Explanation

Melvin Calvin used radioactive $\ ^{14}C$ in algal photosynthesis, which led to the discovery that the first $CO_{2}$ fixation product was a 3-carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him. The first product identified was 3-phosphoglyceric acid or PGA. For this, he was awarded Nobel Prize