BChLs

Bacteriochlorophylls (BChls) are a group of chlorophyll-related pigments used by anoxygenic photosynthetic bacteria to harvest light for energy conversion. Structurally, they resemble chlorophylls but differ in the saturation pattern of the tetrapyrrole macrocycle and in their isoprenoid side chains, which influences their light absorption and membrane binding. As a result, BChls extend the usable light spectrum into the infrared region, enabling photosynthesis under low-light and anaerobic conditions.

Natural BChls come in several forms, commonly designated as BChl a, b, c, d, e, and g. These pigments are distributed among different bacterial groups, most notably among purple photosynthetic bacteria and green sulfur bacteria. In these organisms, BChls function as important antenna pigments and as core components of reaction centers, helping to funnel excitation energy to electron transfer sites.

BChl biosynthesis shares core steps with chlorophyll biosynthesis but includes specific reductions and substitutions that yield the bacteriochlorin-type macrocycle. The genes responsible for BChl production are typically organized in operons within bacterial genomes and are regulated by environmental factors such as light intensity and oxygen availability. The resulting pigments are generally more hydrophobic, aligning with their roles in membrane-bound photosynthetic complexes.

Ecologically, BChls enable photosynthetic bacteria to occupy niches where oxygenic phototrophs are less competitive, contributing to primary production in diverse habitats, including stratified aquatic systems and microbial mats. Their spectral diversity and integration into specialized antenna systems reflect adaptations to distinct light environments.