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

question 2

Multiple Choice

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 metabolising 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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