Anoxygenic Photosynthesis

In this section, we will focus on the process of anoxygenic (non-oxygen producing) photosynthesis. While you may be very familiar with the type of photosynthesis that generates oxygen as a waste product (like cyanobacteria and plants do), it is believed that anoxygenic photosynthesis evolved first. Anoxygenic photosynthesis is unique to bacteria, but is found throughout the Bacterial Domain, particularly in groups like the purple non-sulfur bacteria.

Making ATP

Anoxygenic phototrophs have one photosystem that can be used to generate ATP. When light energy is used to the reaction center, a pair of electrons goes from about E'o =+0.5 volts to E'o=-0.7 volts. This is work done by the system. These low potential electrons are then transferred to a quinone, then to iron-sulfur proteins (cytochrome bc), then to another cytochrome (c2), eventually returning to the reaction center, with an E'o = +0.5 again. Thus, the electron transfer is considered to be cyclical.

During electron transport, protons are pumped outside the cell by quinones, creating a proton motive force. The membrane-bound ATPase can take advantage of the electro-chemical gradient and use it to produce ATP.

Generating Reducing Power

While some purple non-sulfur bacteria get their carbon from organic compounds, most phototrophs are autotrophs-- that is they get their carbon by converting CO2 to reduced sugars (a process called "carbon fixation". It takes a lot of reducing power to reduce CO2. The cells store this reducing power in the form of NADPH.

Different bacteria generate reducing power in different ways. The purple and green NON-SULFUR bacteria use reduced compounds from their environment (like H2 or reduced organic compounds) as a source of electrons to reduce NADP+ to NADPH. If the electron donor has a low enough reducing potential, it can directly donate electrons to NADP+ (which has an E'o = -0.32 volts). For example, many purple non-sulfur bacteria have hydrogenase enzymes that take electrons from H2 (E'o= -0.42 volts).

The purple and green SULFUR bacteria use electron donors that have a higher reduction potential than NADPH (like sulfide or thiosulfate). In this case, electrons from sulfide or thiosulfate are transferred to a high potential electron carrier in the electron transport chain, and energy is used to move the electrons to a lower reduction potential, so that they can be used to reduced NAPD+. This process is referred to as "reverse electron transport."