Question 60·Hard·Command of Evidence
Current estimates of microplastic accumulation in the deepest parts of the ocean rely on box-core samplers and sieves fitted with mesh sizes of 50 µm or larger. Marine chemist Miguel Santos argues that these techniques severely undercount the true burden of microplastics in trench sediments. He notes that (1) polymer fibers thinner than 50 µm can slip through the meshes, and (2) the alkaline digestion commonly used to remove organic matter can partially dissolve certain biodegradable plastics before they are measured. Consequently, Santos concludes that published tallies likely represent only a small fraction of the microplastics actually present on the seafloor.
Which statement, if true, would most strongly support Santos’s conclusion regarding the underestimation of deep-sea microplastic levels?
For “most strongly supports” questions, first restate the conclusion in your own words, then identify the specific reasons/mechanisms the author gives. Choose the option that most directly tests or confirms that mechanism—ideally with controlled data (known inputs, recovery rates, before/after comparisons). Treat broader background facts or evidence from different settings as weaker unless they clearly connect to the same methods and conclusion.
Hints
Locate the conclusion to be supported
Reread the final sentence of the passage. What does Santos ultimately claim about the published tallies of microplastics in trench sediments?
Focus on the measurement methods
Santos criticizes two parts of the method: the 50 µm mesh and the alkaline digestion. The best support will tell you whether those steps cause plastics to be missed.
Prefer direct, method-testing evidence
The strongest support is usually an experiment that starts with a known amount of microplastic and then checks how much the standard protocol actually recovers.
Watch for evidence that only indirectly relates
Be cautious with choices that focus on different deep-ocean settings (not trench sediments) or that discuss broad composition of plastics without showing that the trench sampling/digestion protocol misses a large share.
Step-by-step Explanation
Identify Santos’s main conclusion
Santos’s conclusion is stated at the end: he says that published tallies likely represent only a small fraction of the microplastics actually present on the seafloor—that is, current methods greatly underestimate deep-sea microplastic levels.
Link the conclusion to the reasons given
Santos gives two reasons the published tallies could be too low:
- Mesh size issue: Fibers thinner than 50 µm can pass through the mesh and go uncounted.
- Digestion issue: Alkaline digestion can partially dissolve some biodegradable plastics before measurement.
So the strongest support will show that these very procedures miss microplastics in practice.
Decide what kind of evidence would be strongest
The best supporting evidence would directly test the standard protocol by comparing what is known to be present in sediment to what the protocol actually recovers. That kind of recovery-rate evidence most directly establishes undercounting.
Evaluate each choice for how directly it supports underestimation
- Choice A directly tests the protocol by adding known amounts of the kinds of plastics Santos says are missed (thin fibers and biodegradable fragments) and then measuring how much the protocol recovers.
- Choice B discusses deep-ocean sediments but not trench sediments and emphasizes that many particles are large enough to be caught, which does not show major undercounting in trenches.
- Choice C compares 10 µm vs. 50 µm processing in deep-sea sediments but reports only a small difference, which does not provide strong evidence of severe underestimation.
- Choice D gives a general claim about the proportion of biodegradable plastics detected overall; it does not demonstrate that the trench-sediment protocol substantially misses plastics.
Choose the statement that most strongly supports Santos’s conclusion
Choice A is strongest because it is a controlled test: when researchers add known quantities of the relevant microplastics to sediment and then apply the standard sampling and digestion protocol, they recover only about 35 percent.
Recovering far less than what is known to be present directly supports Santos’s conclusion that published tallies based on that protocol likely capture only a small fraction of true microplastic levels. Therefore, the correct answer is:
When researchers spiked sediment samples with known quantities of microplastic fibers thinner than 50 µm and fragments of polylactic acid, the standard sampling and digestion protocol recovered only about 35 percent of the added material.