The design of this experiment is described in the previous post.
The upper graph shows that, in the absence of arsenic, growth is phosphate-limited and nicely reproducible. The red lines are replicate cultures with no added phosphate (two in glass screw-capped tubes, one in plastic (square symbols) ); the blue lines are replicates with 3 µM phosphate added (three in glass, one in plastic) and the purple ones are replicates with 1500 µM phosphate added (two in glass, one in plastic).
The lower graph shows that growth is much less reproducible in the presence of 40 mM sodium arsenate. The blue, red and purple lines are the same phosphate treatments as in the left panel; again square symbols indicate cultures in plastic rather than glass tubes. The most striking result is, as seen before, that cultures in the plastic tubes (the three lowest lines) grew very little, even with abundant phosphate. The two no-added-phosphate cultures in glass tubes (red) grew identically and slightly better than their no-arsenate controls, as did two of the three 3 µM phosphate cultures (blue). The third 3 µM phosphate culture grew to only half the density of the others. The two 1500 µM phosphate cultures (purple) grew to high density, one slower than the other.
The growth differences between replicates are unlikely to be due to differences in inoculum size, since all cultures began with 10^5 cells/ml.
I have no idea why growth in polypropylene tubes makes cells arsenic-sensitive. Googling 'arsenate' plus 'polypropylene' didn't suggest anything.
Maybe now I'll try cultures in glass flasks.