BARBELL BACTERIUM—Cyanobacteria have mechanisms that allow two antagonistic physiological processes to coexist in the same organism: oxygen-producing photosynthesis and dinitrogen fixation, which is inhibited by oxygen. In Richelia (above), the two processes are separated by space: Dinitrogen fixation occurs only in the bulbous, specialized cells (heterocysts) at the end of a 60-micrometer-long, filamentous cyanobacterium. (Photo by John Waterbury, WHOI)[back]
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