This study is another comparison of Azotobacter chroococcum strains, one with and three without an uptake hydrogenase, in a variety of conditions.
What They Saw
As the dilution rate increased in carbon-limited nitrogen-fixing conditions, the wild-type strain's growth yield remained relatively constant, while the three mutants' yields were noticeably lower at most rates. There was no noticeable difference when fixed nitrogen was provided. When oxygen or sulfate were limiting, there didn't seem to be much difference between strains.
When the strains were mixed together in equal densities in continuous culture, the mutants seem to overtake the wild-type a couple times at lower dilution rates, but the wild-type always took over at higher rates. With ammonium added, there was no consistent pattern.
What This Means
The mutants in this study were obtained by random mutagenesis, so the lack of hydrogenase wasn't necessarily the only difference from wild-type. Still, the three strains seemed to give some fairly consistent results.
The advantage seen in the wild-type occurred in low-density populations or high dilution rates, so it seems like the hydrogenase helps recover energy (or protect nitrogenase from oxygen) rather than preventing inhibition by hydrogen itself.
Aguilar, O. M., Yates, M. G. & Postgate, J. R. The Beneficial Effect of Hydrogenase in Azotobacter chroococcum Under Nitrogen-Fixing, Carbon-Limiting Conditions in Continuous and Batch Cultures. J. Gen. Microbiol. 131, 3141–3145 (1985).