bacteria, the important nitrogenase enzymes contains an Fe–Mo–S cluster and is a catalyst that performs the important function of nitrogen fixation, converting atmospheric nitrogen to ammonia that can be used by microorganisms and plants to make proteins, DNA, RNA, alkaloids, and the other organic nitrogen compounds necessary for life. Deficiency In humans methionine is an essential amino acid, cysteine is conditionally essential and may be synthesized from non-essential serine (sulfur donator would be methionine in this case). Dietary deficiency rarely happens in common conditions. Artificial methionine deficiency is attempted to apply in cancer treatment, but the method is still potentially dangerous. There is a rare fatal genetic disease connected with sulfite oxidase impairment an enzyme metabolizing sulfur-containing amino acids. Precautions Though elemental sulfur is only minimally absorbed through the skin and is of low toxicity to humans, inhalation of sulfur dust or contact with eyes or skin may cause irritation. Excessive ingestion of sulfur can cause a burning sensation or diarrhea, and cases of life-threatening metabolic acidosis have been reported after patients deliberately consumed sulfur as a folk remedy. Toxicity of sulfur compounds Most of the soluble sulfate salts, such as Epsom salts, are non-toxic. Soluble sulfate salts are poorly absorbed and laxative. When injected parenterally, they are freely filtered by the kidneys and eliminated with very little toxicity in multi-gram amounts. Aluminium sulfate is used in the purification of drinking water, wastewater treatment plants and papermaking. When sulfur burns in air, it produces sulfur dioxide. In water, this gas produces sulfurous acid and sulfites; sulfites are antioxidants that inhibit growth of aerobic bacteria and a useful food additive in small amounts. At high concentrations these acids harm the lungs, eyes, or other tissues. In organisms without lungs such as insects or plants, sulfite in high concentration prevents respiration. Sulfur trioxide (made by catalysis from sulfur dioxide) and sulfuric acid are similarly highly acidic and corrosive in the presence of water. Sulfuric acid is a strong dehydrating agent that can strip available water molecules and water components from sugar and organic tissue. The burning of coal and/or petroleum by industry and power plants generates sulfur dioxide (SO2) that reacts with atmospheric water and oxygen to produce sulfuric acid (H2SO4) and sulfurous acid (H2SO3). These acids are components of acid rain, lowering the pH of soil and freshwater bodies, sometimes resulting in substantial damage to the environment and chemical weathering of