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Barbell Bacterium

One group of bacteria—the cyanobacteria—has completely transformed Earth’s environment through their long history. Three billion years ago, ancestors of cyanobacteria infused Earth’s ancient atmosphere with the byproduct of their photosynthesis—oxygen—changing the chemistry of the planet and setting the stage for entirely new oxygen-breathing life forms to evolve. Richelia (above) are cyanobacteria that live symbiotically inside single-celled marine plants called diatoms. The cyanobacteria have specialized dinitrogen-fixing cells that provide nitrogen to their hosts. (Photo by John Waterbury, Woods Hole Oceanographic Institution)