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Difficulty: MediumLogical Inferences

In subglacial lakes beneath the Antarctic Ice Sheet, primary production cannot rely on photosynthesis due to the absolute absence of sunlight. Instead, chemosynthetic microbes oxidize inorganic compounds, such as iron or sulfur, to generate energy. In Lake Mercer, researchers discovered high concentrations of dissolved organic carbon (DOC) alongside these chemosynthetic communities. Because the ice sheet has isolated the lake from the atmosphere for thousands of years, this DOC cannot have originated from recent photosynthetic activity. However, analysis revealed that the DOC molecules are structurally similar to those produced by marine phytoplankton, which require sunlight. Which choice most logically completes the text?

  1. the dissolved organic carbon currently in the lake is likely ancient material that was deposited before the ice sheet isolated the lake from the ocean.Answer
  2. B
    the chemosynthetic microbes in the lake have evolved to synthesize organic compounds that structurally mimic those produced by marine phytoplankton.
  3. C
    the microbes in the lake obtain most of their energy by consuming dissolved organic carbon rather than by oxidizing inorganic compounds.
  4. D
    Antarctic subglacial lakes generally contain higher concentrations of phytoplankton-derived organic carbon than open marine environments do.

Answer

The dissolved organic carbon currently in the lake is likely ancient material that was deposited before the ice sheet isolated the lake from the ocean.
The correct option logically completes the passage by explaining the origin of the dissolved organic carbon (DOC) in Lake Mercer. Since the lake has been isolated for thousands of years, the DOC cannot be from recent photosynthetic activity. However, because the DOC is structurally similar to that produced by marine phytoplankton (which require sunlight), it must have been deposited during a past period when the area was connected to the ocean and exposed to sunlight, before the ice sheet isolated it.

Step-by-Step Solution

1
Identify the premises regarding Lake Mercer's environmental conditions and findings.
The lake is isolated from the atmosphere and sunlight, meaning no recent photosynthesis occurs. Chemosynthetic microbes generate energy from inorganic compounds, yet the lake has a high concentration of dissolved organic carbon (DOC) that is structurally similar to DOC produced by marine phytoplankton.
This establishes the constraints: the DOC must have a source, but it cannot be from recent photosynthesis or from the chemosynthetic microbes' primary energy generation.
2
Analyze the structural similarity of the DOC to marine phytoplankton.
Since marine phytoplankton require sunlight, and the lake is currently dark and isolated, the phytoplankton must have produced this DOC when the region was connected to the marine environment.
This connects the marine origin of the DOC with the historical state of the lake before isolation.
3
Evaluate the choices to find the one that logically completes the explanation of the DOC's presence without introducing outside assumptions.
The conclusion that the DOC is ancient material deposited before isolation is the only direct, non-speculative option supported by all premises.
This completes the logical chain: ancient marine deposition explains both the phytoplankton structure and the lack of recent photosynthetic sources.

Key Concept

Logical Inferences
Estimated Time:1m 30s
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Logical Inferences

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