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purple bacteria[′pər·pəl bak′tir·ē·ə]
photosynthesizing bacteria living in fresh or salt water. The bacteria contain red pigments, or caro-tenoids, which give colonies of the bacteria or accumulations of bacterial cells a dark red coloration. Carotenoids absorb energy (from the blue and green bands of the spectrum that penetrate the water to great depths) and transfer it to the bacteriochloro-phyll. Unlike green plants, sulfureous purple bacteria use hydrogen sulfide rather than water as a hydrogen donor, whereas nonsulfureous purple bacteria use organic substances. Molecular oxygen does not form during photosynthesis.