Bacterial photosynthesis

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Phototrophism is the biological process of converting (capturing) light energy into chemical energy. The process by which green Bacteria and the purple Bacteria carry out phototrophism share a common basic mechanism, usually referred to as photosynthesis, which is only briefly reviewed here.

Cyclic photophosphorylation

Photosynthetic Bacteria generally perform cyclic photophosporylation. Light energy, captured by antenna chlorophyls and accessory pigments, is channeled to reaction center complexes. This light energy excites an electron in the reaction center chlorophyl, and makes the reaction center a strong electron donor (reductant). An electron is transfered from the reaction center (which becomes oxidized) to the electron transport chain. The electron traverses the electron transport chain, resulting in the pumping of protons from the cytoplasm to the periplasm or lumen of the thylakoid. At the end of the electron transport chain the electron is transferred back to the oxidized reaction center in the ground state. The reduced reaction center is now ready to accept another packet of light energy and repeat the cycle. With each cycle comes an increase in the proton motive force, which is used by ATPase to generate ATP from ADP and phosphate. The net reaction of cyclic photophosphorylation is: light + ADP + phosphate -> ATP. All of the components of the cycle are regenerated within the cycle; no other inputs are required.

cyclic photophosphorylation

Obtaining reducing power for carbon fixation

Cyclic photophosphorylation generates only ATP. Photosynthetic organisms have several approaches to obtain NADH; organic compounds (photoheterotrophs), reverse electron flow (purple photoautotrophs and Chloroflexi), from ferridoxin from the electron transport chain (Chlorobi and heliobacteria), or oxygenic photosynthesis (cyanobacteria).

Photoheterotrophs use light only for their energy needs; reducing power and carbon are from organic compounds. This is independent of cyclic photophosphorylation.

Purple photoautotrophic Bacteria and Chloroflexi rely on a source of a strong chemical reductant, such as sulfide, thiosulfate, elemental sulfur, ferrous cation, or hydrogen as a source of electrons. These electrons are used to reduce cytochrome c, and the electrons traverse the electron transport chain in reverse, at the expense of the proton motive force (and so ultimately at the expense of ATP), and used to reduce NAD+ to NADH used for carbon fixation. This uses the same electron transport chain as photophosphorylation, but is otherwise an independent process.

reverse electron flow

Chlorobi have reaction centers that are more strongly reducing that are those of other organisms. As a result, the electrons transferred to the electron transport chain pass first through iron-sulfur proteins that can be used to generate reduced ferridoxin as a source of reducing power for carbon fixation. Electrons removed from the electron transport chain to reduce ferridoxin are replaced in the system from external sources of reductant, typically sulfide, sulfur, thiosulfate, ferrous cation, or hydrogen. Although these electrons are transferred to cyctochrome c, as in the purple photoautotrophs, they subsequently are transferred to the oxidized reaction center, rather than undergoing reverse electron flow. Therefore, photosynthesis in these organisms generations both ATP and reducing power for carbon fixation, but requires a chemical reductant as well as light.

sulfur-dependent photosynthesis

Cyanobacteria, including chloroplasts, carry out oxygenic photosynthesis; the traditional “Z-scheme” of photosynthesis. This process is based on the same cyclic photophophorylation used by other photosynthetic Bacteria. As in the other green Bacteria, reducing power (NADH or NADPH) is siphoned out of the system at the stage of ferridoxin. These electrons must be replaced for cyclic photophosphorylation to continue. Rather than directly use an external chemical reductants, however, cyanobacteria use a second reaction center (photosystem II; photosystem I is the reaction center used for cyclic photophosphorylation). This reaction center transfers electrons to the electron transport chain via reduced quinone. These electrons must be replaced, but photosystem II is so strongly oxidizing that it can accept electrons from water, generating molecular oxygen in the process.

oxygenic photophosphorylation

Rhodopsin phototrophy

It has recently been discovered that a wide range of Bacteria can capture light energy for ATP production using rhodopsin, a simple light-driven proton pump composed on a single protein (rhodopsin) and photopigment (retinal). This is apparently a significant form of phototrophy in the ocean, and perhaps other ecosystems, but is best known only from genes found in uncultivated organisms. This form of phototrophy seems to have spread widely, by horizontal transfer, from its origin in the halophilic Archaea. The mechanism of rhodopsin phototrophy is therefore discussed in later.