Versicoloured microbial mats (Farbstreifen-Sandwatt) in the intertidal zone of the North Sea island of Mellum were investigated with respect to oxygenic photosynthesis and nitrogen fixation. In laboratory cultures of the cyanobacterium Oscillatoria limosa a temporary separation of both incompatible processes occurs. Also in situ a negative correlation of sedimentary oxygen concentration and nitrogenase activity was found. If the sediment turned anaerobic during the night, no acetylene reduction was measurable. In systems, however, which stayed oxygenated during the night, nitrogenase activity was found at night. Peaks of nitrogenase activity were often detected at sunrise. Sometimes acetylene reduction was measured during the day when oxygenic photosynthesis was interrupted or depressed, due to light conditions (clouding effects). The diurnal pattern of nitrogenase activity as well as the acetylene reduction rate was influenced by the variation of all environmental conditions. The highest rate of nitrogenase activity (25 µmol C2H4/mg Chl a/h) detected in situ agreed well with maximal nitrogenase activity found in pure cultures of Oscillatoria limosa
