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Index >> Bacterial Photosynthesis >> Rhodospirillaceae (Formerly Athiorhodaceae ) Purple Non Sulphur Bacteria

Rhodospirillaceae (Formerly Athiorhodaceae ) Purple Non Sulphur Bacteria

	Rhodospirillaceae (Formerly Athiorhodaceae ) Purple Non Sulphur Bacteria - The purple non-sulphur bacteria do not form globules of elemental sulphur inside the cells. There are some purple bacteria which oxidize sulphide to extracellular sulphur and further to sulphate. Pfennig-(1977) has placed such species in the non-sulphur bacteria. The principal photosynthetic pigments are bacteriochlorophylls a and b and carotenoids of groups I, 2 arid 4. There is one photocentre.The sources of hydrogen are mainly organic molecules. In some cases hydrogen sulphide serves a the source of hydrogens, but not sulphur.
The purple non-sulphur bacteria differ from the Chromatiaceae the Chlorobiaceae in that they can grow either anaerobically, in light (photosynthesis) or aerobically in the dark (oxidative respiration). This switch from photophosphorylation to oxidative phosphorylation requires the presence of both electron transport systems. Light is absorbed by the bacteriochlorophyll molecule P800, and transferred to the reaction centre that is P890 in Rhodospirillum rubrum or P870 in Rhodopseudomonas sphaeroides. The ratio between P800 and the reaction centre molecule is 2: 1.
It bas been postulated that an electron acceptor X, and possibly a second electron acceptor Y, are reduced when P890 is oxidized. X is probably an iron-sulphur (Fe-S) protein. Two different types of ferredoxins type (and type II, have been isolated from Rhodospirillum rubrum. Ferredoxin type II is found in cells grown heterotrophically in the dark and light. Type I is found along with type II only when cells are grown in light. Whether connection exists between X and the two types of ferredoxins is not known.
Among the quinones, ubiquinones- 10 (UQ-10) is, predominant in the Rhodospirillaceae: The other quinones found are ubiquinone If; (Rhodopseudomonas gelatinosa) and rhodoquinone. The quinones are reduced by the reaction centre P890. The main types of cytochromes found in the Rhodospirillaceae are the, c-type cytochromes (c550, 5, c553 and c558) and b type cytochrome. Oxidation of c553 takes place mainly at low light intensities, while the other two c type cytochromes are equally well oxidized at high light intensities.
	Rhodopseudomonas palustris can couple energy dependent NAD reduction in the dark to oxidize thiosulphate to sulphate. Electrons released from thiosulphate oxidation probably enter the electron transport chain at cytochrome c. The energy linked reverse flow of electrons resembles that of aerobic respiration in Thiobacillus. Thus in, the presence of light and under anaerobic conditions, there is ATP-generating cyclic electron flow and photoreduction of NAD. Under aerobic conditions in the dark there is aerobic respiration with reversed electron flow.

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