All organisms require food and energy to survive. Food can be broken down and reassembled to make various structural components of cells. Or it can be converted into energy that powers the activities of the cells. Metabolism is the sum total of these two kinds of chemical changes that occur in an organism. Like plants and animals, all microbes are metabolically active, meaning they undergo chemical changes to process food and obtain energy.
Viruses are the only exception. These microbes, however, require a metabolically active host to survive.
Food contains energy in chemical form, similar to how a battery stores electricity. Some organisms, however, obtain energy from non-food sources, such as sunlight or chemical compounds in the environment. Plants and some microbes are able to capture the energy of the sun, and convert it to chemical energy. This process is called photosynthesis. The energy is then stored as compounds in the cells. This energy is then available to other organisms that eat these plants and microbes.
Microbe Food Sources
Organisms obtain nutrients either by eating other organisms or making their own food. Ones that harness sunlight to make their own food are called autotrophs, or primary producers. Those that eat other organisms are called heterotrophs. The heterotrophs can be further divided into herbivores, that eat autotrophs; and carnivores, that eat other heterotrophs. Decomposers, such as fungi, break down dead plant or animal material. They are also considered heterotrophs because they rely on other organisms for their food.
Most microbes are heterotrophs, meaning they obtain their food — and energy — from other organisms. Microbes are usually decomposers, breaking down dead plant and animal material in their environment. Autotrophic microbes also exist, and include algae and blue-green bacteria. They use photosynthesis to create their own food. A few species of autotrophic microbes use inorganic — nonliving — compounds instead of sunlight; these microbes tend to live in extremely hot environments, such as near deep-sea vents.
Microbe Energy Sources
Organisms are like living machines. They need energy to power their bodies. For heterotrophs, this energy comes from the organisms they eat. Chemical energy is transferred to them when they digest food. Autotrophs, however, are able to make their own food using components found in the environment. As they do this, they capture energy from the environment and store it in their own bodies as chemical energy.
The most common method that autotrophic microbes — algae and blue-green bacteria — use for capturing energy is photosynthesis. They capture the energy of the sun using special structures in the cell called chloroplasts. During photosynthesis, these autotrophs take in carbon dioxide and release oxygen. A large part of the oxygen in the atmosphere is produced in this way. Instead of using sunlight, a few autotrophs capture energy stored in the environment as sulfur compounds, or similar.
Microbes & Oxygen
Like humans, many bacteria cannot live without oxygen. These aerobic bacteria use oxygen to release the chemical energy stored in their food. This process is called respiration. Other bacteria, however, can survive without oxygen. These anaerobic organisms survive in places with a low oxygen content, such as the digestive system of organisms, certain types of wounds and parts of sewage treatment plants. They use other processes to release the energy stored in food.
Some anaerobic bacteria thrive in the absence of oxygen, but can also tolerate it. Others are actually killed by oxygen. Certain fungi — such as the yeasts used to make bread and beer — can switch between aerobic and anaerobic states. This allows them to take advantage of oxygen when it is available, but to survive without it. The yeasts convert sugars and starches to alcohol and carbon dioxide gas. This process of fermentation gives beer and wine its alcohol content, and causes bread dough to rise.
Microbe Food Webs
A food web is a diagram of the connections between organisms living in an ecosystem. The food web also shows the flow of energy from one organism to the next. Organisms store chemical energy in their bodies as various compounds. As autotrophs make food, energy captured from the sun enters the ecosystem. When an autotrophs is eaten, the energy passes to the heterotroph that just consumed it. This continues throughout the ecosystem. When organisms die, their nutrients — and energy — are recycled by microbes through a process called decomposition.
In land-based ecosystems, plants are the autotrophs. They harness the energy of sunlight using photosynthesis. In aquatic environments, microbes called phytoplankton play this role. Phytoplankton include algae and blue-green bacteria, as well as protists with symbiotic algae or blue-green bacteria living inside them. Phytoplankton are the primary food source in the ocean. They are eaten by organisms, which are in turn eaten by other ones. Without phytoplankton, the food webs of oceans, rivers and lakes would fall apart.
Microbe Nutrient Cycles
In addition to energy passing from organism to organism, nutrients also cycle throughout ecosystems. The most common cycles are carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. These elements make up 95% of the mass of organisms on the Earth. As the elements cycle, they pass between living and non-living forms — such as compounds that make up organisms, and soil and rocks, respectively. Microbes play an important role in many of these cycles.
Without them, nutrients that organisms need to survive would not be available in the environment.
Nitrogen is an essential nutrient for many organisms, including plants and microbes. Nitrogen is very common in the atmosphere, but few organisms can use it in that form. Some bacteria, however, can convert gaseous nitrogen to another form that plants can use. Other bacteria are responsible for returning nitrogen to its gaseous state. Microbes also play a large part in the cycling of carbon. They convert carbon from non-living sources — such as carbon dioxide in the air — to compounds that they, and other organisms, can use.
