Microcystis aeruginosis is a freshwater photosynthetic cyanobacterium. When temperatures increase and nutrients are readily available in its pond habitat, M. aeruginosis undergoes rapid cell division and forms an extremely large, visible mass of cells called an algal bloom. M. aeruginosis has a short life span and is decomposed by aerobic bacteria and fungi. Identify the metabolic pathway and the organism that is primarily responsible for the change in oxygen level in the pond between times I and II AND between times III and IV.

Complete question:

You will find the figure with cyanobacteria, decomposers, and oxygen concentration curves in the attached files.

- between times I - II: cyanobacteria produce oxygen through the procees of photosynthesis.

- between times II - III and III - IV: decomposers consume oxygen through the process of respiration.

Explanation:

Cyanobacteria, also known as green-blue algae, inhabits the euphotic stratum, producing oxygen through the process of photosynthesis. Probably they produce among 20 - 30% of the global photosynthetic activity. When they die, they sink to deeper water strata.

At the time I, the cyanobacteria population experiences a fast growth, which can be seen through the positive sharp increase of the curve slope. The curve reaches a maximum point just before time II, where it begins to decrease. Cyanobacteria reproduce, and the population grows in size. The more cells there are, the more photosynthetic activity, and the higher amount of oxygen is produced. Following cyanobacteria growth, there is an increase in oxygen concentration, which is reflected by the sharp increase in the curve slope. Both curves are very similar, but they are separated on time, meaning that first cyanobacteria need to reproduce and photosynthesize to posteriorly observe a proportional increase in oxygen concentrations.

Just before time II, the cyanobacteria population stops growing, and at time II begins a sharp decrease in size, reflected in the curve slope. Living cells still produce oxygen, but as there are fewer cells, oxygen concentration also decreases. Cyanobacteria are decomposed by aerobic bacteria and fungi. These last organisms consume oxygen through the process of respiration. As cyanobacteria die and are decomposed, aerobic cells can reproduce at higher rates, increasing their population size. This also can be seen reflected in the decomposers curve, which is stable until time II where it shows a sharp positive increase in its slope. As decomposers' population increases, they consume more oxygen, which concentration decreases inversely proportional to the decomposers' increase. This can be seen in the curves: from time II to time III, the aerobic organisms curve increases and the oxygen curve decreases, so as the cyanobacteria curve.

Just before time III cyanobacteria population stabilizes and during time IV keeps equal. Oxygen concentration keeps following, from time II to IV. When cyanobacteria stabilize, before time III, decomposers population starts to decrease, but they still consume oxygen.