第22章原核生物：細菌と古細菌

Prokaryotes are metabolically diverse organisms. In many cases, a prokaryote may be placed into a species clade by its defining metabolic features: Can it metabolize lactose? Can it grow on citrate? Does it produce H₂S? Does it ferment carbohydrates to produce acid and gas? Can it grow under anaerobic conditions? Since metabolism and metabolites are the product of enzyme pathways, and enzymes are encoded in genes, the metabolic capabilities of a prokaryote are a reflection of its genome. There are many different environments on Earth with various energy and carbon sources, and variable conditions to which prokaryotes may be able to adapt. Prokaryotes have been able to live in every environment from deep-water volcanic vents to Antarctic ice by using whatever energy and carbon sources are available. Prokaryotes fill many niches on Earth, including involvement in nitrogen and carbon cycles, photosynthetic production of oxygen, decomposition of dead organisms, and thriving as parasitic, commensal, or mutualistic organisms inside multicellular organisms, including humans. The very broad range of environments that prokaryotes occupy is possible because they have diverse metabolic processes.

The diverse environments and ecosystems on Earth have a wide range of conditions in terms of temperature, available nutrients, acidity, salinity, oxygen availability, and energy sources. Prokaryotes are very well equipped to make their living out of a vast array of nutrients and environmental conditions. To live, prokaryotes need a source of energy, a source of carbon, and some additional nutrients.

Cells are essentially a well-organized assemblage of macromolecules and water. Recall that macromolecules are produced by the polymerization of smaller units called monomers. For cells to build all of the molecules required to sustain life, they need certain substances, collectively called nutrients. When prokaryotes grow in nature, they must obtain their nutrients from the environment. Nutrients that are required in large amounts are called macronutrients, whereas those required in smaller or trace amounts are called micronutrients. Just a handful of elements are considered macronutrients—carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. (A mnemonic for remembering these elements is the acronym CHONPS.)

Why are these macronutrients needed in large amounts? They are the components of organic compounds in cells, including water. Carbon is the major element in all macromolecules: carbohydrates, proteins, nucleic acids, lipids, and many other compounds. Carbon accounts for about 50 percent of the composition of the cell. In contrast, nitrogen represents only 12 percent of the total dry weight of a typical cell. Nitrogen is a component of proteins, nucleic acids, and other cell constituents. Most of the nitrogen available in nature is either atmospheric nitrogen (N₂) or another inorganic form. Diatomic (N₂) nitrogen, however, can be converted into an organic form only by certain microorganisms, called nitrogen-fixing organisms. Both hydrogen and oxygen are part of many organic compounds and of water. Phosphorus is required by all organisms for the synthesis of nucleotides and phospholipids. Sulfur is part of the structure of some amino acids such as cysteine and methionine, and is also present in several vitamins and coenzymes. Other important macronutrients are potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na). Although these elements are required in smaller amounts, they are very important for the structure and function of the prokaryotic cell.

In addition to these macronutrients, prokaryotes require various metallic elements in small amounts. These are referred to as micronutrients or trace elements. For example, iron is necessary for the function of the cytochromes involved in electron-transport reactions. Some prokaryotes require other elements—such as boron (B), chromium (Cr), and manganese (Mn)—primarily as enzyme cofactors.

Prokaryotes are classified both by the way they obtain energy, and by the carbon source they use for producing organic molecules. These categories are summarized in Table 22.3. Prokaryotes can use different sources of energy to generate the ATP needed for biosynthesis and other cellular activities. Phototrophs (or phototrophic organisms) obtain their energy from sunlight. Phototrophs trap the energy of light using chlorophylls, or in a few cases, bacterial rhodopsin. (Rhodopsin-using phototrophs, oddly, are phototrophic, but not photosynthetic, since they do not fix carbon.) Chemotrophs (or chemosynthetic organisms) obtain their energy from chemical compounds. Chemotrophs that can use organic compounds as energy sources are called chemoorganotrophs. Those that can use inorganic compounds, like sulfur or iron compounds, as energy sources are called chemolithotrophs.

Energy-producing pathways may be either aerobic, using oxygen as the terminal electron acceptor, or anaerobic, using either simple inorganic compounds or organic molecules as the terminal electron acceptor. Since prokaryotes lived on Earth for nearly a billion years before photosynthesis produced significant amounts of oxygen for aerobic respiration, many species of both Bacteria and Archaea are anaerobic and their metabolic activities are important in the carbon and nitrogen cycles discussed below.