Bacteriochlorophyll (BChl) is a type of pigment that enables various phototrophic bacteria to perform photosynthesis. Unlike chlorophyll, which is the primary pigment in plants, algae, and cyanobacteria, BChl does not produce oxygen as a byproduct of photosynthesis. Instead, it uses wavelengths of light that are not absorbed by plants or cyanobacteria, such as infrared or near-infrared light.

BChl was discovered by C. B. van Niel in 1932 . It has a similar chemical structure to chlorophyll, but with some modifications. Both pigments have a porphyrin ring with a central metal atom (magnesium for chlorophyll and bacteriochlorophyll), but BChl has two reduced pyrrole rings (B and D) that form a bacteriochlorin ring. This ring has a lower energy gap than the porphyrin ring, allowing BChl to absorb longer wavelengths of light.

There are several types of BChl, each with a different absorption spectrum and molecular structure. The most common ones are BChl a, b, c, d, e, f, and g. They are found in different groups of bacteria, such as purple bacteria, green sulfur bacteria, heliobacteria, chloroflexi, and chloracidobacteria . Some bacteria can have more than one type of BChl in their photosynthetic apparatus, forming complex antenna systems that capture light efficiently.

BChl plays a vital role in the energy conversion process of photosynthesis. It acts as an electron donor and acceptor in the photochemical reactions that generate ATP and NADPH. These molecules are then used to fix carbon dioxide into organic compounds. BChl also transfers the excitation energy from light to the reaction center, where the charge separation occurs.

BChl is an example of how nature has evolved diverse solutions to harness solar energy for life. By studying the properties and functions of BChl, scientists can gain insights into the origin and evolution of photosynthesis, as well as develop novel applications for biotechnology and renewable energy.

One of the most intriguing aspects of BChl is its ability to absorb light in the infrared region of the spectrum. This allows some bacteria to thrive in environments where visible light is scarce or absent, such as deep-sea hydrothermal vents, hot springs, or the subsurface of rocks and soils. These bacteria can use geothermal or chemical energy sources to supplement their photosynthesis, or even switch to other metabolic modes when light is unavailable.

Another fascinating feature of BChl is its structural diversity and adaptability. Depending on the environmental conditions, such as pH, temperature, or salinity, some bacteria can modify their BChl molecules by adding or removing functional groups, such as methyl, ethyl, vinyl, or acetyl groups. These modifications can alter the absorption and fluorescence properties of BChl, as well as its interactions with other pigments and proteins. This allows the bacteria to optimize their photosynthetic performance and efficiency under different scenarios.

BChl is not only important for bacteria, but also for other organisms that live in symbiosis with them. For example, some marine animals, such as sponges, corals, clams, or worms, host BChl-containing bacteria in their tissues or cells. These bacteria provide them with organic nutrients and oxygen derived from photosynthesis, while the animals offer them protection and access to light. Some of these symbiotic associations are so intimate that the bacteria can transfer their BChl genes to the animal genome, creating a new type of photosynthetic organism.