The answer is A, but canbody explain why is it a? The data were collected from t

Question

The answer is A, but canbody explain why is it a?

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. Enough O2 enters heterocysts during hours of peak photosynthesis to have a somewhat-inhibitory effect on nitrogen fixation. Light-dependent reaction rates must be highest between 1800 hours and 0600 hours. Atmospheric N2 levels increase at night because plants are no longer metabolizing this gas, so they are not absorbing this gas through their stomata. Heterocyst walls become less permeable to N2 influx during darkness. 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).

Explanation / Answer

Nitrogen-fixing organisms utilize nitrogenase enzyme to fix the nitrogen, but this enzyme is extremely sensitive to oxygen. Microbes have developed various mechanisms to protect their nitrogenase from oxygen sensitivity. Cyanobacteria are the only oxygenic nitrogen-fixing photosynthetic organisms, capable of fixing nitrogen under aerobic conditions.

Nitrogen fixation has been reported for all three major morphological groups of cyanobacteria: heterocystous filamentous, non-heterocystous filamentous and unicellular forms. One of the oxygen protection mechanisms is the formation of morphologically different specialized cells called heterocysts under nitrogen-fixing conditions. In such arrangement, nitrogen-fixing occurs in heterocysts, and photosynthesis takes place in vegetative cells.

As nitrogenase is sensitive to oxygen, entering of oxygen into the heterocysts during peak hours of photosynthesis will have an inhibitory effect on nitrogen fixation.

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