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 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. References