## 2 Structure of the Fenna-Matthews-Olson Light-Harvesting Complex
### Green Sulfur Bacteria and the FMO Complex  
The FMO protein is found in green sulfur bacteria (GSB) and chloroacidobacteria (excluding filamentous anoxygenic phototrophs) that contains a Type I reaction center [4]. Sulfur bacteria have evolved to use sulfur in higher amounts as an electron donor or electron acceptor for energy production, taking advantage of its properties as redox reaction mediator [15]. These prokaryotic organisms belong to the _Chlorobiceae_ family. See Figure 3 including the excellent microscopic approaches reported in [16]. Photosynthetic sulfur bacteria are divided in green sulfur bacteria and purple sulfur bacteria due to the photosynthetic pigments being present in each one. Their distribution and growth are determined by the light availability and sulfide concentrations in their media [15]. GSB have been found in a variety of environments ranging from sulfur springs and deep-sea hydrothermal vents to the anoxic hypolimnia of lakes, where they grow under either high or low but stable sulfide concentrations [15; 16]. The depths at which these bacteria are found, where there is little if any access to light, and the vast range of temperatures at which they grow and adapt, contributes to the possibility that the efficiency of their photosynthetic mechanisms take advantage of quantum effects at relatively high temperatures.  
Green sulfur bacteria are anoxygenic photosynthetic sulfur bacteria that use light as energy source for carbon fixation, mainly using hydrogen sulfide (H\({}_{2}\)S) as electron donor [15]. These bacteria are obligate anaerobic photolithotrophs that only grow under strict anoxic conditions using (depending on species) sulfide, elemental sulfur, thiosulfate, molecular hydrogen or even reduced iron and other organic substances as electron donors for anoxygenic photosynthesis [15]. The electrons from the reduced form of sulfur are used for CO\({}_{2}\) fixation via the reverse tricarboxylic acid cycle, while the oxidation of sulfide results in the formation of sulfur globules deposited outside the cell [4,15], GSB have adapted to survive the low light intensity conditions of their habitats [15].  
Initially known as the \"bacteriochlorophyll \(a\) protein\", the Fenna-Matthews-Olson (FMO) protein gets its name from [PERSON] and [PERSON], who first determined its structure [7,8], and [PERSON], who discovered the protein [33]. This LHC is located between the chlorosome (another LHC) and the reaction center as shown in the representation included in Figure 4. It participates in the excitation energy transfer process during photosynthesis by funneling that excitation from the baseplate of the chlorosome to the reaction center [4]. This is possible due to its pigment-protein couplings which tune the optical properties of the complex [1].  
Figure 4: Schematic representation of the spatial arrangement of the Chlorosome, the FMO protein and the reaction center in the cytoplasmic membrane. Light is absorbed by photopigments in the chlorosome and its excitation energy is transferred down to the baseplate which then transfers it to the FMO complex and sequentially to the reaction center.  
Figure 3: Taxonomy of Green Sulfur Bacteria (GSB). (**A**) Scanning electron micrograph of _Escherichia coli_. (**B**) GSB1 negatively stained cells viewed by transmission electron microscopy. (**C**) GSB1 cells deposited on a filter and viewed by scanning electron microscopy. (**D**) Thin section through GSB1 cells viewed by transmission electron microscopy with electron-transparent structures characteristic of chlorosomes. Adapted with permission from [16]. National Academy of Sciences, USA, 2005.