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.