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Difficulty: MediumIdentifying Unstated Assumptions in Passages

In deep-sea hydrothermal vent ecosystems, chemosynthetic bacteria form the foundation of the food web by utilizing hydrogen sulfide emitted from vents to synthesize organic compounds. Higher organisms, such as giant tubeworms (Riftia pachyptila), rely entirely on symbiotic bacteria within their specialized organs for nutrition, as mature tubeworms lack a functional digestive tract. Marine biologists recently observed that during periods of volcanic quiescence, when hydrogen sulfide emissions drop dramatically, populations of Riftia pachyptila experience rapid physiological decline and high mortality within weeks. However, neighboring populations of another vent species, the bathymodiolin mussel (Bathymodiolus thermophilus), sustain stable population densities for several months under identical low-sulfide conditions.

Researchers hypothesized that B. thermophilus survives prolonged sulfide deprivation primarily because of its dual endosymbiotic capacity: it harbors both thiotrophic (sulfide-oxidizing) bacteria and methanotrophic (methane-oxidizing) bacteria. The researchers concluded that the presence of methanotrophic endosymbionts allows B. thermophilus to switch to methane as a primary energy source when sulfide availability diminishes, thereby granting the species a distinct competitive survival advantage during volcanic lulls.

Which of the following is an unstated assumption upon which the researchers' conclusion depends?

  1. Methane emissions at hydrothermal vent sites do not drop to levels insufficient to support methanotrophic chemosynthesis during periods of volcanic quiescence.Answer
  2. B
    Methanotrophic bacteria generate significantly more cellular energy per unit of substrate oxidized than do thiotrophic bacteria under typical vent conditions.
  3. C
    Hydrogen sulfide emissions regularly resume normal baseline levels before the metabolic energy reserves of B. thermophilus are completely exhausted.
  4. D
    Bathymodiolin mussels acquire their methanotrophic endosymbionts directly from environmental seawater rather than through maternal inheritance.
  5. E
    The total basal metabolic demand of B. thermophilus increases substantially during periods of low volcanic activity.

Answer

The researchers' conclusion relies on the unstated assumption that methane emissions at hydrothermal vent sites do not drop to levels insufficient to support methanotrophic chemosynthesis during periods of volcanic quiescence.
The conclusion asserts that switching to methane chemosynthesis enables bathymodiolin mussels to survive when hydrogen sulfide emissions drop during volcanic quiescence. For this mechanism to function, methane must actually be present and accessible in usable quantities when sulfide emissions decline. If methane levels also fell below the threshold required for chemosynthesis during volcanic lulls, the presence of methanotrophic bacteria would afford no survival benefit. Thus, the argument strictly requires that methane emissions remain sufficient during these periods.

Step-by-Step Solution

1
Identify the author's core premise and main conclusion.
Premise: B. thermophilus harbors methanotrophic bacteria and survives sulfide drops, whereas Riftia lacks methanotrophic symbionts and dies. Conclusion: The ability of methanotrophic symbionts to use methane as an energy source allows B. thermophilus to survive low-sulfide periods.
Establishing the logical gap between the observation (dual symbionts) and the conclusion (methane switching enables survival) is necessary to uncover unstated premises.
2
Apply the Negation Test to candidate assumptions.
Negating the statement regarding methane availability yields: 'Methane emissions drop to levels insufficient to support methanotrophic chemosynthesis during volcanic quiescence.'
If methane is not available in sufficient quantities during volcanic quiescence, having methanotrophic symbionts cannot explain how the mussels survive during that period.
3
Evaluate the impact of the negated statement on the conclusion.
The negated statement completely breaks the argument's causal mechanism.
A statement whose negation invalidates the conclusion represents a strictly necessary unstated assumption.

Key Concept

Identifying Unstated Assumptions in Passages
Estimated Time:2m 0s
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Identifying Unstated Assumptions in Passages

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