Imagine you want to use an enrichment culture to attempt to isolate individual species of cyanobacteria from Greenlake. You set up a flask with an appropriate nutrient solution and a small sample of lake water, cover the flask loosely with a cap that is not airtight, and place the flask under a source of light. Which of the following would you NOT include in your culture medium so that you would encourage the growth of cyanobacteria relative to other microorganisms? (A) Phosphorus (B) Nitrogen (C) Iron (D) Carbon (E) Sulfate (F) Potassium

B. Nitrogen.

Explanation:

Nitrogen is a crucial element for life because they play a key role in amino acids that end up building proteins. Proteins have several important features like being enzymes, structural components of cell membranes, etc. hence nitrogen is an element that limits cell growth in freshwater ecosystems. I would not add nitrogen because it can be fixed from the atmosphere by cyanobacteria, in other words, they can extract it from the air (air is 78% composed of nitrogen). Since nitrogen is so important for cell growth only cyanobacteria would have an advantage over other types of cells. The other nutrients, phosphorus, iron, carbon, sulfate, and potassium have to be added to the growth medium because they don't have a gaseous form present in the atmosphere.