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).

Question

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).

Quizplus · Accepted Answer

During hours of peak photosynthesis, oxygen enters the heterocysts, which inhibits nitrogen fixation. This means that nitrogen fixation is lower during those hours, resulting in the lower point on the curve. As oxygen levels decrease, nitrogen fixation increases, resulting in the peak of nitrogen fixation during hours of lower photosynthesis.

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