Calcium requirement in nitrogen fixation in the cyanobacterium Synechococcus RF-1

Under diurnal 16/8-h light-dark cycles, ethyleneglycol-bis-(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) at 1 mM completely blocked the appearance of rhythmic N2-fixing activity in Synechococcus RF-1. Ca(2+) at 2 mM, when supplied either together with or several hours after the EGTA application, restored the nitrogenase activity, whereas, when Ca(2+) was supplied several hours later, the peak of nitrogenase activity was shifted from the dark to the light period in which the activity is normally suppressed. Sr(2+) also reversed the inhibition by EGTA, but only partially. When O2 in the gas phase above the culture was below 1%, the inhibition of nitrogenase activity by EGTA was reduced to less than 20% of the control value without EGTA. Thus Ca(2+) appears to be required by the cell to protect its nitrogenase from inactivation by O2. In media without EGTA, a close correlation between nitrogenase activity and concentrations of Ca(2+) was also observed.