The first surprising result is that the presence of 40 mM arsenate had little or no effect on growth, regardless of the level of phosphate provided (compare the blue, red and green bars for each phosphate level). Previously I've only sometimes seen very slow growth in 40 mM arsenate cultures with high phosphate (1.5 mM) or with no added phosphate, and other times seen no growth at all.
Second, some cultures with little or no added phosphate grew to much higher densities than previously or than parallel cultures with similar amounts of phosphate (e.g. 4 µM and 5 µM phosphate with no arsenate, and 3 µM and 7.5 µM phosphate with 10 mM arsenate). Based on colony counts of previous experiments, I expect the 3 µM phosphate cultures to have about 100-fold fewer cells than cultures with 1500 µM phosphate
Third, the culture densities differed dramatically from tube to tube, with little correlation with the amount of phosphate I added. Part of the explanation for this might be that the culture tubes were heavily and variably contaminated with phosphate (they had been previously used, many years ago, and I had rinsed them and their caps well but not acid-cleaned them). But the contamination would have to have been consistently lower in the tubes that I added no or little phosphate to (because other cultures that differed by only 1 µM added phosphate differed by OD's of 0.2, equivalent to about 5 x 10^8 cells/ml).
(I looked at some of these cultures under the microscope and saw no evidence of contamination with other bacteria, nor did I see any unusual colonies when I plated some of them.)
Fourth, I plated the cells from a few cultures, and one's cfu counts were way lower than expected from its OD.
Fifth, by mistake I initially set up these cultures in base medium with 2 mM phosphate rather than in base medium with no added phosphate. I didn't throw this set of cultures out but incubated them with the correct cultures. These grew to higher densities than the low-phosphate cultures, but most didn't grow as dense as similar cultures had previously in flasks.
But again there was substantial tube-to-tube variation in density, and no effect of arsenate at either 10 mM or 40 mM. Phosphate contamination of the culture tubes can't explain this variation, as all the cultures had ample phosphate.
To rule out effects of contamination with phosphate (or whatever), I'm now going to set up some similar cultures in new plastic screw-top tubes. I won't bother with 10 mM arsenate and I'll reduce the number of different phosphate concentrations. I'll also test the effect of filling the tubes to different levels, in case reduced aeration is a factor.
Yes, I have no idea what's going on!