Mutations Affecting Heterocyst Differentiation and Nitrogen Fixation in Blue-Green Algae

Stable spontaneous and NTG induced mutants of the blue-green alga, Cylindrospermum sp., affecting heterocyst differentiation were isolated. Mutants with complete loss of heterocyst differentiation formed long filaments unlike the parent algae and were unable to fix nitrogen. The second type of mutants formed long filaments where only terminal heterocysts were present. The third type of mutants showed differentiation of paired incipient heterocysts, defective in further development, at almost regular intervals in long filaments and rarely fully differentiated terminal heterocysts. The fourth type of mutants produced more mucilage where heterocysts were detached. The decrease in heterocyst percentage in mutants showed less growth in nitrogen-free medium and nitrogen fixation than parent algae.

NTG-induced short trichome forming mutants of Wollea bharadwajae with loss of nitrogen fixation and heterocyst differentiation were isolated. The mutants of this algae producing defective heterocysts were also not able to fix nitrogen. The mutational studies have confirmed that heterocysts are the sites of nitrogen fixation in heterocystous forms.