Basis for Diel Variation in Nitrogenase Activity in the Marine Planktonic Cyanobacterium Trichodesmium thiebautii

Natural populations of the nonheterocystous marine cyanobacterium Trichodesmium thiebautii exhibit a diel periodicity in nitrogenase activity (NA). NA "turns on" near dawn and "turns off" near dusk, independent of photic conditions. Chloramphenicol (CAP) and ammonium prevented turn on of NA in T. thiebautii when added to samples collected before dawn but were progressively less effective in inhibiting NA in samples collected later in the morning. In samples collected after turn on, activities declined with time with both CAP and ammonium treatments, with ammonium having a stronger effect. In contrast, CAP added to samples collected in late afternoon prolonged NA, compared with controls, which turned off. Direct analysis of the presence of the Fe protein of nitrogenase in T. thiebautii by using a Western immunoblot procedure found a strong protein band present in samples collected after 0800 h through the late evening but little or no Fe protein in samples collected within the 2 to 4 h preceding dawn. We conclude that the diel cycle of NA in T. thiebautii results from de novo synthesis of nitrogenase each morning and from the inactivation and degradation of nitrogenase in the late afternoon and night.