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Difficulty: HardMain Idea and Primary Purpose

Until the late 1970s, marine ecology was anchored in the paradigm that all deep-ocean benthic ecosystems were ultimately dependent upon surface photosynthetic production for organic carbon inputs. The discovery of hydrothermal vent communities along the Galápagos Rift dramatically challenged this foundational assumption, revealing thriving ecosystems supported by chemoautotrophic bacteria that oxidized reduced sulfur compounds. Initial interpretations of this phenomenon cast hydrothermal vents as isolated biological anomalies—fascinating curiosities of extreme environments with minimal relevance to broader oceanic carbon budgets. However, subsequent quantitative surveys across mid-ocean ridge systems demonstrated that chemosynthetic primary production plays an essential, highly integrated role in global marine biogeochemistry. By demonstrating that ecosystems could originate and sustain complex biomass independently of solar radiation, vent ecology forced a profound reconfiguration of models governing planetary carbon cycles, while simultaneously expanding the theoretical parameters for astrobiological searches for life on icy ocean worlds.

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

  1. A
    To detail the specific biochemical pathways utilized by chemoautotrophic bacteria in extreme benthic environments.
  2. To describe how a scientific discovery overturned an established ecological paradigm and reshaped theoretical models of carbon cycles.Answer
  3. C
    To contend that chemosynthetic primary production has surpassed photosynthetic production as the main driver of planetary carbon cycles.
  4. D
    To severely criticize mid-twentieth-century marine ecologists for their short-sighted reliance on light-dependent models.
  5. E
    To present a comprehensive survey of astrobiological techniques for detecting life on icy planetary bodies.

Answer

The primary purpose of the passage is to describe how a scientific discovery overturned an established ecological paradigm and reshaped theoretical models of carbon cycles.
The correct answer accurately summarizes the overall rhetorical progression of the passage: explaining how the discovery of hydrothermal vent communities challenged the prevailing assumption that all deep-sea life depends on photosynthesis, ultimately forcing a major revision in ecological models.

Step-by-Step Solution

1
Analyze the structural arc of the passage.
The passage moves from the pre-1970s consensus (photosynthesis dependency) to the discovery of hydrothermal vents (chemoautotrophy), initial mischaracterization as anomalies, and finally the realization of their fundamental role in planetary biogeochemistry and astrobiology.
Identifying the overarching narrative structure isolates the primary purpose from mere background details.
2
Evaluate the author's main thesis.
The author emphasizes that vent ecology forced a 'profound reconfiguration of models governing planetary carbon cycles.'
Main idea questions require identifying what the text as a whole seeks to establish.
3
Distinguish central thesis from supporting details and extrapolations.
Specific biochemical mechanisms, astrobiological implications, and claims of total superiority are either too narrow, tangential, or extreme.
Eliminating scope errors and unwarranted extrapolations yields the accurate primary purpose.

Key Concept

Main Idea and Primary Purpose in Reading Comprehension
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Topics

Main Idea and Primary Purpose

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