Growth and nitrogen fixation of mutants defective in heterocyst differentiation in the blue-green alga Cylindrospermum sp.

Three classes of spontaneous and N-methyl-N'-nitro-N-nitrosoguanidine (NTG)-induced mutants of the blue-green alga Cylindrospermum sp. were isolated in the presence of combined nitrogen. Mutants with complete loss of heterocyst differentiation formed longer filaments than the parent alga and were unable to grow in nitrogen-free medium and to fix nitrogen. The second type of mutants formed long filaments with terminal heterocysts. In the third type of mutants, paired proheterocyst differentiation occurred at regular intervals in long filaments and these were defective in their further development into mature heterocysts. Mutants with decreased heterocyst frequency due to defective differentiation and development showed less growth and nitrogen fixation in nitrogen-free medium than the parent alga.