Green and Purple Bacteria

 

The Green and Purple Bacteria

The purple bacteria are a small group of gram-negative eubacteria. They are unicellular and reproduce by binary fission or, in a few species by budding. Most are motile by flagella; a few are immotile. Purple bacteria contain pigments on thylakoids, but they are different from the cyanobacteria in that they contain a special type of chlorophyll called bacterio-chlorophyll, they do not give off oxygen, and they live in habitats that lack oxygen. These bacteria are named for their predominant colors. They exist as single cells of many different shapes, and are frequently motile. These bacteria utilize sulphur compounds in their metabolism, and some may deposit intracellular granules of sulphur or sulfates. These organisms live in various aqueous habitats, including sulphur springs, fresh water lakes and swamps. All purple bacteria are, at least potentially photoautotrophs, capable of growing anaerobically in the light with CO₂ as the carbon source and reduced inorganic compounds as the electron donor. Under these conditions, the Calvin-Benson cycle is the principal pathway of carbon assimilation. However, the purple bacteria can also develop photo-heterotrophically under anaerobic conditions in the light at the expense of organic compounds, of which acetate is the most widely utilized.

                Purple sulphur bacteria can sometimes be seen growing as colored masses in sulphur-rich habitats such as sulphur springs. The cells are relatively large, sometimes in excess of 5 Jlm in diameter, and some are motile by flagella. They may also have gas vesicles, which enable them to move up or down to their preferred level in the water column. The purple bacteria, such as Chromatium, also use sulphur, sulphur compounds or hydrogen gas to reduce carbon dioxide. They are distinguished from the green bacteria by their type of chlorophyll, location of stored sulphur and ribosomal RNA.

Many species of purple sulphur bacteria are strict anaerobes and phototrophs, but some can grow in absence of light aerobically, oxidizing reduced inorganic or organic compounds as a source of energy. Representative genera of purple sulphur bacteria include Chromatium, Thiospirillum, and TlziodichJon.

The purple non-sulphur bacteria are found in a wide variety of aquatic habitats, including moist soils, bogs and swampy areas. One important characteristic that distinguishes them from the purple sulphur bacteria is that they preferentially use a variety of organic molecules rather than hydrogen sulfide as a source of electrons for reducing power. Purple non sulphur bacteria are remarkably versatile metabolically. These bacteria can grow aerobically in the absence of light using chemotrophic metabolism. Representative genera of purple non sulphur bacteria include Rhodobacter and Rhodopseudomonas.

Green sulphur bacteria use hydrogen sulfide as a source of electrons for reducing power and they form sulphur granules. These granules, however, form outside of the cell. The green sulphur bacteria lack flagella, but many have gas vesicles. All are strict anaerobes and none can use a chemotrophic metabolism. Representative genera include Chlorobium and Pelodictyon.

Green noonsulhur bacteria are characterized by their filamentous growth. Metabolically, they resemble the purple non-sulphur bacteria, preferentially using organic compounds to generate reducing power. These bacteria can use hydrogen gas or hydrogen sulfide. They can grow in the dark aerobically using chemotrophic metabolism. Clzloroflexus is the representative genus in this group.