LectNotes&ReadingC-Cycle - Microbial Carbon...

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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. Fig. 1. 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 cycling? 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. Fig. 2. Atmospheric CO2 and CH4 concentrations have been increasing in recent years (Fig. 3). Fig. 3. Microbial Contribution to the Global Carbon Cycle CO2 Fixation 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 material. 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. Footnotes: 1 Tabita FR. Microbiol Rev. 1988. 52:155-89. 3 ...
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This note was uploaded on 03/06/2012 for the course MIMG 100 taught by Professor Lazazzera during the Summer '10 term at UCLA.

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