I belatedly realized that the data I posted yesterday are quite important, in that they contradict the growth results in Figure 1 of Wolfe-Simon et al.'s paper.
Here's their Fig. 1B (growth curves based on cell counts), annotated with a table of the levels of phosphate and arsenate they found in their media.
In my blog and in my published Technical Comment I argued that the ~3 µM phosphate contaminating the medium was indeed sufficient to support the observed growth of ~ 2 x 10^7 cells/ml. The calculation underlying this argument is shown below. Given the uncertainty in genome size and in the % of cellular P needed for DNA, my new results nicely support this argument - adding 3 µM phosphate to the -P medium supported growth to 1.7 x 10^7 colony-forming units/ml (equal to at least 1.7 x 10^7 cells/ml).
I've now set up some new cultures, testing whether the presence of 40 mM arsenate affects growth in media with no added phosphate or with 3 µM or 1.5 mM phosphate. I hope the sudden introduction of 40 mM isn't too big a shock to the cells; they've been growing without any arsenate for the past two months.
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