the light-harvesting complex; together with the reaction center, this one makes up the photosynthetic apparatus of Chromatiaceae, which is localized within an intracytoplasmic membrane system arranged as vesicles or tubules, depending on the species. Those able to synthesize okenone are advantaged in deeper water layers, since water absorbs the longer wavelengths in the upper 10 meters thus limiting the use of bacteriochlorophylls for light harvesting. Metabolism Out of a metabolic point of view, Chromatiaceae bacteria are divided into two main groups: specialized and versatile species. The specialized species are obligate anaerobic photolithoautotrophs, they use reduced sulfur compounds as electron donors and photoassimilate exclusively acetate and pyruvate (or propionate). This group does not include chemotrophs. The versatile species photoassimilate a wider range of organic substrates and most of them do not require the presence of reduced sulfur compounds for their growth, since they are also capable of alternative metabolic strategies. Various metabolic activities are observed in Chromatiaceae: Sulfur metabolism Chromatiaceae bacteria carry out anoxygenic photosynthesis, during which they oxidize reduced sulfur compounds to sulfate (SO42-). All the species are able to use elemental sulfur (S0) and hydrogen sulfide (H2S) as electron donors. As early as 1931, the stoichiometric link between photosynthetic CO2 fixation and sulfide oxidation was demonstrated: the overall reaction contains the reduction of 2 carbon dioxide molecules to 2 carbohydrates molecules coupled with the oxidation of 1 sulfide molecule to 1 sulfate molecule. 2CO2 + H2S + 2H2O -> 2CH2O + H2SO4 Many species utilize thiosulfate (S2O32-) as electron donor, whereas only few of them oxidize sulfite (SO32-) and tetrathionate (S4O62-). In the following example reaction, thiosulfate appears as sodium thiosulfate (Na2S2O3) and is oxidized to sulfuric acid (H2SO4) and sodium sulfate (Na2SO4). 2CO2 + Na2S2O3 + 3H2O -> 2CH2O + H2SO4 + Na2SO4 The final goal of those enzymatic reactions is the oxidation to sulfate. Meanwhile, metabolic intermediates can be formed as sulfur and sulfite. Sulfur is stored as granules inside the cells and can be used as an electron reserve in dark growth conditions during respiration and fermentation. Conversely, sulfite does not accumulate intracellularly since it is readily oxidized to sulfate. Nitrogen metabolism Most of the species can perform biological nitrogen fixation, during which atmospheric nitrogen or dinitrogen (N2) is converted into ammonia by a nitrogenase. N2 + 16ATP + 16H2O + 8e- + 8H+ -> 2NH3 + H2 + 16ADP + 16Pi The favorite nitrogen source