Ammonium excretion by a mutant of the nitrogen-fixing cyanobacterium Anabaena siamensis

Anabaena siamensis isolated from rice fields in Thailand is a fast growing cyanobacterium with a high nitrogen-fixing activity. Mutant strains resistant to the l-glutamate analogue, l-methionine sulfoximine (MSX) were isolated by ethyl methanesulfonate mutagenesis. A stable mutant named A. siamensis SS1, which released ammonium to the medium, was studied further. In batch cultures the rate of ammonium production peaked at the early log phase and gradually decreased until the 4th day of growth when the cultures reached a density of 90 μg chl ml^-1. To obtain constant release of ammonium by SS1, continuous culture experiments were performed at a cell density of 5 μg chl ml^-1 and the following results were obtained: (1) growth rate as the parent (μ:0·123 h^-1) in the presence and absence of 500 μm MSX; (2) 48% GS transferase activity when compared with the parent; (3) ammonium excretion at a rate of 8 μmol (mg chl)^-1 h^-1 as measured up to 20 generations (120 h); (4) depressed nitrogenase activity; and (5) 30% higher nitrogenase activity than that of the parent. SS1 immobilized in alginate beads (5 μg chl ml^-1) exhibited values of glutamine synthetase and nitrogenase activity similar to those of free cells. However, ammonium excretion at the rate of 11·61 μmol (mg chl)^-1 h^-1 was obtained only up to 20 h after loading in bioreactors, due to the fast growth of SS1 as also occurred in batch cultures.