What They Saw
They grew A. chroococcum in chemostats, similar to previous studies (163). In their allegedly oxygen-limited culture, biomass levels fell as dilution rate increased, while carbon dioxide production fell and then rose again for some reason; they claim it's just random, and that CO2 was actually fairly constant (or blame it on consistency of electricity in Britain). They also observed that as dilution rate decreased, the amount of PHB in the cells rose. They could tell the cells were oxygen-limited because when more oxygen was added, it disappeared pretty quickly.
When cells were phosphate-limited, extra oxygen was pretty toxic; the cells couldn't handle it. Plating cells out after this challenge revealed that there was very little viability, if any. In contrast, cells that had been oxygen-limited didn't seem to mind extra oxygen much, especially when they were grown on medium with fixed nitrogen.
What This Means
It fits pretty well with previous results. PHB seems to be an electron sink, useful when cells don't have enough oxygen to receive all their electrons.
The fact that oxygen didn't reduce cells' viability when plated on medium with fixed nitrogen suggests that the cells' problem with extra oxygen is lack of ability to fix nitrogen immediately, rather than loss of viability. Then the cells ramp up respiration to consume the extra oxygen—respiratory protection.