Chlorosome

A Chlorosome is a photosynthetic antenna complex found in green sulfur bacteria (GSB) and some green filamentous anoxygenic phototrophs (FAP) (Chloroflexaceae, Oscillochloridaceae). They differ from other antenna complexes by their large size and lack of protein matrix supporting the photosynthetic pigments. Green sulfur bacteria are a group of organisms that generally live in extremely low-light environments, such as at depths of 100 metres in the Black Sea. The ability to capture light energy and rapidly deliver it to where it needs to go is essential to these bacteria, some of which see only a few photons of light per chlorophyll per day. To achieve this, the bacteria contain chlorosome structures, which contain up to 250,000 chlorophyll molecules. Chlorosomes are ellipsoidal bodies, in GSB their length varies from 100 to 200 nm, width of 50-100 nm and height of 15 - 30 nm, in FAP the chlorosomes are somewhat smaller.

Inside the GSB cell, the chlorosomes are thought to be attached to the reaction centers in the cell membrane via FMO-proteins and a chlorosome baseplate composed of csmA proteins. Chlorosomes from FAP lack the FMO complex. The composition of the chlorosomes is mostly bacteriochlorophyll with small amounts of carotenoids and quinones surrounded by a galactolipid monolayer with ten different proteins attached to it.

Bacteriochlorophyll and carotenoids are two molecules responsible for harvesting light energy. Current models of the organization of bacteriochlorophyll and carotenoids (the main constituents) inside the chlorosomes have put them in a lamellar organization, where the long farnesol tails of the bacteriochlorophyll intermix with carotenoids and each other, forming a structure resembling a lipid multilayer.

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