Dr Bergersen is distinguished for his research in symbiotic nitrogen fixation. He is a world authority on the physiology and biochemistry of nitrogen fixation in the rhizobium/legume symbiotic system. He was the first to develop methods for preparing active bacteroids from nodules. This led to the purification of nitrogenase components and the identification of the active site on the Mo-Fe-protein. Bergersen was a major contributor in the elucidation of the physiological role of leghemoglobin in nodules supporting efficient respiration at low free oxygen concentrations. His work defined the complex terminal oxidase system in bacteroids. He also contributed information on the structure of legume root nodules, an important recent result being in the localization of leghemoglobin. In current research he is contributing in the bioenergetics of nodule activity and in developing improved techniques for measuring nitrogen fixation in the field.