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.