In.1930 Bortels published a paper entitled "Molybdenum as a Catalyst in the Biological Fixation of Nitrogen." Using Azotobacter chroococcurn, a freeliving nitrogen-fixing bacterium, he showed that if grown in a nitrogen-free nutrient solution, its growth was stimulated by the addition of a very small amount of sodium molybdate. In another paper in 1933 he reported loo-fold increase in the fixation of nitrogen by this and other species of bacteria when supplied with molybdenum. Portels was thus the first to establish the biological importance of molybdenum.