Anoxygenic photosynthesis

Anoxygenic photosynthesis is a special way some tiny living things called bacteria make food. It is different from the way plants and blue-green algae make food.

In anoxygenic photosynthesis, bacteria use things like hydrogen sulfide instead of water. They make sulfur instead of oxygen.

These bacteria can use two different ways to turn carbon dioxide into food. Plants usually use just one way. They also use a special material called bacteriochlorophyll to catch light energy. This is different from the chlorophyll that plants use.

Anoxygenic photosynthesis came before the kind that makes oxygen. It developed after another way of getting energy called chemolithoautotrophy. It uses one of two types of special centers to change light into energy.

The rise of anoxygenic photosynthesis

Long ago, during the Archean Era, our Earth had very little oxygen in its atmosphere. Instead, it had lots of iron and strong harmful ultraviolet (UV) rays because there was no ozone layer to block them. Scientists studying sedimentary rock from the oceans believe that a special kind of photosynthesis, called anoxygenic photosynthesis, existed before the kind we usually learn about in plants. They think this because certain iron minerals were not around, which suggests oxygen wasn’t being made back then.

Some special places under the sea, called serpentinizing hydrothermal vents, have been around for about 4.2 billion years. These vents create hydrogen-based compounds like H₂ and H₂S. Before photosynthesis existed, these vents helped make simple organic matter using a process that relied on hydrogen. Later, special bacteria learned to use H₂S instead of oxygen in their chemical reactions. This helped them survive and grow where there was no oxygen.

Bacteria

Some special kinds of bacteria can do a type of photosynthesis called anoxygenic photosynthesis. These include green sulfur bacteria, Chloracidobacterium, heliobacteria, acidobacteriota, red and green filamentous phototrophs like Chloroflexia, and purple bacteria. Most of these bacteria use one kind of system for photosynthesis, while others use a different system. Some bacteria in the Myxococcota group might also be able to do this because they have the genes needed for photosynthesis.

Pigments

The photopigments used by some bacteria for a special kind of photosynthesis are like chlorophyll but a little different. They work best with a type of light called the near-infrared, which is not the same light that plants or cyanobacteria use. These pigments, called bacteriochlorophylls, are mostly in bacteria that live without oxygen. The light they absorb changes depending on the type of bacteria and where the pigment is in the cell's membrane. Usually, they absorb light between 800 and 1040 nm, but some bacteria, like green sulfur bacteria, use different types of bacteriochlorophylls that absorb light between 720 and 755 nm.

Reaction centers

There are two main types of special light-processing systems in bacteria. The type I systems are found in certain green bacteria and other types, while the type II systems are found in FAPs and purple bacteria.

Type I reaction centers

In green sulfur bacteria, a special pair of molecules called P800/P840 works with other molecules to move energy. When light hits these molecules, they become excited and pass energy along a chain. This helps create a special energy difference across the cell's membrane, which is used to make ATP, a key energy source for the cell.

Type II reaction centers

Type II systems work in a way that is similar to the systems in plants, but they do not produce oxygen. In purple bacteria, a special pair of molecules called P870 gets excited by light and passes energy along a chain. This helps create an energy difference across the cell's membrane, which is used to make ATP. Hydrogen in the environment often provides the energy for this process.