Despite some studies suggesting that hydrogen stimulates hydrogenase, other data suggested it does not. So the people who generated this data did this study on various species, including Azotobacter vinelandii CA, and claimed that low oxygen stimulated hydrogenase activity.
What They Saw
They grew A. vinelandii with ammonium chloride and measured hydrogenase activity with different electron acceptors (oxygen, methylene blue, etc). As A. vinelandii grew, it used up the dissolved oxygen, and hydrogenase activity went up but then back down after the oxygen was gone (when oxygen or iron cyanide were the electron acceptors, it went to zero; otherwise it didn't go all the way to zero). This was all in the presence of ammonium.
What This Means
Based on other studies, I wouldn't expect much activity from hydrogenase in general when growing with fixed nitrogen. I'm not sure how to interpret these results, especially with electron acceptors other than oxygen, but I guess it would make sense if hydrogenase were somewhat downregulated in low-oxygen conditions, even if other acceptors were present.
Reference:
What They Saw
They grew A. vinelandii with ammonium chloride and measured hydrogenase activity with different electron acceptors (oxygen, methylene blue, etc). As A. vinelandii grew, it used up the dissolved oxygen, and hydrogenase activity went up but then back down after the oxygen was gone (when oxygen or iron cyanide were the electron acceptors, it went to zero; otherwise it didn't go all the way to zero). This was all in the presence of ammonium.
What This Means
Based on other studies, I wouldn't expect much activity from hydrogenase in general when growing with fixed nitrogen. I'm not sure how to interpret these results, especially with electron acceptors other than oxygen, but I guess it would make sense if hydrogenase were somewhat downregulated in low-oxygen conditions, even if other acceptors were present.
Reference:
Pinkwart, M., Bahl, H., Reimer, M., Wölfle, D. & Berndt, H. Activity of the H2-oxidizing hydrogenase in different N2-fixing bacteria. FEMS Microbiology Letters 6, 177–181 (1979).
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