Many marine phytoplankton species rely on dissolved iron to synthesize chlorophyll for photosynthesis. In the Southern Ocean, low iron concentrations typically limit phytoplankton growth. However, researchers observed rapid increases in phytoplankton blooms during periods of high continental dust deposition. Some scientists hypothesized that the dust directly supplies iron to the water, while others proposed that the dust stimulates local iron-producing bacteria. Analyses of the dust revealed that it releases highly bioavailable iron upon contacting seawater, whereas local bacterial populations showed no change in density or activity during the blooms.
Which choice most logically completes the text?
- The increase in phytoplankton blooms is primarily driven by the iron directly released from the dust rather than by the activity of local bacteria.Answer
- BContinental dust deposition is the sole source of dissolved iron in the Southern Ocean throughout the entire year.
- CLocal bacterial populations in the Southern Ocean do not require dissolved iron to survive.
- DThe phytoplankton blooms would grow even larger if the density of local iron-producing bacteria were artificially reduced.
Answer
The increase in phytoplankton blooms is primarily driven by the iron directly released from the dust rather than by the activity of local bacteria.
The correct answer logically completes the text because the passage sets up two possible explanations for why dust deposition increases phytoplankton blooms: direct supply of iron by the dust, or stimulation of iron-producing bacteria. Since the dust was shown to release iron directly and the bacteria showed no changes in activity or density, the evidence supports the direct-supply hypothesis and refutes the bacterial-stimulation hypothesis.
Step-by-Step Solution
Key Concept
Drawing logical conclusions that are directly supported by premises in the text and evaluating competing hypotheses based on evidence.
Estimated Time:1m 30s