longer wavelengths than the chlorophyll used by all other photosynthesizing organisms does. As a result, bacteria using this pigment can grow in water beneath fairly dense layers of algae, using light of wavelengths that are not absorbed by the algae (Figure 27.7).
Photoheterotrophs use light as their source of energy, but must obtain their carbon atoms from organic compounds made by other organisms. They use compounds such as carbohydrates, fatty acids, and alcohols as their organic "food." The purple nonsulfur bacteria, among others, are photo-heterotrophs.
Chemolithotrophs (chemoautotrophs) obtain their energy by oxidizing inorganic substances, and they use some of that energy to fix carbon dioxide. Some chemolithotrophs use reactions identical to those of the typical photosynthetic cycle (see Figure 8.3), but others use other pathways to fix carbon dioxide. Some bacteria oxidize ammonia or nitrite ions to form nitrate ions. Others oxidize hydrogen gas, hydrogen sulfide, sulfur, and other materials. Many archaea are chemo-lithotrophs.
Deep-sea hydrothermal vent ecosystems are based on chemolithotrophic prokaryotes that are incorporated into large communities of crabs, mollusks, and giant worms, all living at a depth of 2,500 meters, below any hint of light from the sun. These bacteria obtain energy by oxidizing hydrogen sulfide and other substances released in the near-boiling water that flows from volcanic vents in the ocean floor.
Finally, chemoheterotrophs obtain both energy and carbon atoms from one or more complex organic compounds. Most known bacteria and archaea are chemoheterotrophs— as are all animals and fungi and many protists.
NITROGEN AND SULFUR METABOLISM. Many prokaryotes base important parts of their metabolism on reactions involving nitrogen or sulfur. For example, some bacteria carry out respiratory electron transport without using oxygen as an electron acceptor. These organisms use oxidized inorganic ions such as nitrate, nitrite, or sulfate as electron acceptors. Examples include the denitrifiers, bacteria that release nitrogen to the atmosphere as nitrogen gas (N2). These normally aerobic bacteria, mostly species of the genera Bacillus and Pseudomonas, use nitrate (NO3-) as an electron acceptor in place of oxygen if they are kept under anaerobic conditions:
Nitrogen fixers convert atmospheric nitrogen gas into a chemical form usable by the nitrogen fixers themselves as well as by other organisms. They convert nitrogen gas to ammonia:
All organisms require nitrogen for their proteins, nucleic acids, and other important compounds. The vital process of nitrogen fixation is carried out by a wide variety of archaea and bacteria, including cyanobacteria, but by no other organisms. (We'll discuss this process in detail in Chapter 37.)
Ammonia is oxidized to nitrate in the soil and in seawater by chemolithotrophic bacteria called nitrifiers. Bacteria of two genera, Nitrosomonas and Nitrosococcus, convert ammonia to nitrite ions (NO2-), and Nitrobacter oxidizes nitrite to nitrate (NO3-).
What do the nitrifiers get out of these reactions? Their chemosynthesis is powered by the energy released by the oxidation of ammonia or nitrite. For example, by passing the electrons from nitrite through an electron transport chain, Ni-trobacter can make ATP, and using some of this ATP, it can also make NADH. With this ATP and NADH, the bacterium can convert CO2 and H2O to glucose.
Numerous bacteria base their metabolism on the modification of sulfur-containing ions and compounds in their environments. As examples, we have already mentioned the photoautotrophic bacteria and chemolithotrophic archaea that use H2S as an electron donor in place of H2O. Such uses of nitrogen and sulfur have environmental implications, as we'll see in the next section.
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WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.