Prokaryotic Metabolism

Section Summary

As the oldest living inhabitants of Earth, prokaryotes are also the most metabolically diverse; they flourish in many different environments with various energy and carbon sources, variable temperature, pH, pressure, oxygen and water availability. Nutrients required in large amounts are called macronutrients, whereas those required in trace amounts are called micronutrients or trace elements. Macronutrients include C, H, O, N, P, S, K, Mg, Ca, and Na. In addition to these macronutrients, prokaryotes require various metallic elements for growth and enzyme function. Prokaryotes use different sources of energy to assemble macromolecules from smaller molecules. Phototrophs obtain their energy from sunlight, whereas chemotrophs obtain energy from chemical compounds. Energy-producing pathways may be either aerobic or anaerobic.

Prokaryotes play roles in the carbon and nitrogen cycles. Producers capture carbon dioxide from the atmosphere and convert it to organic compounds. Consumers (animals and other chemoorganotrophic organisms) use organic compounds generated by producers and release carbon dioxide into the atmosphere by respiration. Carbon dioxide is also returned to the atmosphere by the microbial decomposers of dead organisms. Nitrogen also cycles in and out of living organisms, from organic compounds to ammonia, ammonium ions, nitrite, nitrate, and nitrogen gas. Prokaryotes are essential for most of these conversions. Gaseous nitrogen is transformed into ammonia through nitrogen fixation. Ammonia is anaerobically catabolized by some prokaryotes, yielding N2 as the final product. Nitrification is the conversion of ammonium into nitrite. Nitrification in soils is carried out by bacteria. Denitrification is also performed by bacteria and transforms nitrate from soils into gaseous nitrogen compounds, such as N2O, NO, and N2.