Distribution and Ecological Roles

Although bacteria may appear simple, they excel in the diversity and complexity of their metabolic capabilities and they are able to survive in many places. Bacteria are found everywhere on Earth where life is able to exist. They are plentiful in soils, bodies of water, on ice and snow, and are even found deep within Earth's crust. They often take advantage of living in and on other organisms in symbiotic relationships and can be found inhabiting the intestinal tracts and surfaces of animals, including humans. For the most part, the bacteria in and around us bring us more benefit than harm. Sometimes however, bacteria can be pathogenic, or disease causing. This can happen for a number of reasons, such as when the host has a compromised immune system or when a bacterium acquires genes that make it grow more aggressively or secrete toxins into its host environment.

Oxygen-producing photosynthesis, which is so familiar in plants, is actually a bacterial invention. Many bacteria are photosynthetic and use light energy to turn CO2 from the atmosphere into cell material. Among these only the cyanobacteria produce oxygen during photosynthesis. Plastids, the photosynthetic organelles found in plants and algae, evolved from cyanobacteria through a process called endosymbiosis, in which cyanobacteria lived inside the cells of other organisms that were the ancestors of green algae. Mitochondria, found in most eukaryotic cells, also evolved from nonphoto-synthetic respiring bacteria in this way.

Bacteria are crucial for the cycling of elements necessary for all life. Through various processes, which we generally call decomposition, bacteria break down the cell materials of dead organisms into simpler carbon-, phosphorus-, sulfur-, and nitrogen-containing nutrients that can be used again by other organisms for growth. Without bacteria to recycle these essential nutrients, they would remain within the dead organisms or sediments and would thus be unavailable for use by other organisms. SEE ALSO Archaea; Biogeochemical Cycles; Cyanobacteria; Decomposers; Endosymbiosis; Evolution of Plants; Nitrogen Fixation.

J. Peter Gogarten and Lorraine Olendzenski

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