Hydrogen Cycling by the Unicellular Marine Diazotroph Crocosphaera watsonii Strain WH8501

The hydrogen (H-2) cycle associated with the dinitrogen (N-2) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H-2 production and acetylene (C2H2) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H-2 production was coincident with the maximum rates of C2H2 reduction. Theoretical stoichiometry for N-2 fixation predicts an equimolar ratio of H-2 produced to N-2 fixed. However, the maximum rate of net H-2 production observed was 0.09 nmol H-2 mu g chlorophyll a (chl a)(-1) h(-1) compared to the N-2 fixation rate of 5.5 nmol N-2 mu g chl a(-1) h(-1), with an H-2 production/N-2 fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H-2 production was hypothesized to be a result of H-2 reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H-2 production rates similar to 40-fold to a maximum rate of 3.5 nmol H-2 mu g chl a(-1) h(-1). We conclude that the reassimilation of H-2 by C. watsonii is highly efficient (>98%) and hypothesize that the tight coupling between H-2 production and consumption is a consequence of fixing N-2 at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H-2 cycle provides unique insight into N-2 fixation and associated metabolic processes in C. watsonii.