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Question

Difficulty: MediumIdentifying Primary Purpose and Main Idea

Passage:
For decades, oceanographers posited that primary biological productivity in oceanic oxygen minimum zones (OMZs) was strictly bottlenecked by the availability of fixed nitrogen, which is rapidly depleted through microbial denitrification. According to classical biogeochemical models, as oxygen levels decline, denitrifying bacteria convert bioavailable nitrate into inert dinitrogen gas, creating an energetic desert incapable of supporting complex planktonic communities. However, recent genomic sequencing of microbial assemblages in the Eastern Tropical South Pacific OMZ has challenged this long-standing paradigm. Researchers discovered abundant populations of hitherto uncharacterized chemolithoautotrophic bacteria that couple anaerobic ammonium oxidation with the reduction of trivalent iron species. Rather than acting purely as nutrient sinks, these OMZs host self-sustaining metabolic networks wherein iron cycling continuously replenishes bioavailable nitrogen intermediates. Consequently, while traditional frameworks viewed oxygen depletion exclusively as a driver of marine ecological degradation, the emerging perspective recognizes OMZs as dynamic biogeochemical engines that maintain regional primary productivity despite extreme hypoxia. By demonstrating that microbial metabolic plasticity buffers nitrogen loss, these findings necessitate a substantial revision of global oceanic carbon and nitrogen budget models.

Which of the following best expresses the primary purpose of the passage?

  1. A
    detail the specific genomic sequencing procedures used to identify chemolithoautotrophic bacteria in marine environments.
  2. B
    assert that traditional biogeochemical frameworks have successfully accounted for all metabolic pathways in marine ecosystems.
  3. present recent scientific findings that challenge a long-held model of oceanic nutrient depletion and propose a revised ecological perspective.Answer
  4. D
    advocate for immediate industrial interventions to prevent nitrogen loss in hyper-oxygenated coastal zones.
  5. E
    condemn oceanographers for intentionally misrepresenting the ecological role of denitrifying bacteria in marine sediments.

Answer

The primary purpose of the passage is to present recent scientific findings that challenge a long-held model of oceanic nutrient depletion and propose a revised ecological perspective.
The passage opens by describing a long-standing traditional view regarding oxygen minimum zones and nutrient depletion, introduces recent genomic research showing a newly discovered metabolic pathway using the pivot 'However', and concludes by explaining how these findings require a substantial revision of classical biogeochemical models. The option stating that the passage presents recent scientific findings that challenge a long-held model accurately captures this overarching structure and main objective.

Step-by-Step Solution

1
Analyze the structural organization of the passage.
The first two sentences outline a traditional scientific consensus (oceanic OMZs are nutrient deserts due to nitrogen depletion).
Establishing the starting context reveals what baseline idea is being addressed.
2
Identify key structural pivots and shifts in tone.
The transition word 'However' introduces new genomic discoveries regarding chemolithoautotrophic bacteria and iron cycling.
Pivots signal the main point or counter-thesis the author intends to highlight.
3
Synthesize the conclusion and overall rhetorical goal.
The final sentences explain how these findings revise classical models by showing OMZs as dynamic engines maintaining productivity.
Combining the new evidence with its broader theoretical implication provides the complete primary purpose.

Key Concept

Identifying Primary Purpose and Main Idea
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Identifying Primary Purpose and Main Idea

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