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Difficulty: EasyDirect Factual Retrieval

In marine microbiology, the structural role of microbial mats in coastal sediment stabilization has been extensively documented. Microbial mats are multi-layered functional communities of microorganisms, predominantly cyanobacteria and microalgae, that form cohesive organic films over benthic surfaces in tidal environments. Early ecological surveys suggested that these microbial mats protected sediments against hydrodynamic erosion solely through mechanical binding, a physical mechanism wherein long filaments of cyanobacteria physically entangle with surrounding mineral grains. However, more recent biochemical analyses demonstrate that extracellular polymeric substances (EPS)—complex high-molecular-weight polymers secreted by cyanobacteria—play a far more pivotal stabilization role than mechanical binding alone.

Specifically, EPS acts as a sticky organic matrix that coats sediment particles and increases critical shear stress, thereby significantly elevating the threshold water flow velocity required for sediment mobilization during routine tidal cycles. Furthermore, research indicates that the chemical composition of EPS varies systematically according to seasonal shifts. During periods of peak summer photosynthetic activity, cyanobacterial production of acidic polysaccharides within the EPS matrix increases markedly. This chemical shift enhances ionic cross-linking with divalent cations, such as calcium and magnesium, which are abundantly present in surrounding seawater. Consequently, this seasonal biochemical modification strengthens the overall structural cohesion of the sediment bed, rendering coastal mudflats remarkably resilient to erosive storm surges during late summer and early autumn months.

According to the passage, which of the following describes how extracellular polymeric substances (EPS) elevate the threshold velocity required for sediment mobilization?

  1. By forming a cohesive matrix that coats mineral particles and elevates critical shear stressAnswer
  2. B
    By physically entangling mineral grains with long cellular filaments during low tide
  3. C
    By decreasing the concentration of calcium and magnesium cations in surrounding seawater
  4. D
    By replacing photosynthetic activity as the primary driver of microbial mat growth during summer
  5. E
    By serving as the main structural support for coastal mudflats during early spring storm surges

Answer

Extracellular polymeric substances (EPS) elevate threshold velocity by forming a cohesive matrix that coats sediment particles and increases critical shear stress.
The passage explicitly states in the second paragraph that EPS acts as an organic matrix that coats sediment particles and increases critical shear stress, which directly elevates the threshold velocity needed for sediment mobilization.

Step-by-Step Solution

1
Locate the explicit discussion of EPS and threshold velocity in the passage.
Identified sentence in paragraph 2: 'EPS acts as a sticky organic matrix that coats sediment particles and increases critical shear stress, thereby significantly elevating the threshold water flow velocity...'
Direct factual retrieval requires pinpointing the exact passage location addressing the specific mechanism requested.
2
Match the passage statement with the semantic equivalent among the answer choices.
The statement directly matches the choice describing the formation of a cohesive matrix coating mineral particles to elevate critical shear stress.
Correct GMAT Reading Comprehension factual answers paraphrase explicit passage details without altering meaning.

Key Concept

Direct Factual Retrieval
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