated carbon dioxide boosting photosynthetic efficiency enough to create a energy surplus that covers calcification costs under acidifying conditions. The option stating that individual cells show no increase in photosynthetic output directly breaks the first premise. The option stating that calcification energy costs exceed the extra energy produced breaks the second premise. Both statements successfully undermine the hypothesis.

Step-by-Step Solution

1
Identify the core premises of the researchers' hypothesis
The hypothesis relies on two crucial links: (1) elevated CO2 enhances photosynthetic efficiency to yield surplus energy, and (2) this surplus energy is sufficient to offset the increased energetic cost of calcification under acidic conditions, provided ambient nutrients are adequate.
To undermine a causal argument, evidence must attack one of its essential assumptions or mechanisms.
2
Evaluate the impact of the statement regarding laboratory cultures displaying no increase in photosynthetic output
If CO2 exposure produces no increase in photosynthetic output, link (1) is broken.
Without enhanced photosynthetic efficiency, the organism cannot generate the surplus energy posited to drive calcification.
3
Evaluate the impact of the statement regarding nutrient-depleted regions
This statement is consistent with the argument rather than a counterexample.
The passage explicitly bounds the hypothesis with the qualifier that nutrient levels must remain above a critical threshold.
4
Evaluate the impact of the statement regarding metabolic energy expenditure exceeding energy gains
If the energy cost of calcification exceeds the extra photosynthetic energy, link (2) is broken.
The proposed offsetting mechanism fails if the net energy balance remains negative.

Key Concept

Evaluating underlying premises and scope qualifiers in causal hypotheses
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Topics

Argument Analysis and Evaluating Claims

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