Extremely low D/H ratios of photoproduced hydrogen by cyanobacteria

Cyanobacteria, having primary photosynthetic reactions similar to higher plants, are capable of producing large quantities of molecular hydrogen by nitrogenase and/or hydrogenase delivering electrons to hydrogen ions via ferredoxin or oxidation of NADPH. We measured the deuterium/hydrogen (D/H) ratios of the hydrogen gas photoproduced by Synechococcus sp. Miami BG 043511 and Anabaena sp. TU 37-1, and demonstrate that ΔD values of their hydrogen gas are extremely low (about - 600%) when compared with that of available water (-7%).This depletion gives a mean fractionation factor (a) of 0.43, which is similar to that calculated for hydrogen ions at equilibrium with water (0.35) and hydrogen produced by electrolysis of water (0.24) but significantly different from those of carbon bound hydrogens (>0.83). Thus hydrogen ions available for protonation of NADP⁺ may be extremely deuterium depleted. Our results may explain why D/H ratios of nitrated cellulose or lipids from most plants are always depleted relative to water available for photosynthesis.