BChl

BChl, short for bacteriochlorophyll, refers to a family of chlorophyll-related pigments used by various photosynthetic bacteria. These pigments are essential for light harvesting and energy conversion in organisms ranging from purple bacteria to green sulfur bacteria and Chlorobi. BChls differ from plant and algal chlorophylls in their ring structure and tuning of light absorption, enabling photosynthesis in habitats with different light quality and intensity.

Structure and spectral properties: Like chlorophylls, BChls are magnesium-containing tetrapyrroles with a long hydrophobic phytol tail. Their macrocycle is a bacteriochlorin, a reduced form of the porphyrin ring, which shifts absorption toward longer wavelengths into the near-infrared. This red-shift allows bacteria to exploit light that is weak or largely absorbed by other organisms. There are several BChl homologs, commonly designated BChl a, b, c, d, e, and f, which differ in substituents on the macrocycle and in their side chains. The different homologs are associated with distinct light-harvesting structures and cellular niches.

Occurrence and function: BChl pigments are core components of photosynthetic systems in many anaerobic and microaerophilic bacteria. In purple bacteria, BChl a participates in reaction centers and in the LH1 and LH2 antenna complexes, forming characteristic B800 and B850 absorption bands. In green sulfur bacteria and related groups, BChl c, d, and e predominate in chlorosomes, enabling efficient light capture in very low-light environments and transferring energy to reaction centers through accessory pigments.

Biosynthesis and regulation: BChl biosynthesis proceeds from porphyrin precursors through specialized enzymatic steps that reduce the macrocycle and modify side chains to produce the various BChl homologs. Gene clusters such as those encoding bch enzymes coordinate production in response to light, oxygen, and ecological niche.

Significance: By extending absorptive capacity into the near-infrared, BChls broaden the ecological and evolutionary versatility of photosynthetic bacteria and inform studies of natural light harvesting and bio-inspired energy conversion.