title: Hydrogen cycling by the unicellular marine diazotroph Crocosphaera watsonii strain WH8501.
abstract: The hydrogen (H₂) cycle associated with the dinitrogen (N₂) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H₂ 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 H₂ production was coincident with the maximum rates of C₂H₂ reduction. Theoretical stoichiometry for N₂ fixation predicts an equimolar ratio of H₂ produced to N₂ fixed. However, the maximum rate of net H₂ production observed was 0.09 nmol H₂ μg chlorophyll a (chl a)⁻¹ h⁻¹) compared to the N₂ fixation rate of 5.5 nmol N₂ μg chl a⁻¹ h⁻¹, with an H₂ production/N₂ fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H₂ production was hypothesized to be a result of H₂ reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H₂ production rates ∼40-fold to a maximum rate of 3.5 nmol H₂ μg chl a⁻¹ h⁻¹. We conclude that the reassimilation of H₂ by C. watsonii is highly efficient (> 98%) and hypothesize that the tight coupling between H₂ production and consumption is a consequence of fixing N₂ at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H₂ cycle provides unique insight into N₂ fixation and associated metabolic processes in C. watsonii.
authors: Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822, USA. stwilson@hawaii.edu
authors: Samuel T
authors: ST
authors: Wilson
authors: Sasha
authors: S
authors: Tozzi
authors: Rachel A
authors: RA
authors: Foster
authors: Irina
authors: I
authors: Ilikchyan
authors: Zbigniew S
authors: ZS
authors: Kolber
authors: Jonathan P
authors: JP
authors: Zehr
authors: David M
authors: DM
authors: Karl
mesh_terms: D000114:Acetylene / Q000378:metabolism; D000458:Cyanobacteria / Q000378:metabolism*; D005470:Fluorometry; D006859:Hydrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000032:analysis
chemical_list: D006859:Hydrogen; D000114:Acetylene; D010100:Oxygen