Green bacteria denote strict anoxygenic phototrophic bacteria containing chlorosomes as light-harvesting system. Chlorosomes are very efficient light-harvesting structures. This structure is a giant antenna in which bacteriochlorophyll molecules are not bound to proteins and functions instead like a solid-state circuit. Chlorosome bacteriochlorophylls absorb light and transfer the energy to the bacteriochlorophyll located in the reaction center on the cytoplasmatic membrane. This arrangement is highly efficient for absorbing light at low intensities. Chlorosomes allow green bacteria to grow at the lowest light intensities of all known phototrophs. The green photosynthetic bacteria are divided into two distinct phylogenetic groups: the green sulfur bacteria and the green nonsulfur bacteria. The green sulfur bacteria utilize H2S as an electron donor, so they can tolerate this toxic molecule. Most of the species are...