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| Anaerobic Photosynthesis |
The photosynthetic process in all plants and algae, as well as in specific types of bacteria, involves the reduction of carbon dioxide to carbohydrate and the removal of electrons from water, resulting in the release of oxygen.
This process is known as oxygenic or aerobic photosynthesis. Water is oxidized by a multi-subunit protein located in the photosynthetic membrane. This is a molecular protein feature shared among more than 500,000 species of plants on earth.
While this is a common feature among nearly every form of plant life on earth, some photosynthetic bacteria can use light energy to extract electrons from molecules other than water. These bacteria are of ancient origin and are believed to have evolved before aerobic photosynthetic organisms.
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These anaerobic photosynthetic organisms occur in the domain Bacteria. Anaerobic photosynthetic bacteria, also known as anoxygenic photosynthetic bacteria, differ from aerobic organisms in that each species of these bacteria has only one type of reaction center.
In some photosynthetic bacteria the reaction center involves the oxidation of water and the reduction of the aromatic molecule plastoquinone. In other species it involves the oxidation of plastocyanin and the reduction of ferredoxin protein.
Photosynthetic bacteria are typically aquatic microorganisms inhabiting marine and freshwater environments, including wet and muddy soils, stagnant ponds, sulfur springs, and still lakes. They are classified into five groups based on pigment composition, metabolic requirements, and membrane structure: green bacteria, purple sulfur bacteria, purple nonsulfur bacteria, heliobacteria, and halophilic archaebacteria.
Some of these organisms are strict anaerobes; that is, they can grow only in the complete absence of oxygen. They cannot use water as a substrate, and they do not produce oxygen during photosynthesis. Facultative anaerobes, on the other hand, can grow either in the presence or in the absence of oxygen.
Green bacteria include two families, the Chloroflexaceae and the Chlorobiaceae. The Chlorobiaceae are strict anaerobes that grow by utilizing sulfide, thiosulfate, or organic hydrogen as an electron source.
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| Anaerobic Photosynthesis Process |
Chloroflexaceae are facultative aerobes which use reduced carbon compounds as electron donors. Purple sulfur bacteria uses an inorganic sulfur compound, such as hydrogen sulfide, as a photosynthetic electron donor.
Purple nonsulfur bacteria depend on the availability of simple organic compounds such as alcohols and acids as electron donors, but they can also use hydrogen gas. Purple sulfur bacteria must fix carbon dioxide to live, whereas nonsulfur bacteria can grow aerobically in the dark by respiration on an organic carbon source.
Heliobacteria are anaerobic photosynthetic bacteria that contain a special type of bacteriochlorophyll, BChl g, that works as both antenna and reaction center pigment. Halobacteria are very unusual. They cannot grow in low salt concentrations (or their cell walls collapse).
Typically, they are heterotrophs with an aerobic electron-transport chain, but they can also respire anaerobically, with nitrate or sulfur. In the absence of suitable electron acceptors they can ferment carbohydrates.
Halobacteria, when exposed to light in the absence of oxygen, can synthesize a purple membrane containing a single photosensitive protein called bacteriorhodopsin which, when illuminated, begins cyclic bleaching and regeneration, extruding protons from the cell. This light-stimulated proton pump operates without electron transport.
The mechanism by which halobacteria convert light is fundamentally different from that of higher organisms because there is no oxidation/reduction chemistry, and halobacteria cannot use carbon dioxide as their carbon source. As a result, some scientists do not consider halobacteria as being photosynthetic.
Process
The common features to both aerobic and anaerobic photosynthesis have been known since the mid-twentieth century:
Green plants:
CO2 + 2H2O + light → (CH2O) + O2 + H2O
Green sulfur bacteria:
CO2 + 2S + H2O + light → (CH2O) + 2S + H2O
In each case, inorganic carbon (CO2) is fixed into organic carbon (CH2O), the source of reductant is hydrogen in either water or hydrogen sulfide, and the chemical energy required for this activity is derived from light energy. The sulfur produced anaerobically is analogous to the oxygen produced by the oxygenic photosynthesis of green plants.
Photochemical processes in photosynthetic bacteria require three major components: an antenna of light-harvesting pigments, a reaction center within an intra-cytoplasmic membrane containing at least one bacteriochlorophyll, and an electron transport chain.
All photosynthetic bacteria can transform light energy into a trans membrane proton gradient used for the generation of adenosine triphosphate (ATP) and production of oxidase, but none of the anaerobic photosynthetic bacteria are capable of extracting electrons from water, so they do not evolve oxygen.
Many species can only survive in low-oxygen environments. To provide the necessary electrons for carbon dioxide reduction, anoxygenic photosynthetic bacteria must oxidize inorganic or organic molecules from their immediate environment.
Despite basic differences, the principles of energy transductions are the same in anaerobic and aerobic photsynthesis. Anaerobic photosynthetic bacteria depend on bacteriochlorophyll, a group of molecules similar to chlorophyll, that absorbs in the infrared spectrum between 700 and 1,000 nanometers. The antenna systems in these bacteria consist of bacteriochlorophyll and carotenoids, serving a reaction center where primary charge separation occurs.
Electron carriers include quinone and cytochrome bc complex. Electron transfer is coupled to the generation of electrochemical potential that drives phosphorylation by ATP synthase, and the energy required for the reduction of carbon dioxide is provided by ATP and dehydrogenase.
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