What They Saw
They grew A. vinelandii OP (aka CA) in chemostats with different levels of oxygen, with 3 or 15 g/L sucrose. They extracted enzymes from samples and assayed them for SOD or catalase activity, and also by electrophoresis.
As oxygen saturation increased from 1% to 90%, SOD activity increased linearly (when standardized to total protein); the increase was slightly faster at lower oxygen when standardized by number of cells (probably because cell size increases as oxygen increases, 098). The sucrose concentration didn't affect things. When cells were given ammonia, SOD activity was about 2x lower.
Based on electrophoresis, they concluded that the SOD is iron-containing, rather than manganese. They tried adding manganese but still didn't see any Mn-SOD.
It didn't seem like catalase activity increased with increasing oxygen, standardized by protein. The increase standardized by cells was much more apparent.
What This Means
It seems like SOD at least might contribute to A. vinelandii's protection of its nitrogenase enzyme from oxygen.