The presence of which of the following elements is required for the formation of leghemoglobin in N₂ fixing bacteria?

Cobalt is required for the synthesis of leghemoglobin and for the growth of legumes relying on symbiotically fixed nitrogen. It is an essential micronutrient. It has been established that rhizobium and other N₂ fixing microorganisms have an absolute cobalt requirement whether or not they are growing within nodules and regardless of whether they are dependent on a nitrogen supply from N₂ fixation or from mineral nitrogen.