A culture of Spirogyra (an autotrophic 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?

a. Carbohydrates: same

b. Proteins: decrease

c. Lipids: same

d. Nucleic Acids: decrease

b) Proteins: decrease

Explanation:

Out of all the biomolecules required for the survival of an organism, proteins and nucleic acids are the ones containing Nitrogen. In nucleic acids, the building blocks of DNA, nitrogen is present in the nitrogenous bases, which are Adenine, Guanine, Cytosine, Thymine, and Uracil. In the proteins, the amino acids contain the amine group or - NH₂ which forms the peptide bond (-NH-CO-) with a carboxylic group or - COOH of the next amino acid. As the culture of Spirogyra is being maintained, there would be no replication of DNA, which would have required nitrogen to synthesize the new DNA strand. But, to synthesize proteins, the RNA synthesis would require to happen which would also require nitrogen. Moreover, the formation of proteins after the translation of the mRNA would have also required Nitrogen. Thus, protein level in the cells would decrease.