Azotobacter Chroococcum has the ability to fix atmospheric nitrogen, and was the first aerobic, free-living nitrogen fixer discovered.

A. chroococcum is a microaerophilic bacterium, which is able to fix nitrogen under aerobic conditions. To do so, it produces three enzymes (catalase, peroxidase, and superoxide dismutase) to "neutralise" reactive oxygen species.

It also forms the dark-brown, water-soluble pigment melanin at high levels of metabolism during the fixation of nitrogen, which is thought to protect the nitrogenase system from oxygen.
​Source: Wikipedia