A Number of Prokaryotes Carry Out Photosynthesis:

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A number of prokaryotes carry out photosynthesis. For example, Anaerobic photosynthetic bacteria fix carbon dioxide from air but unlike more familiar photosynthetic organisms, many use hydrogen sulphide(H₂S) rather than water as a source of electrons and hydrogen atoms. They are called Anaerobic because they do not survive in oxygen. In fact, instead of producing oxygen, they produce elemental sulfur as a waste product.

Cyanobacteria: Cyanobacteria are prokaryotes that are believed to be among the most primative organisms on the planet. Unlike the Anaerobic photosynthetic bacteria, Cyanobacteria can survive in oxygen.

Curiously, like many of the Anaerobic photosynthetic bacteria, at least some cyanobacteria can use hydrogen sulphide as a source of hydrogen and electrons. Some scientists hypothesize that hydrogen sulphide was much more abundant in the atmosphere and dissolved in water and was used as the main source of hydrogen and electrons since they are easier to extract from hydrogen sulphide rather than water. Since the "waste" product is innsoluable sulfur and H₂S less abundant than water it eventually was largely exhausted forcing the opportunistic cyanobacteria to switch to water.

This picture shows Oscillatoria, a common and important cyanobacteria. Many cyanobacteria are important becuase they can "fix" nitrogen; that is convert nitrogen gas into nitrates which are usable by plants.

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pgd: 06/26/02