Fundamentals of Microbiology

2.4.2 Photoautotrophs

Photoautotrophs use light as a source of energy and carbon dioxide as their chief source of carbon. They include photosynthetic bacteria (green sulfur and purple sulfur bacteria and cyanobacteria), algae, and green plants. In the photospynthetic reactions of cyanobacteria, algae, and green plants, the hydrogen atoms of water are used to reduce carbon dioxide, and oxygen gas is given off. Because this photosynthetic process produces O₂, it is sometimes called oxygenic.

Some bacteria cannot use H₂O to reduce CO₂ and cannot carry on photoysyntheis when oxygen is present (they must have an anaerobic environment). Consequently, their photosynthetic process does not produce O₂ is called anoxygenic. Two of the families are photoatuotrophs: the green sulfur and purple sulfur bacteria. The green sulfur bacteria, such as Chlorobium use sulfur (S), sulfur compounds (such as hydrogen sulfide, H₂S), or hydrogen gas (H₂) to reduce carbon dioxide and form organic compounds. Applying the energy from light and the appropriate enzymes, these bacteria oxidize sulfide (S^2-) or sulfur (S) to sulfate (SO₄^2-), or hydrogren gas to water (H₂O). The purple sulfur bacteria, such as Chromatium, also use sulfur, sulfur compounds, or hydrogen gas to reduce carbon dioxide. They are distinguished from the green sulfur bacteria by their type of chlorophyll and location of stored sulfur.

The chlorophyll is used by these photosynthetic bacteria are called bacteriochlorophylls, and they absorb light at longer wavelengths than that absorbed by chlorophyll a. Bacteriochlorophylls of green sulfur bacteria are found in vesicles called chlorosomes or chlorobium vesicles) underlying and attached to the plasma membrane. In the purple sulfur bacteria,the bacteriochlorophylls are located in invaginations of the plasma membrane (intracytoplasmic membrane)