What They Saw
They grew A. vinelandii OP (aka CA) in Burk without Mo, with added ammonium phosphate. Because it's really hard to get rid of every little bit of Mo, they added lots of tungsten (W) to make sure that they could see it if it got incorporated into enzymes. Some of the W was radioactive.
W didn't inhibit growth when ammonia was present, which makes sense. But it did inhibit it, about the same, with N2, nitrate, or urea. The enzymes that use these N sources all need Mo. When Mo was about 0.1 μM, it took 20 μM W to inhibit growth 50%; when Mo was 10 μM, it took 4 mM W. When just a little ammonia was added, it took about 5000 times more W than Mo to stop growth.
When they purified nitrogenase from these W-grown cells, they actually did see acetylene reduction activity, though not nearly as much as with normal Mo nitrogenase. The W content of extracts was very high, though it seemed to be easily removable. Specifically purifying Fe-W protein and comparing to the Fe-Mo version, all activities seemed relatively low: acetylene reduction, hydrogen production, and ATP hydrolysis.
What This Means
A. vinelandii might treat W the same as it treats Mo: taking up as much as it can and storing what it doesn't incorporate. But it does seem to incorporate some into the Mo nitrogenase. This seems to result in a poorly functional enzyme, but is that enough to stop cells from growing entirely? Maybe W's effects on other enzymes cause some problems too.