13.5: Metabolic diversity among the bacteria is extreme.

One important attribute that makes bacterial diversity possible is that bacteria can metabolize almost anything. (Not all bacteria can metabolize everything. Rather, there is a huge variety of bacteria, each type with its own particular set of metabolic specializations.) Some can even use energy from light to make their own food, just as plants do. Microbiologists place bacteria into different “trophic” (feeding) categories that reflect their metabolic specialization.

Chemical organic feeders (chemoorganotrophs) are bacteria that consume organic molecules, such as carbohydrates. You probably see the products of organic feeders every time you take a shower—they are responsible for the pink deposits on the shower curtain and other discolorations on shower tiles (FIGURE 13-9). Most of the bacteria that live in and on your body are also organic feeders. Some compete with you to metabolize the food you eat. Others digest things you can’t digest.

Chemical inorganic feeders (chemolithotrophs, meaning “rock feeders”) are able to use a completely different type of food as their source of energy: inorganic molecules such as ammonia, hydrogen sulfide, hydrogen, and iron. The most common inorganic feeders are the iron bacteria responsible for the brown stains that form on plumbing fixtures in regions where tap water contains high levels of iron. Sulfur bacteria are associated with iron bacteria, and these are responsible for the slimy black deposits that you will probably find if you lift the stopper out of the drain in your bathroom sink.

On a larger scale, inorganic feeders are responsible for the acidic drainage that is a by-product of mining. The desirable ore makes up only a small part of the total amount of rock that is removed from a mine. The portion that does not contain ore is discarded on the ground surface in piles called “tailings.” This material is often rich in minerals such as pyrites (iron sulfides). Inorganic feeders can gain energy by oxidizing these minerals and, in the process, they release compounds that combine with rainwater to produce strong acids, such as sulfuric acid. When this acidic water drains into streams, it can kill fish and aquatic plants and insects.

Bacteria that use the energy from sunlight (photoautotrophs, or “light self-feeders”) contain chlorophyll and use light energy to convert carbon dioxide to glucose by photosynthesis. The floating mats of gooey green material that you see in roadside ditches are a type of photoautotroph called cyanobacteria.

The cyanobacteria living today closely resemble the first photosynthetic organisms that appeared on earth about 2.6 billion years ago. Cyanobacteria could use solar energy to build organic compounds from carbon dioxide, and in the process they broke down water molecules to release free oxygen. Before cyanobacteria, the earth’s atmosphere contained no free oxygen—instead, air consisted almost entirely of nitrogen and carbon dioxide. The accumulation of oxygen released by cyanobacteria is called the Oxygen Revolution. Oxygen—which humans depend on—now makes up about 21% of the volume of air, and cyanobacteria still release important quantities of oxygen into the atmosphere. One of the common ways that bacteria are classified is as aerobic or anaerobic, depending on whether they require or do not require oxygen for growth—although some bacteria, called facultative anaerobes, utilize oxygen if it is present but can also switch to anaerobic respiration when oxygen is absent.