The Data Were Collected from the Heterocysts of a Nitrogen-Fixing

question 11

Multiple Choice

The data were collected from the heterocysts of a nitrogen-fixing cyanobacterium inhabiting equatorial ponds. Study the following graph and choose the most likely explanation for the shape of the curve.

• A) Enough O₂ enters heterocysts during hours of peak photosynthesis to have a somewhat-inhibitory effect on nitrogen fixation.
• B) Light-dependent reaction rates must be highest between 1800 hours and 0600 hours.
• C) Atmospheric N₂ levels increase at night because plants are no longer metabolizing this gas, so they are not absorbing this gas through their stomata.
• D) Heterocyst walls become less permeable to N₂ influx during darkness.
• E) The amount of fixed nitrogen that is dissolved in the pond water in which the cyanobacteria are growing peaks at the close of the photosynthetic day (1800 hours).

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