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: the photic zone in sediments is less extensive compared to that in the water, in fact, while in the latter the penetration depth of light is many meters, sediment particles strongly reduce the amount of light entering, and this is the reason why phototrophic bacteria in sediments often form thinner layers within the uppermost millimeters. sulfate present in the sediments is metabolised to sulfide by sulfate-reducing bacteria and the sulfide thus produced diffuses upwards into the water column. then, sulfide forms a vertical gradient with an opposite trend to that of the light, which on the contrary decreases going downwards. this is the reason why these bacteria can find their optimal growth conditions in small areas of overlap between the countercurrent gradients of the above-mentioned factors. the bacteria inhabiting meromictic lakes generally grow at a quite fixed depth and in a relatively stable way over time, as in these lakes there is a permanent stratification due to the higher salinity of the bottom water layers; in particular, photosynthetic purple sulfur bacteria are located at the level of the chemocline, where they can benefit from both the sunlight from above and the sulfide produced by the underlying anaerobic bacteria. since the chemocline is usually relatively deep (from a few centimeters up to several meters, depending on the lake), the blooms of these bacteria tend not to be visible at the water surface. regarding the holomictic lakes, here occurs a seasonal, thermal stratification maintained by temperature differences (especially during summer), which also provides a stable enough layering for purple sulfur bacteria growth; in this case, they cluster at the level of the anoxic and sulfidic hypolimnion, that is to say the dense, bottom water layer of the lake. purple and green sulfur bacteria in a winogradsky column ectothiorhodospiraceae and chlorobiaceae are the only other families of phototrophic bacteria which thrive under similar environmental conditions: despite their dissimilarities from a genetic and evolutionary point of view, both purple and green sulfur bacteria depend on reduced inorganic sulfur compounds for their growth and it allows them to play similar ecological roles. the metabolic flexibility of the versatile species belonging to the chromatiaceae family provides a better adaptation to the environmental changes, such as those occurring in the intermittently oxygenated areas; anyway, the growth they obtain through photosynthesis is greater than that obtained through any other metabolic strategy, regardless of the fact that using forms of metabolism different from photosynthesis means having to compete with those nonphototrophic bacteria and facultative chemotrophic purple nonsulfur bacteria which share the same habitat. generally, in the multi-layering of a microbial mat, the green sulfur bacteria are placed immediately underneath layers of phototrophic purple sulfur bacteria (and of algae and cyanobacteria); this coexistence is strategic for several reasons, just to name a few: the green sulfur bacteria are typically much more sulfide tolerant than the purple sulfur bacteria; the green sulfur bacteria require lower light intensities than the purple sulfur bacteria and they also absorb different light wavelengths; since they are physically covered by one or more layers of purple sulfur bacteria, the green sulfur bacteria are protected from oxygen, as they are obligate anaerobes; in return, the green sulfur bacteria supply extracellular elemental sulfur to the purple sulfur bacteria. this is a full-fledged form of syntrophy which has been observed among several species, for example chromatium vinosum and chlorobium limicola. applications the pigment molecule okenone is diagenetically altered and preserved as the partially saturated okenane. use as biomarkers the diagenetic end product of okenone, okenane, is considered a valid biomarker. when bacteria die, they sink to the seafloor together with some of their pigment molecules, including okenone. under conditions of preservation, the environment is often anoxic and reducing, leading to the chemical loss of functional groups and the partial saturation of okenone to okenane, which in this form can evade microbial and chemical degradation and therefore is buried in marine sediments. this molecule allows to carry out paleoenvironmental reconstructions, since its discovery in marine sediments implies the presence of purple sulfur bacteria during the time of burial and thus a past euxinic environment (that is to say, anoxic and sulfidic waters). use as bioremediators an anaerobic lagoon is a shallow, man-made, covered basin designed to hold and pretreat industrial, urban or other kinds of wastewaters. here chromatiaceae (and purple sulfur bacteria in general) are the predominant phototrophic bacteria, since they play a key role in metabolizing and thus lowering the concentration of undesirable compounds present in the influent wastewater stream. this is an example of in situ bioremediation, during which the bacteria inhabiting the lagoon exploit these compounds to support their photoheterotrophic growth. for example, they can use the toxic and bad-smelling hydrogen sulfide as the electron source and the greenhouse gas methane as the carbon source for anoxygenic photosynthesis; in this way, the contaminants are removed from the lagoon thus reducing odour and toxicity as well as atmospheric pollution, respectively.