used for taxonomic sample preservation. It was also used in embalming fluids historically. Arsenic was used as an opacifier in ceramics, creating white glazes. Until recently, arsenic was used in optical glass. Modern glass manufacturers, under pressure from environmentalists, have ceased using both arsenic and lead. In computers; arsenic is used in the chips as the n-type doping Biological role Bacteria Some species of bacteria obtain their energy in the absence of oxygen by oxidizing various fuels while reducing arsenate to arsenite. Under oxidative environmental conditions some bacteria use arsenite as fuel, which they oxidize to arsenate. The enzymes involved are known as arsenate reductases (Arr). In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just as ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that, over the course of history, these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive. One strain PHS-1 has been isolated and is related to the gammaproteobacterium Ectothiorhodospira shaposhnikovii. The mechanism is unknown, but an encoded Arr enzyme may function in reverse to its known homologues. In 2011, it was postulated that a strain of Halomonadaceae could be grown in the absence of phosphorus if that element were substituted with arsenic, exploiting the fact that the arsenate and phosphate anions are similar structurally. The study was widely criticised and subsequently refuted by independent researcher groups. Essential trace element in higher animals Arsenic is understood to be an essential trace mineral in birds as it is involved in the synthesis of methionine metabolites, with feeding recommendations being between 0.012 and 0.050 mg/kg. Some evidence indicates that arsenic is an essential trace mineral in mammals. However, the biological function is not known. Heredity Arsenic has been linked to epigenetic changes, heritable changes in gene expression that occur without changes in DNA sequence. These include DNA methylation, histone modification, and RNA interference. Toxic levels of arsenic cause significant DNA hypermethylation of tumor suppressor genes p16 and p53, thus increasing risk of carcinogenesis. These epigenetic events have been studied in vitro using human kidney cells and in vivo using rat liver cells and peripheral blood leukocytes in humans. Inductively coupled plasma mass spectrometry (ICP-MS) is used to detect precise levels of intracellular arsenic and other arsenic bases involved in epigenetic modification of DNA. Studies investigating arsenic as