Parallel and sequential processing of visual information in man: investigation by evoked potential recording.

Publisher Summary This chapter describes comparative nitrogen fixation in photosynthetic bacteria and in other microorganisms. The photosynthetic bacteria are among the metabolically most primitive currently living forms. Photosynthetic bacteria are classified into three major groups: the purple sulfur bacteria (Thiorhodaceae), the green sulfur bacteria (Chlorobacteriaceae), and the purple and brown nonsulfur bacteria (Athiorhodaceae). These bacteria depend on the presence of external electron donors, such as reduced sulfur and/or organic compounds, and thrive in moist and muddy soils, ponds, rivers, lakes, sulfur springs, and marine environments. Some photosynthetic bacteria have been known to evolve hydrogen in the dark at a slow rate via well-known mechanisms such as the ferredoxin-dependent phosphoroclastic reaction typical of the Clostridia. Hydrogen has been seen to inhibit nitrogen fixation in a wide variety of nitrogen-fixing systems and all nitrogen-fixing organisms contain a hydrogen-activating enzyme. When molecular nitrogen is available, the electrons can be used to reduce the dinitrogen molecule. The photoproduction of hydrogen is a control system for regulating the redox potential and the energy charge of the cell so that the intracellular environment is suitable for biosynthesis. The detailed mechanism of nitrogenase activity remains unreported, while little direct evidence is available on the binding of N2 and other electron acceptors to nitrogenase and the role of Mo and Fe in the binding.

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