What They Saw
They grew A. vinelandii strain OP (aka CA) in chemostats, at D = 0.1 h-1, limited in oxygen or nitrogen, then broke up the cells and isolated membrane vesicles anaerobically.
The P/O ratio is how much ATP is produced by moving 2 electrons through the electron transport chain to reduce one oxygen atom. The pattern of P/O over a range of oxygen levels is similar for different electron donors (NADH, malate, hydrogen, and NADH + hydrogen): it goes up to a peak, then falls as dissolved oxygen levels rise above the limit of detection. ATP production mostly levels off near that peak too (except with hydrogen, where it declines). The peak occurs at higher oxygen levels with NADH and malate than with hydrogen though. The height of the peak is 0.7 (ATPs per O reduced) for NADH and hydrogen, but only 0.5 with malate.
They tried adding acetylene up to 20%, but didn't see any indication that hydrogenase was inhibited. Previous studies showed that 40% acetylene was required to show inhibition, so it's not surprising. They also found that the hydrogen branch seemed to be very efficiently coupled to phosphorylation, and doesn't seem to involve flavoproteins.
What This Means
The lower values from hydrogen at high oxygen is probably due to inactivation of the hydrogenase, which is sensitive to oxygen.
The fall in P/O ratio is likely due to excess oxygen going through a different cytochrome branch, with cytochrome b to cytochrome d, which isn't involved in ATP production, so respiration and ATP are decoupled.
It also appears that the electrons from hydrogen oxidation don't travel through the same branch of the electron transport chain as electrons from carbon sources; hydrogenase has its own branch.