Unformatted text preview: Microbial Carbon Cycling, Global Warming, and Bioremediation
Overview of the Global Carbon Cycle
Carbon Dioxide Cycle
Carbon cycles between oxidized and reduced
forms (Fig. 1). Light is the major energy
source driving the reduction of CO2 (i.e. CO2
fixation). Biomass is mineralized to CO2,
completing the cycle with the release of heat.
At a minimum, you have probably learned this
cycle as plants take CO2 and incorporate it into
their biomass. Animals, including us, eat the plants, break them down for energy, and release
CO2. But, the cycle is much more complex and microbes play a major role. Why care about carbon
CO2 is linked to global
warming, as it is a greenhouse
gas. And just in case you didn’t
already know: Greenhouse
gases trap heat (e.g. heat
released in the process of
mineralizing reduced carbon
(Fig 2.)). Atmospheric Methane
(CH4), which cycles with CO2,
is an ~20 times more effective
greenhouse gas than CO2.
Atmospheric CO2 and CH4
concentrations have been
increasing in recent years (Fig. 3). Fig. 3. Microbial Contribution to the Global Carbon Cycle
Microbes are the major source of CO2 fixation in marine environments.
Organisms that posses the Calvin cycle are “the diverse group aerobic lithotrophy bacteria,
virtually all of the photosynthetic bacteria, including the cyanobacteria, as well as various
Pseudomonas species, Rhizobium species, actinomycetes, certain methylotrophic bacteria,
and perhaps several other prokaryotes, as well as the eukaryotic algae and green plants, are
all capable of assimilating carbon dioxide via the Calvin cycle.”1 1 Textbook Reading Sections 10.2 (pages 258-263) The points to be remembered from this text:
CO2 fixation is the process of reducing CO2 and incorporating it into cellular
There are three known mechanisms for CO2 fixation.
CO2 fixation is energetically expensive, requiring a large number of ATP
molecules. Degradation of Biomass
The vast majority of CO2 returning to the atmosphere is the result of microbial degradation of
biomass. Do you think that being able to reduce this microbial degradation could be a
reasonable mechanism to reduce the level of CO2 in the atmosphere? (Think about this
question for lecture.)
The majority of biomass is large molecular weight compounds (e.g. cellulose, lignin, pectin,
starch, DNA, RNA, lipids, protein). For some of these compounds, cellulose and lignin, only
microbes are able to degrade these compounds. Microbial strategies for degrading these
compounds are similar. The textbook reading provides examples of degradation of these
large molecular weight compounds.
Textbook Reading Page 322-328, Section 12.2 The points on which to focus from this text:
1. Know what is similar about how the first step in the microbial degradation of large
molecular weight compounds is carried out.
2. Know, in general terms, how CO2 and ATP are generated from these large
molecular weight compounds. Methane Cycle Methogenesis
Methanogenesis is carried out by Methanogens, which are members of the Archaea.
Methane is a large component of the natural gas that we use to heat homes and stoves,
etc. In the future, will Methanogens be our primary source of this fuel?
Textbook Reading Page 523-528, “Methanogenesis” The points on which to focus from this text:
1. Be able to define methanogenesis.
2. Know what the substrates for methanogenesis have in common (i.e. how
many carbon atoms they have). Knowing this will help you to understand
under what environmental conditions these microbes are active.
3. Know the purpose that methanogenesis serves for Methanogens (i.e. is it
carbon assimilation or ATP generation or both). 2 Syntrophy: Generation of Substrates for Methanogenesis
The points which to know from the lecture:
1. Be able to explain why syntrophic organisms can’t grow alone.
2. Know why the presence of a hydrogen consuming organism allows growth of
a syntrophic organism.
3. Know why syntrophic organisms are important for the return of reduced
carbon to CO2. Methanotrophy
Methanotrophs carry out the oxidation of methane to CO2. These organisms thus
represent an important potential mechanism to reduction methane emissions.
Textbook Reading Pages 586-587 The point on which to focus from this text:
Know what purpose methane oxidation serves for Methanotrophs. Can Microbes Degrade Xenobiotic Compounds (i.e. Bioremediation)?
“Xenobiotic compounds are synthetic chemicals that are not naturally occurring substances”2.
Examples of xenobiotic compounds include pesticides, plastics, and solvents. While these
xenobiotic compounds generally have a simple hydrocarbon backbone, which could be oxidized
to CO2 by microbes, xenobiotic compounds have unusually chemical bonds or substitutions, such
as chlorine, that block oxygenation reactions. Bioremediation refers to the cleanup of oil, toxic
chemicals, and other pollutants by microorganisms.
Tabita FR. Microbiol Rev. 1988. 52:155-89. 3 ...
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- Summer '10
- Photosynthesis, CO2, xenobiotic compounds, molecular weight compounds