Growth of GFAJ-1 under phosphate limitation (correction)

Erika Check Hayden's otherwise-excellent Nature News report on our work contained one error, the statement that "Redfield was unable to grow any cells without adding a small amount of phosphorus".

Here's the email I had sent her in response to an earlier query about phosphorus concentrations:

Hi Erika, 

The amount of phosphate in the medium used by Wolfe-Simon et al for their published growth analysis is indeed uncertain.  Their ICP-MS analysis found that most of their media preparations contained 3-4 µM phosphorus, but one batch contained <0.3 µM and a solution containing only the AML60-medium salts had 7.8 µM.  Because we don't know which batch was used for the results in their Figure 1, 3-4 µM is a good estimate of the phosphorus contamination, but the actual amount could have been substantially lower or higher. 

My cells did grow in medium with no added phosphorus*, to about 5 x 10^6 cells/ml.  This is about 1/4 of the density reached by GFAJ-1 in Wolfe-Simon et al's '-P/+As' medium.  Adding 3 µM phosphorus to my medium increased GFAJ-1 growth fourfold, to the same density as reported in Wolfe-Simon et al's experiments.    Simple algebra thus suggests that my unsupplemented medium contained about 1 µM phosphorus.  The correspondence of the cell densities reached in my supplemented (3 µM) and their unsupplemented medium supports the estimate of 3-4 µM contaminating phosphorus in their medium.   

My cells, like theirs, were clearly phosphorus-limited, because they grew to much higher densities when additional phosphorus was provided (see my recent RRResearch post and their Fig. 1). 

I think this is the best that can be done, since Wolfe-Simon et al. apparently did not keep good enough records to determine the actual phosphorus concentration of the medium they used for their reported experiments. 

Hope this helps, 

Rosie 

*The initial growth problem was not due to a lack of phosphorus but to the need for an amino acid, which I solved by supplementing the medium with a small amount of glutamate.

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