Photosynthesis
All organisms need energy and a source of carbon to make more biomass and, thus, grow. Some bacteria (organotrophs) get their energy from reduced organic molecules, while other bacteria can convert light energy to ATP (the phototrophs). Some bacteria (the heterotrophs) get their carbon from reduced organic molecules, while other bacteria (the autotrophs) can use CO2 as their carbon source. While there are certainly exceptions in the microbial world, most phototrophs tend to be autotrophs, while most organotrophs are heterotrophs.
Photosynthesis is the process by which cells convert light energy to cellular energy (ATP). Light-harvesting pigments embedded in membranes capture light energy and transfer that energy to a protein-complex called a reaction center. Here, the energy is converted into excited, low potential electrons. These low potential electrons are fed into an electron transport chain, where they "fall" through a series of electron carriers, generating a proton motive force. Membrane-bound ATPases then use the proton motive force to make ATP.
The Light Reactions
The first step in photosynthesis is capturing light energy. Most organisms use a molecule called "chlorophyll" (in its many forms) to capture light energy. There are a number of different types of chlorophyll, most of them varying by only a small side group, and each with the ability to absorb different wavelengths of light. Many phototrophic bacteria also have a number of auxillary light-harvesting pigments, giving them a wide cariety of distinct colors.
Light-harvesting pigments (LHP) are clustered together in groups of 20-30 molecules in the membrane. LHP's feed light into a few special chlorophyll molecules called "reaction centers," to aid in the collection of light energy. Together, these molecules make up a photosystem.
When light energy is funneled into the reaction centers, a special pair of electrons (which exist at a relatively high reduction potential) are excited to a very low (reducing) potential. This is work done by the system. These low potential electrons can then "fall" through an electron transport system containing quinones, iron-sulfur proteins, and cytochromes.
Quinones cannot accept the negative charge of the electron alone, so must also pick up protons (H+) when reduced. They take these protons from the interior of the cell. When the quinones give the electrons up to the next electron carrier, the protons are released outside the cell. This way, electron transport drives the build-up of protons outside the cell, that is, it builds a proton motive force (PMF). Once a PMF is established, the proton electro-chemical gradient drives the protons back into the cell through a membrane-bound ATPase. This allows the ATPase to make ATP from ADP and inorganic phosphate (PO4 -3). Thus, these light-driven electron transport reactions result in the production of ATP for the bacterial cell.
So why have bacteria gone through the trouble of evolving different types of pigments? Let's look at a classic example to give some insight into this question. In the late 1880s, the Russian microbiologist Sergei Winogradsky revealed with his "Winogradsky column" that chemolithic autotrophy (the use of inorganic compounds and light energy to make organic carbon) was prevelent in nature. From clever enrichment techniques, he discovered bacteria that could use inorganic compounds (like H2S) as electron donors and carry out photosynthesis. In the Winogradsky column, different colored layers exist which are representative of different types of bacteria. At the top of the column where the oxygen concentration and intensity of light are greatest, the cyanobacteria and algae grow. These microbes carry out oxygenic photosynthesis, which generates oxygen. Below the cyanobacteria and algae are the purple and green non-sulfur bacteria. These bacteria can grow using either anoxygenic photosynthesis or aerobic respiration, so are perfectly suited for the dynamic oxic-anoxic interface. Next come the purple and green sulfur bacteria. These microorganisms use hydrogen sulfide (H2S) as an electron donor while the photons of light move the electrons through a chain to generate ATP (see above).
The vertical placement of the different types of bacteria in the column is not an accident. The cyanobacteria didn't just happen to miss the train to Winogradsky column, in which case managed to barely crawl in at the top!! The bacteria essentially "migrate" to certain zones within the column for special reasons. Okay, so you think you have an idea of why they are found at different levels?