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Unit Chapters
Genomics
Proteins & Proteomics
Evolution & Phylogenetics
Microbial Diversity
Introduction
Microbes as the First Organisms
The Diversity of Microbial Metabolism
Archaea and Bacteria
The Universal Tree of Life
Studying Unculturable Microbes with PCR
Microbes and the Carbon Cycle
Microbes and the Cycling of Nitrogen
Biofilms
Biofilms Formation and Bacterial Communication
Impact of Biofilms in Humans
Communication Between Bacteria and Eukaryotes
Microbes in Mines
Microbial Leaching of Ores
Coda
Emerging Infectious Diseases
HIV & AIDS
Genetics of Development
Cell Biology & Cancer
Human Evolution
Neurobiology
Biology of Sex & Gender
Biodiversity
Genetically Modified Organisms
Microbes and the Carbon Cycle

We have classified microorganisms, including archaea, based on their sources of energy and carbon. The cycling of carbon between carbon dioxide and organic compounds is of considerable ecological importance. In addition to eukaryotes (such as plants and algae), autotrophic bacteria (such as cyanobacteria) play an important role in the fixation of carbon dioxide into organic compounds. Consumers, in turn, use organic compounds and release carbon dioxide. Decomposition of plants and animals and their constituent organic compounds is carried out by a large number of bacteria and fungi.

What is taking place in a swamp where you see marsh gas bubbling up though the ooze? A carbon cycle, based on one-carbon compounds, is taking place in the sediments and overlaying water of such freshwater environments. The anoxic sediments harbor archaea, which produce methane as a byproduct of energy metabolism. The methane rises from the sediment
Figure 5. The methane cycle
and moves into the zone above it. This upper area contains enough oxygen to support methane oxidizers, bacteria that use methane as a source of carbon as well as an energy source.

Methane (CH4) is a greenhouse gas and, according to international agreement, its emissions are controlled. Although it is produced by burning fossil fuel, most enters the atmosphere because of microbial action. How can the latter be limited? One strategy is to drain rice paddies more often, limiting the action of methane producers. Another is to add a layer of soil to landfills to encourage methane-oxidizers. Such approaches to reducing this harmful greenhouse gas are being studied.

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