Powerful mutagenicity of a bipyridylium herbicide in a nitrogen-fixing blue-green alga Nostoc muscorum.

The herbicide, paraquat (1,1'-dimethyl-4,4'-bipyridylium ion), was found to be toxic and lytic to Nostoc muscorum in N2 (at the expense of elemental nitrogen, i.e. unsupplemented with any combined nitrogen source) and NO-3 media, without any apparent inhibitory or stimulatory effect on its nitrogen-fixing apparatus, i.e. heterocyst formation. At a dose of paraquat resulting in 20, 50 and 75% survival, induction of reverse mutations (from het- nif- auxotrophy to het+ nif+ prototrophy), forward mutations (for streptomycin [St]-resistance), and auxotrophic mutations (carbon-auxotrophy through methylamine [MA]-resistance) were observed with frequencies comparable to those obtained through induction with the well known mutagen MNNG (N-methyl-N'-nitro-N-nitrosoguanidine).