Bacteriochlorophylls

Bacteriochlorophylls are a group of photosynthetic pigments found in certain anaerobic bacteria. They are chemically related to chlorophylls but differ in the structure of their central ring and in their side chains, giving them distinct light-absorbing properties. Like chlorophylls, bacteriochlorophylls contain a magnesium ion coordinated within a porphyrin-like ring, but the ring is a bacteriochlorin, which is more reduced and shifts absorption toward longer wavelengths. The hydrophobic side chains vary among homologs, and in some forms the phytol tail is shortened or modified.

These pigments absorb light in the near-infrared region, extending into wavelengths that largely escape absorption by plant chlorophylls. This spectral tuning enables photosynthetic bacteria to harvest light in environments where light is scarce or filtered, such as deep water, microbial blankets, or densely pigmented habitats. The ability to absorb far-red and near-infrared light complements other photosynthetic pigments within the same organism or community, facilitating efficient energy capture under low-light conditions.

Bacteriochlorophylls occur primarily in anaerobic photosynthetic bacteria, including purple bacteria and green sulfur bacteria. In purple bacteria, they participate in light-harvesting complexes that transfer energy to reaction centers for charge separation and electron transport. In green sulfur bacteria, certain bacteriochlorophylls predominate in chlorosomes, large antenna structures that can contain very high pigment-to-protein ratios and function without a conventional outer membrane.

Several homologs are known, notably bacteriochlorophyll a and b in many purple bacteria, and bacteriochlorophylls c, d, and e associated with chlorosomes in green sulfur bacteria. The diversity of BChLs reflects adaptation to different light environments and ecological niches, making them key pigments for studying microbial photosynthesis and energy capture in low-light ecosystems.