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

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

A) Enough oxygen (O₂) enters heterocysts during hours of peak photosynthesis to have a somewhat inhibitory effect on nitrogen fixation.
B) Atmospheric nitrogen (N₂) levels increase at night because plants are no longer metabolizing this gas, so they are not absorbing this gas through their stomata.
C) Heterocyst walls become less permeable to nitrogen (N₂) influx during darkness.
D) 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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