Background extinction See extinction

background radiation radiation coming from sources other than that under investigation. The background radiation may increase at altitude, owing to the greater intensity of cosmic rays, and in areas where the rock or soil contains radium, whose decay causes release of the radioactive gas radon. See radioactive decay.

Bacteria In the Five Kingdoms system of classification, a kingdom containing all the prokaryotes, formerly called the Prokary-otae or Monera. Most taxonomists now divide the prokaryotes into two distinct kingdoms or domains (a taxonomic level above kingdom), the Bacteria and the ar-chaea.

Bacteria are found over almost all the planet, from the interstices of rocks deep underground to icebergs, mountains, deserts, deep oceans, and the insides of other organisms. Most are single cells, ranging from rounded cocci to rods and spirochetes. A few, such as the cyanobac-terium Anabaena, form filaments of cells, clusters, or dense colonies (e.g. the cyanobacteria that form stromatolites). Bacteria play an important role in the recycling of nutrients in soil and water, in decomposition, and in nitrogen fixation (e.g. the symbiotic bacterium Rhizobium in the root nodules of legumes).

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