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Chlorobaculum tepidum strain TLST (formerly Chlorobium tepidum) is a green sulfur bacterium (GSB; phylum Chlorobi), which was first isolated from a sulfidic hot spring stream near Rotorua, New Zealand. Similar to other GSB, the photosynthetic apparatus of Cba. tepidum comprises homodimeric type-1 photochemical reaction centers, the Fenna-Matthews-Olson BChl a-binding protein, and chlorosomes. Chlorosomes are sac-like structures that can contain up to 250,000 BChl c molecules, and it is the presence of these light-harvesting structures that allow GSB to thrive under astonishing low irradiance conditions. Cba. tepidum is a moderate thermophile (~50°C) and a strict anaerobe, and it was the second photosynthetic bacterium to have its genome sequenced. Cba. tepidum is an obligate photoautotroph and fixes fixes CO2 by the reverse TCA cycle, and it also can reduce dinitrogen to ammonia. It uses thiosulfate, sulfide, polysufide, sulfur and sulfite as electron donors for photosynthesis. Because Cba. tepidum can be genetically manipulated by natural transformation, it has become the model organism for GSB. This capability has been exploited to study many aspects of GSB metabolism and physiology. Notably, the biochemical pathways for the synthesis of bacteriochlorophyll (BChl) c and chlorobactene were established in Cba. tepidum. The ability to manipulate the BChl c composition of the organism allowed the structure of BChls in chlorosomes to be determined for the first time.