Bacteriochlorophylls are a class of pigments used by specific groups of bacteria to harness light energy for photosynthesis. Discovered in 1932, these molecules are structurally related to the chlorophylls found in plants and algae. Unlike plant-based photosynthesis, the process in these bacteria follows a different pathway. This allows them to occupy unique environmental niches where other photosynthetic life cannot survive.
Bacteriochlorophyll vs. Chlorophyll
The structural differences between bacteriochlorophyll and chlorophyll lead to significant functional distinctions. Both pigment types feature a central magnesium ion held within a complex ring structure, but variations in the side groups alter their properties. Bacteriochlorophylls possess a bacteriochlorin ring with two reduced pyrrole rings (B and D), whereas chlorophylls have a chlorin ring with only one reduced pyrrole ring (D).
Plant chlorophylls absorb red and blue portions of the visible light spectrum, reflecting green light. In contrast, bacteriochlorophylls capture light at longer wavelengths in the far-red and near-infrared regions, from around 800 to over 1000 nanometers. This allows bacteria to photosynthesize where visible light is scarce.
This difference in light absorption also affects coloration. Instead of the familiar green of plants, these phototrophic bacteria often appear purple, brown, or green. Their color is a direct result of reflecting the visible light they do not absorb for energy.
The Role in Anoxygenic Photosynthesis
Bacteriochlorophyll facilitates anoxygenic photosynthesis, which does not produce oxygen as a byproduct. This is a departure from the oxygen-releasing photosynthesis performed by plants, algae, and cyanobacteria, and it shapes the environments where these organisms can thrive.
In plant photosynthesis, water (H₂O) serves as the electron donor, and its splitting releases oxygen. Bacteria using bacteriochlorophyll rely on different substances as electron donors, such as hydrogen sulfide (H₂S), elemental sulfur, or hydrogen gas (H₂). The choice of donor is specific to the bacterial species and its habitat.
Despite the different inputs and byproducts, the goal is the same as in plants: to convert light energy into chemical energy, stored as adenosine triphosphate (ATP). The energy captured by bacteriochlorophyll initiates a series of electron transfers that drive this conversion, allowing these bacteria to act as primary producers in oxygen-depleted environments.
Varieties and Bacterial Hosts
Bacteriochlorophyll is not a single molecule but a family of related pigments, with major forms designated as BChl a, b, c, d, e, and g. This diversity, which arises from small modifications to the pigment’s side chains, allows different bacterial species to fine-tune their light-harvesting capabilities.
Different pigment types are associated with specific groups of phototrophic bacteria. For instance, purple bacteria predominantly utilize BChl a and BChl b. Green sulfur bacteria employ BChl c, d, and e, which are housed in unique antenna structures called chlorosomes.
The specific type of bacteriochlorophyll determines the precise wavelengths of infrared light the bacterium can use. For example, BChl b absorbs light at slightly longer wavelengths than BChl a. This specialization enables different species to coexist by partitioning the available light spectrum, preventing direct competition for light.
Ecological and Technological Importance
In nature, bacteria using bacteriochlorophyll are primary producers in anaerobic (oxygen-free) environments. They thrive at the bottom of stratified lakes, in marine and freshwater sediments, and around hydrothermal vents. In these anoxic zones, they form the base of unique food webs by converting light energy into biomass that supports other organisms.
Beyond their ecological roles, these pigments have potential technological applications. The efficiency of bacteriochlorophylls in absorbing near-infrared light makes them promising for developing new types of solar cells. These could capture a broader spectrum of solar energy than conventional silicon-based cells.
Bacteriochlorophyll derivatives are also being investigated for medical uses, particularly in photodynamic therapy for cancer. In this treatment, a light-sensitive drug derived from the pigment is administered to a patient. When the drug accumulates in tumor tissue, it is activated by infrared light, which penetrates tissue deeper than visible light, to generate reactive oxygen species that destroy cancer cells.