Domain: Bacteria Phylum: Proteobacteria Class: Gammaproteobacteria Order: Chromatiales Family: Chromatiaceae Genus: Chromatium Species: Chromatium okenii
Chromatium were first described by Swiss botanist, Maximilian Perty, in 1852. He introduced the genus name and called them "pigment bacteria". Theodor W. Engelmann first cultured and experimented with these bacteria to discover the light excitation of the purple bacteria in 1883.
Chromatium is a gram negative bacteria found in marine environments. They tend to be flagellated straight rod shaped or even slightly curved rods of ~1 µm in diameter and 3-4 µm long. These bacteria belong to the purple photosynthetic sulfur bacteria that oxidize sulfide into sulfur which is deposited in intracellular granules in their cytoplasm.
Chromatium okenii is a gram negative purple sulfur bacteria that is capable of anoxygenic photosynthesis. It uses hydrogen sulfide as an electron.
Ecology and Significance
Little genetic information is available about the photosynthetic purple sulfur bacteria. They may represent one of the most primitive photosynthetic organisms that are capable of carbon fixation. Researchers hope to sequence more strains of purple sulfur bacteria in hopes that it will provide a better understanding of the process of photosynthesis in these organisms as well as an overall process. To date only one strain has been fully sequenced but a complete species wide genomic data would provide information on the intricacies of their metabolic processes. Genomic information will also provide insight to the intricacies of global carbon and sulfur cycles. Additionally, some of these bacteria may be members of the phototrophic mats in geothermal environments, playing a key role in the microbial communities. 
Chromatiaceae is closely related to Ectothiorhodospiraceae but is distinguished by its lack of lamellar intracytoplasmic membrane structures. Ectothiorhodospiraceae have a significant differences in polar lipid composition and dependence on saline and alkaline growth conditions.
There are two major physiological groups of Chromatium: metabolically versatile and metabolically specialized species.
The versatile species ( Allochromatium vinosum and Allochromatium minutissimum) use thiosulfate with elemental sulfur as electron donor. They can grow photoorganoheterotrophically in the absence of reduced sulfur compounds or as chemolithotrophs on reduced sulfur compounds.
The specialized species such as Chromatium okenii, Allochromatium warmingii, and Isochromatium budfer. These species depend on strict anaerobic conditions and are obligate phototrophs. Sulfide is required for their metabolic processes but only acetate and pyruvate are photoassimilated in the presence of CO2 and sulfide.
These bacteria grow photolithoautotrophically, photolithoheterotrophically, and/or photoorganogeterotrophically.
They grow in the presence of light under anaerobic conditions. They can grow on an inorganic medium using CO2 as the sole carbon source in the presence of reduced inorganic sulfur. They use sulfur and sulfide as sole photosynthetic electron donor. Purple sulfur bacteria have to fix CO2 in order to grown and survive.
Chromatium can perform anoxygenic photosynthesis and contain bacteriochlorophyll a and carotenoids of spirilloxanthin group.
 "Bacterial Metabolism & Photosynthesis." Bacterial Photosynthesis. N.p., n.d. Web. 11 May 2015.
 Golyshin, Peter N. “Genome Sequence Completed of Alcanivorax borkumensis, a Hydrocarbon-degrading Bacterium That Plays a Global Role in Oil Removal from Marine Systems.” 3 (2003): 215-20. Print.
 Neyra, Carlos A. Carbon Metabolism. Boca Raton, FL: CRC Pr., 1985. Web. 11 May 2015.
 "Sulfur Metabolism in Phototrophic Sulfur Bacteria." Sulfur Metabolism in Phototrophic Sulfur Bacteria. N.p., n.d. Web. 11 May 2015.
 "Why Sequence Purple Sulfur Bacteria? - DOE Joint Genome Institute." DOE Joint Genome Institute. N.p., 07 Nov. 2013. Web. 11 May 2015.
Page authored by Sukhwinder Kaur, student of Prof. Katherine Mcmahon at University of Wisconsin - Madison.