What They Saw
They grew A. vinelandii CA fixing nitrogen, and isolated the membrane fraction from the cells. They looked at oxygen uptake, and saw that unless there were oxidizable substrates, there was no consumption of oxygen, which makes sense. Hydrogen fulfilled the requirement though, and there were two hydrogen molecules taken up for each molecule of oxygen, which makes sense: two hydrogen atoms for each atom of oxygen, to make H2O.
Using spectrophotometry, they observed peaks that occurred when components in the membrane were reduced with hydrogen, malate, or dithionite. Hydrogen affected cytochrome d (showing a peak at 627nm), b (shoulder at 559nm) and c (peak at 550), but not a (595). The other reductants affected b a lot more, and a somewhat.
With carbon monoxide added, hydrogen only reduced cytochrome d. The others showed a new peak, cytochrome o, at 417nm, but hydrogen didn't. The same seemed true with low levels of cyanide; so hydrogenase's terminal oxidase seems to be cytochrome d type. Not really sure how they prevented these inhibitors from inhibiting the hydrogenase itself, like they seem to in other studies.