The Mechanisms Involved in the Removal of Oxygen and Oxygen Radicals in the N₂-Fixing Cyanobacterium Nostoc Muscorum

Abstract

Cyanobacteria (blue green algae) consist of the largest group of photosynthetic prokaryotes in nature. They differ from other photosynthetic bacteria by their oxygenic photosynthetic activity employing water as primary electron donor for CO₂ fixation, (Binder 1982). The cyamobacteria are therefore combining the simplicity of the prokaryotic cell structure, with the highly developed photosynthetic functions of the higher plants, (Papageorgiou and Tzani, 1980). The cyanobacterium Nostoc muscorum is also capable of fixing molecular nitrogen to ammonia under aerobic conditions. The cells are organized in a multicellular filament including the vegetative cells responsible for oxygenic photosynthesis and CO₂ fixation, and fewer heterocysts which accomodate the N₂-fixing enzyme, nitrogenase, and its supporting systems providing reductants, ATP and protecting mechanisms. Heterocysts are formed by differentiation of vegetative cells during grown in media deficient of combined nitrogen compounds, and frequency of heterocysts in air-grown cultures approaches 5–8 percent (Wolk 1982, Peterson and Burris 1976). The frequency of heterocysts in nitrate-grown cultures is lower, and is very low in ammonia-grown cultures (Wolk 1979, Neuer et al. 1983). The heterocyst is considered to be a specialized cell for N₂-fixation, under aerobic conditions, acquiring unique structure and functions throughout the process of differentiation, providing the suitable conditions for the expression and activity of nitrogenase (Stewart et al. 1969, Haselkorn 1978).