Is it possible to isolate methionine auxotrophs in Chlamydomonas reinhardtii? Consideration of photodynamic action of the amino acid.

In order to address the problem of amino acid auxotroph scarcities in photosynthetic organisms, an attempt was made to recover methionine auxotrophs in a unicellular green alga, Chlamydomonas reinhardtii. Evidence of methionine permeation into the algal cells was provided by the successful competition of the amino acid with its antimetabolite, methionine sulfoximine. No methionine auxotrophs were isolated despite the frequent recovery of both nicotinamide and arginine auxotrophs, selected as controls, and of methionine sensitive mutants. The use of either N-methyl-N'-nitro-N-nitrosoguanidine or ultraviolet light as mutagens appeared not to alter the mutation spectrum. Medium containing methionine was shown to be inhibitory to the growth of cells in culture under fluorescent light and this toxicity was further stimulated in the presence of riboflavin. In view of our results, and the observations of other studies, the non-recoverability of methionine auxotrophs is discussed as a function of the photodynamic action of methionine.