Evaluation of argon-induced hydrogen production as a method to measure nitrogen fixation by cyanobacteria

The production of dihydrogen (H-2) is an enigmatic yet obligate component of biological dinitrogen (N-2) fixation. This study investigates the effect on H-2 production by N-2 fixing cyanobacteria when they are exposed to either air or a gas mixture consisting of argon, oxygen, and carbon dioxide (Ar:O-2:CO2). In the absence of N-2, nitrogenase diverts the flow of electrons to the production of H-2, which becomes a measure of Total Nitrogenase Activity (TNA). This method of argon-induced hydrogen production (AIHP) is much less commonly used to infer rates of N-2 fixation than the acetylene reduction (AR) assay. We provide here a full evaluation of the AIHP method and demonstrate its ability to achieve high-resolution measurements of TNA in a gas exchange flow-through system. Complete diel profiles of H-2 production were obtained for N-2 fixing cyanobacteria despite the absence of N-2 that broadly reproduced the temporal patterns observed by the AR assay. Comparison of H-2 production under air versus Ar:O-2:CO2 revealed the efficiency of electron usage during N-2 fixation and place these findings in the broader context of cell metabolism. Ultimately, AIHP is demonstrated to be a viable alternative to the AR assay with several additional merits that provide an insight into cell physiology and promise for successful field application.

Automated summary

The study investigates the effect of different gas conditions on hydrogen production by cyanobacteria, highlighting the potential of the argon-induced hydrogen production method in evaluating nitrogenase activity. Additionally, comparison of hydrogen production under air and argon gas conditions revealed the efficiency of electron usage during nitrogen fixation process.