Chlorosome in the context of Chromatophore (bacteria)

Chlorobium is a genus of green sulfur bacteria. They are photolithotrophic oxidizers of sulfur and most notably utilise a noncyclic electron transport chain to reduce NAD+. Photosynthesis is achieved using a Type 1 Reaction Centre using bacteriochlorophyll (BChl) a. Two photosynthetic antenna complexes aid in light absorption: the Fenna-Matthews-Olson complex ("FMO", also containing BChl a), and the chlorosomes which employ mostly BChl c, d, or e. Hydrogen sulfide is used as an electron source and carbon dioxide its carbon source.

Chlorobium species exhibit a dark green color; in a Winogradsky column, the green layer often observed is composed of Chlorobium. This genus lives in strictly anaerobic conditions below the surface of a body of water, commonly the anaerobic zone of a eutrophic lake.

Chloroflexales is an order of bacteria in the class Chloroflexia. The clade is also known as filamentous anoxygenic phototrophic bacteria (FAP), as the order contains phototrophs that do not produce oxygen. These bacteria are facultative aerobic. They generally use chemotrophy when oxygen is present and switch to light-derived energy when otherwise. Most species are heterotrophs, but a few are capable of photoautotrophy.

The order can be divided into two suborders. Chloroflexineae ("Green FAP", "green non-sulfur bacteria") is the better-known one. This suborder uses chlorosomes, a specialized antenna complex, to pass light energy to the reaction center. Roseiflexineae ("Red FAP") on the other hand has no such ability. The named colors are not absolute, as growth conditions such as oxygen concentration will make a green FAP appear green, brown, or reddish-orange by inducing changes in pigment composition.

The Fenna–Matthews–Olson (FMO) complex is a water-soluble complex and was the first pigment-protein complex (PPC) to be structurally analyzed by x-ray spectroscopy. It appears in green sulfur bacteria and mediates the excitation energy transfer from light-harvesting chlorosomes to the membrane-embedded bacterial reaction center (bRC). Its structure is trimeric (C3-symmetry). Each of the three monomers contains eight bacteriochlorophyll a (BChl a) molecules. They are bound to the protein scaffold via chelation of their central magnesium atom either to amino acids of the protein (mostly histidine) or water-bridged oxygen atoms (only one BChl a of each monomer).

Since the structure is available, calculating structure-based optical spectra is possible for comparison with experimental optical spectra. In the simplest case only the excitonic coupling of the BChls is taken into account. More realistic theories consider pigment-protein coupling. An important property is the local transition energy (site energy) of the BChls, different for each, due to their individual local protein environment. The site energies of the BChls determine the direction of the energy flow.