of all Chromatiaceae is ammonia; as in many other bacteria, its assimilation is catalyzed by the enzymes GS (glutamine synthetase) and GOGAT (glutamine oxoglutarate aminotransferase), which incorporate the inorganic ammonia into amino acids. Regarding the organic nitrogen sources used by these bacteria, glutamate and aspartate are among the most common. Hydrogen metabolism Hydrogen (H2) is a major electron donor for photoautotrophic Chromatiaceae. It can be obtained in different ways, according to the growth conditions: the membrane-bound enzyme hydrogenase can recycle the hydrogen produced by the enzyme nitrogenase during the biological nitrogen fixation; it can be photoproduced through the process of photosynthesis; it can be produced together with CO2 via the complete degradation of a number of organic carbon compounds (i.e. lactate and acetate) using dinitrogen, glutamate or aspartate as nitrogen source; H2 and CO2 thus obtained can be then used as substrates for photosynthesis. Carbon metabolism The majority of Chromatiaceae species utilize CO2 as the sole carbon source for autotrophic growth. They employ the Calvin cycle with the key enzymes ribulose bisphosphate carboxylase (RubisCO) and phosphoribulokinase. Versatile species are able to metabolize a larger number of organic compounds than specialized species, which instead mainly assimilate acetate and pyruvate. Oxygenic metabolism As mentioned above, although Chromatiaceae largely practice anoxygenic photosynthesis, there are also versatile species able to switch their metabolism in the presence of oxygen. Ecology and distribution As in the case of the other purple sulfur bacteria, Chromatiaceae are mainly found in all those anaerobic habitats where the presence of light and the geochemical or biological production of sulfide can support their growth. These types of habitats occur especially in the anoxic sediments of both fresh and sea water, salt marshes, saline and soda lakes, sulfur springs and in stratified environments such as microbial mats and meromictic lakes. The natural distribution of Chromatiaceae strongly depends on the selective environmental factors described above (anoxic conditions, availability of light and sulfide) and follows daily and seasonal fluctuations, especially in response to the changes in sunlight intensity and temperature. Sulfide concentration tends to fall during the day and rise during the night, this is due to the fact that oxygen production by algae and Cyanobacteria and sulfide oxidation by phototrophic bacteria are diurnal activities; conversely, sulfide production by sulfate reducers and oxygen utilization by respiratory bacteria occur overnight since they are light-independent reactions. Light absorption changes also according to the type of habitat: