The Cycles of Elements

An ultra-simplified, general review including the use of terms
defined on our bacterial nutrition and catabolism pages.

by John Lindquist

The Carbon Cycle:  Organic compounds are used as carbon source by HETEROTROPHS (by definition). As is also the case for the Nitrogen and Sulfur Cycles below, organic compounds are converted from one to another within plants, animals and microorganisms – also animals eating plants, microorganisms decomposing plants and animals, microorganisms undergoing fermentation, etc.

  1. Use of CO2 as carbon source of AUTOTROPHS (by definition).

  2. Release of CO2 as an end product in respiration (and some fermentation) by CHEMOORGANOTROPHS.

  3. Methanogenesis. (We broadly categorize this unique process as a kind of anaerobic respiration.)

  4. Methane oxidation (another unique process).

The Oxygen Cycle: (reduced to its absolute essentials)


  2. O2 production by oxygenic PHOTOTROPHS.

The Nitrogen Cycle: (A similar view is shown here where there are links to more.)

  1. Respiration of ammonium and nitrite by CHEMOLITHOTROPHS. This process is called nitrification.

  2. Dissimilatory nitrate reduction – a form of anaerobic respiration. This process is called denitrification when nitrate is reduced all the way to N2.

  3. Nitrogen-fixation by certain bacteria under aerobic (upper) and anaerobic (lower) conditions
  4. Assimilatory nitrate reduction performed by plants and microorganisms.

  5. Assimilatory ammonium uptake performed by plants and microorganisms.

  6. Ammonification performed by microorganisms.

The Sulfur Cycle:

  1. Oxidation of sulfide and sulfur by CHEMOLITHOTROPHS (by respiration) and PHOTOLITHOTROPHS.

  2. Dissimilatory sulfate reduction – a form of anaerobic respiration.

  3. Assimilatory sulfate reduction performed by plants and microorganisms.

  4. Desulfurylation performed by animals and microorganisms.

Selected General Microbiology Topics
Archived Bacteriology 102 site
Site Outline of our microbial pages

These general microbiology pages have copyright by John Lindquist
and found their permanent sanctuary here circa 2001.
Copies found elsewhere are neither authorized nor up to date.
Page content was last modified on 3/4/02 at 8:30 AM, CST.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison