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SEM of Heliobacterium chlorum cells

SEM of <em>Heliobacterium chlorum</em> cells, a slow-moving bacterium

This scanning electron micrograph (SEM) shows Heliobacterium chlorum cells, slow-moving bacterium that have been isolated from the soil in a lab in Bloomington, Ind. (The bar at the bottom of the image equals 5 micrometers.)

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First discovered in Thai rice paddies, the microbe known as Heliobacillus mobilis belongs to a group of bacteria called Heliobacteria. These bacteria respond to light and contain a form of chlorophyll that is intermediate to that found in green plants and cyanobacteria--the first bacteria to release oxygen on Earth. Some scientists believe that early forms of cyanobacteria used energy from sunlight to transform the Earth's atmosphere into an oxygen-rich blanket suitable for animal life.

Structural similarities between the two bacteria suggest the possibility that the Heliobacterium chlorum microbe could be the direct descendant of the ancient bacterium that produced cyanobacteria. Further evidence to support this possibility includes the comparison of molecules called ribosomal RNA that are found in Heliobacteria and other microbes. Ribosomal RNA, which plays a crucial role in making proteins, has evolved slowly over the years. The type found in Heliobacterium chlorum closely matches the ribosomal RNA found in some older bacteria that do not respond to light, yet the RNA are strikingly different from that residing in other more common light-sensitive bacteria. The Heliobacterium chlorum related to both cyanobacteria and older, more primitive microbes seems clearly marked as an evolutionary link. (Date of image: Unknown)

Credit: F.R. Turner, Indiana University, Bloomington

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