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Question

Difficulty: HardExplicit Causal Relationships

Read the following passage:

Oceanic carbon sequestration relies heavily on the biological carbon pump, a process primarily mediated by the descent of organic detritus known as "marine snow." The formation of these sinking aggregates is initiated by the excretion of transparent exopolymeric particles (TEPs) by diatoms during the terminal phase of an algal bloom. These TEPs effectively glue smaller organic fragments together, significantly increasing their mass and sinking velocity. While early climate models posited that the physical fragmentation of these aggregates by ocean turbulence was the primary factor limiting downward carbon transport, recent microbial research has elucidated a different primary mechanism. According to these researchers, as the heavy aggregates descend through the mesopelagic zone, their colonization by heterotrophic bacteria results in the enzymatic hydrolysis of particulate organic carbon into dissolved organic carbon. The researchers conclude that it is precisely this bacterial enzymatic activity, rather than physical disruption, that diminishes the efficiency of deep-ocean carbon sequestration, because the resulting dissolved carbon is subsequently remineralized and circulated back to the ocean's surface by upwelling currents.

Based on the passage, the recent microbial researchers assert that the diminished efficiency of deep-ocean carbon sequestration is directly caused by which of the following?

  1. The enzymatic conversion of particulate organic carbon into dissolved organic carbon by colonizing bacteriaAnswer
  2. B
    The physical fragmentation of descending organic aggregates driven by turbulence in the mesopelagic zone
  3. C
    The excretion of transparent exopolymeric particles by diatoms during the terminal phase of algal blooms
  4. D
    The circulation of remineralized carbon dioxide back to the ocean's surface via strong upwelling currents
  5. E
    The complete consumption of all descending marine snow by heterotrophic bacteria prior to reaching the benthic floor

Answer

The enzymatic conversion of particulate organic carbon into dissolved organic carbon by colonizing bacteria
The correct answer accurately synthesizes the final two sentences of the passage. The text explicitly states that "recent microbial research" points to the "enzymatic hydrolysis of particulate organic carbon into dissolved organic carbon" by heterotrophic bacteria, and concludes that "it is precisely this bacterial enzymatic activity... that diminishes the efficiency of deep-ocean carbon sequestration."

Step-by-Step Solution

1
Analyze the question stem to identify the specific detail being asked for.
The question asks for the direct cause of the "diminished efficiency of deep-ocean carbon sequestration" specifically according to the "recent microbial researchers."
Focusing on the exact attribution and consequence prevents falling for distractors that describe other phenomena or viewpoints in the passage.
2
Scan the passage for the phrase "diminishes the efficiency" or synonymous concepts linked to the microbial researchers.
The final sentence states: "The researchers conclude that it is precisely this bacterial enzymatic activity... that diminishes the efficiency of deep-ocean carbon sequestration."
Locating the explicit textual evidence ensures the answer is derived directly from the passage, as required by detail-oriented reading comprehension questions.
3
Trace the reference of "this bacterial enzymatic activity" back to its definition in the preceding text.
The previous sentence defines this activity as "the enzymatic hydrolysis of particulate organic carbon into dissolved organic carbon" resulting from colonization by "heterotrophic bacteria."
Resolving referential phrasing is necessary to match the precise causal mechanism with the correct answer choice.
4
Evaluate the options against the identified mechanism.
The correct option directly paraphrases the enzymatic conversion by colonizing bacteria, while eliminating choices that rely on outdated theories, opposing processes, or overstated claims.
This guarantees selection of the single objectively correct answer supported by explicit text.

Key Concept

Identifying explicitly stated causal relationships and attributing them to the correct viewpoint in dense academic text.
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Topics

Explicit Causal Relationships

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