Cultivations Precultivation was performed in a 300-ml conical flask (with a silicon stopper, 200ml medium) for approximately 3 d at 30°C under static-light conditions using two tungsten bulbs (5 klx, 20 W/m2, on the surface of a culture vessel) as described previously (11). The main cultivation (denitrifying culture) was performed at 30°C for 5 d under dark anaerobic conditions in a 1.5-l Roux bottle (with a silicon stopper) containing GM medium with 5 g/f of KN03. After inoculation with the preculture broth (2%, v/v), the medium was sparged with sterile nitrogen gas (99.9%) in order to make it anaerobic. Culture bottles were mixed vigorously by hand at 12-h intervals in order to maintain their homogeneity.