Hydrogen production by Trichodesmium erythraeum Cyanothece sp. and Crocosphaera watsonii

Diazotrophic cyanobacteria are important components of marine ecosystems, where they contribute to primary production and provide a source of fixed nitrogen (N). During biological fixation of atmospheric nitrogen (N 2 ), hydrogen is produced as an obligate by-product. The present study investigated the potential contribution of 4 marine diazotrophs to the pool of dissolved H 2 in the oceans. N 2 fixation, as measured by acetylene reduction, and H 2 production rates were monitored throughout the diel period in cultures of the filamentous Trichodesmium erythraeum strain IMS101, and the unicellular organisms Cyanothece sp. strain ATCC 51142 and Crocosphaera watsonii strains WH8501 and WH0002. H 2 production coincided with diel variations in N 2 fixation for each strain regardless of whether N 2 fixation peaked during the day or night. Chlorophyll-normalized rates of H 2 production ranged 100-fold from a maximum of 3 nmol g chl a -1 h -1 in T. erythraeum IMS101 cultures to 0.03 nmol g chl a -1 h -1 in Crocosphaera watsonii WH0002. Overall, the ratio of net H 2 produced to N 2 fixed varied from 0.05 to 0.003 in the unicellular cyanobacteria, compared to 0.3 in the filamentous T. erythraeum IMS101, indicating that unicellular cyanobacteria produce less, or alternatively, re-assimilate more of the H 2 produced during N 2 fixation. Crocosphaera watsonii has recently been identified as a significant source of fixed N in the marine environment, and an efficient recycling of H 2 would provide a valuable source of energy to their respiratory electron transport chain. Furthermore, the magnitude of H 2 produced by T. erythraeum IMS101 strongly implicates this organism in the production of H 2 in the upper ocean.