The oxygen requirement is high partly for the same reason that carbon is, _i.e._, respiration. Oxygen is one of the constituents of protoplasm, and combined with hydrogen forms water which makes up such a large part of the living cell. Anaërobic bacteria are dependent on so-called “molecular respiration” for their energy. That is, through a shifting or rearrangement of the atoms in the compounds used as food the oxidation of carbon is brought about. Enzymes are probably responsible for this action. Carbon dioxide is produced by anaërobes as well as by aërobes, and frequently in amounts readily collected. A carbohydrate is usually though not always essential for the growth of anaërobes and serves them as the best source of energy. Nitrogen is the characteristic element of living material. Protoplasm is a chemical substance in unstable equilibrium and nitrogen is responsible for this instability. No other of the commoner elements is brought into combination with such difficulty, nor is so readily liberated when combined (all commercial explosives are nitrogen compounds). Bacteria, like other forms of protoplasm, require nitrogen. More marked peculiarities are shown by bacteria with reference to the sources from which they derive their nitrogen than for carbon. Some can even combine the free nitrogen of the air and furnish the only natural means of any importance for this reaction. Some few forms (the nitrite and nitrate formers, Chapter XI) obtain their energy from the oxidation of inorganic nitrogen compounds, ammonia and nitrites respectively, and not from carbon. These latter bacteria use carbon from carbon dioxide and carbonates. A great many bacteria can secure their nitrogen from nitrates but some are restricted to organic nitrogen. Many bacteria obtain their carbon from the same organic compounds from which their nitrogen is derived. Sulphur serves mainly as a constituent of protein compounds in the protoplasmic structure. In some of the _sulphur_ bacteria it is a source of energy, since either free sulphur or H₂S is oxidized by them. Some of these bacteria can obtain their carbon from CO₂ or carbonates, and their nitrogen from nitrates or ammonium salts. Whether the _iron_ bacteria, belonging to the genus _Crenothrix_ of the higher, thread bacteria, use this element or its compounds as sources of energy is still a disputed question. The evidence is largely in favor of this view. Free hydrogen has been shown to be oxidized by some forms which obtain their energy in this way. Whether there is a special class of _phosphorus_ bacteria remains to be discovered. That phosphorus is oxidized during the activity of many bacteria is undoubted, but whether this represents a source of energy or is the accidental by-product of other activities is undetermined. Practically nothing is known about the metabolism of the other elements as such.