Ecological rationale for H₂ metabolism during aquatic blooms of the cyanobacterium Anabaena

Nitrogenase-produced H₂ serves to remove excess intracellular O₂ during vigorous growth periods (blooms) of the nuisance cyanobacterium Anabaena. In two naturally-occurring species, A. oscillarioides and A. spiroides, nitrogen fixation (acetylene reduction) showed a high degree of resistance to O₂ inactivation. Under the influence of supersaturated O₂ concentrations, commonly encountered in lake blooms, elevated cellular ATP levels and enhanced uptake hydrogenase and nitrogenase activities were observed in actively growing filaments. Oxygen enhancement of nitrogenase activity appears mediated through localized uptake hydrogenase reactions. Hydrogen assimilated by hydrogenase is combined with O₂ in a "Knallgas" reaction, leading to the formation of H₂O and ATP via a respiratory chain. This combination of activities appears poised at O₂ removal and allows Anabaena to dominate O₂ supersaturated surface waters while maintaining optimal nitrogenase activity. Hence, instead of being a wasteful dissipation of reducing power, H₂ evolution via nitrogenase ultimately affords protection from O₂ while constituting a source of ATP through subsequent H₂ metabolism.