When limited in sulfate, the hydrogen produced under air (with hydrogenase active) or under argon mix seemed much higher (at least proportional to the amount of protein in cells).
Under argon mixes with different proportions of oxygen, hydrogen evolution seemed highest around 10% oxygen. With nitrogen instead of argon, the peak was similar. Too much or too little oxygen was not good. And despite the high amount of hydrogen, around 10% (actually between 6-12%) was when the most nitrogen was being fixed too, such that the ratio of hydrogen produced to nitrogen fixed was as low as 1 (or 0.5, when sulfate-limited).
In carbon-limited cultures, though, the optimum oxygen value was 3%, though oxygen consumption increased as oxygen increased, at least up to 6%.
In vitro, the hydrogen-nitrogen ratio increased as the ratio of dinitrogenase to dinitrogenase reductase increased, though higher levels of ATP decreased this effect. Sulfate limitation didn't really affect this finding.
What This Means
This makes sense; some oxygen is required to generate ATP to power the nitrogenase, but too much oxygen requires extra carbon to detoxify it, so there's less energy for nitrogenase.
It seems like a lack of dinitrogenase reductase or ATP reduces the ability to fix nitrogen instead of just producing hydrogen. I wonder if dinitrogenase has any effect on its own in the absence of ATP or its other component.
Chemically it's unclear how or why nitrogenase produces hydrogen, but it seems to be an essential part of the nitrogen fixation process.