Question

The answer for this question is “C”, the answer key says “This is the point where the line crosses the x-axis and where respiration therefore exceeds photosynthesis. Imagine a dark, rainy day where algae in a pond are not able to carry out photosynthesis but are still performing cellular respiration. On these days cellular respiration may exceed photosynthesis.” The answer key confused me about the line crossing the x-axis and stuff

Answer 1

Context

The graph in the question show the ratio of respiration and photosynthesis in the aquatic environment, those producers are represented by the amount of oxygen present in the water, and the light intensity necessary for the organism do photosynthesis.

The respiration od those organisms does not need light to happen, therefore they can obtain oxygen from the common process of respiration, however the energy for them to survive is made by the photosynthesis process that needs light and release oxygen in the aquatic environment (in this case).

Graph representation

The y-axis of the graph represents the amount of oxygen produced by photosynthesis, while x-axis shows the light intensity in the environment, both gross and net need the same substrates to obtain energy and produce oxygen.

Why the answer is letter C

The answer is below 10% (light intensity that there is no oxygen production by the Net), because it is the point in the graph where we can see that the net productivity (photosynthesis) is bellow 0. Therefore there is no oxygen production by the Net in the environment.