chlorosomes

Chlorosomes are large light-harvesting antenna complexes found in certain anaerobic photosynthetic bacteria, most notably green sulfur bacteria (Chlorobi) and some green non-sulfur bacteria (Chloroflexi). They play a key role in allowing photosynthesis to proceed under very low light by capturing photons with little reliance on a rigid protein scaffold.

Inside a chlorosome, pigments form densely packed supramolecular aggregates composed primarily of bacteriochlorophylls (mainly types c, d, and e) and carotenoids. The pigment aggregates are enclosed by a lipid-rich envelope and require only a small set of proteins to anchor the complex to the photosynthetic apparatus. Energy absorbed by the chlorosome is transferred to the reaction center through a baseplate protein and the Fenna–Matthews–Olson (FMO) protein complex, which serves as a conduit to the reaction center.

Chlorosomes are notable for their size and the efficiency of energy capture, enabling light harvesting in environments where light is scarce. Their absorption extends into the near-infrared region, allowing bacteria to utilize wavelengths not efficiently absorbed by other photosynthetic systems. They are typically associated with microbial communities in illuminated but low-light habitats such as sediments and mats, where they contribute to photosynthetic productivity.

Ecological and evolutionary significance: chlorosomes represent a distinct adaptation in bacterial photosynthesis, illustrating how self-assembled pigment aggregates can serve as effective light-harvesting antennas. Their study informs understanding of natural light harvesting and has inspired concepts in bioinspired solar energy research.