One of my A2 students is investigating the effect of phosphate concentration on the growth of Chlorella. She is using the medium included from the supplier which contains no phospate, and then adding varying amounts of phosphate, in the form of dihydrogen sodium phosphate (aq) The supplier has not been able to tell us why there is no phosphate in the recommended solution, and the results from the investigation indicate a reduction in growth as the concentration of phosphate increases. Have you any suggestions to help both her and our biology department?

I have followed this up with an academic who works with algae.
He reports that:-

"Our routine alga medium contains 200 mg/l K2HPO4.  Hydroponic (MS-based) medium for Arabidopsis usually contains 170 mg/l, so about the same amount."

Like me, he feels that it is unlikely to be successful to grow Chlorella in the absence of P for any sustainable time. P is, of course, essential for the growth of new cells.

I wonder how your concentrations compare with this?

Your investigation has the appearance of exploring a concentration range which is higher than the optimum.  If your concentrations of phosphate are significantly higher than those quoted above, then I would expect to observe a decreasing growth with increasing concentration.

Another medium used for culture of algae is:-
http://www-cyanosite.bio.purdue.edu/media/table/chu10mod.html which only uses 10mg/l of K2HPO4, so low concentrations of P are sometimes advised.

We believe that a 1% soln of dihydrogen sodium phosphate has a pH below 5 and we wonder whether you controlled the pH of your solutions.  An increasing amount of dihydrogen sodium phosphate could lead to a decreasing pH.

To carry the matter further, we would need to have a look at the recipe which your supplier uses and we would be willing to do that.

We'd be interested to know how this works out.

John Hewitson, Charles Hill, John Love, Barry Meatyard