a culture of spirogyra (a photosynthetic alga) is maintained in a water solution containing dissolved carbon dioxide and a source of phosphates but lacking nitrogen compounds. a researcher determines the rates of synthesis of several organic compounds found in the spirogyra before and after several weeks in the water solution. which of the following graphs best illustrates a likely result of the experiment? hint: think about which of these macromolecules contain nitrogen and how lacking nitrogen would impact the building of these molecules. select only one answer choice.