The study of bacteria. While bacteria are different in some important respects from all other kinds of cells, their basic processes of physiology and genetics are the same as in all forms of life. One unusual property of bacteria as a whole is their physiological diversity: Some live in the total absence of oxygen and convert complex carbohydrates to acids and alcohols (fermentation), sulfate to hydrogen sulfide, nitrate to nitrogen gas, and hydrogen plus carbon dioxide to methane gas; others carry out photosynthesis by mechanisms nearly identical to plants; some bacteria can grow and multiply by using energy obtained from oxidation of sulfur, ammonia, hydrogen, or iron while obtaining carbon for cell synthesis from carbon dioxide; and some can obtain their needed nitrogen from the gas in air.