The global methane cycle
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The Global Methane Cycle. Methane. CH 4 Colorless gas Component of the atmosphere Main component of natural gas CH 4 + 2O 2 → CO 2 + 2H 2 0 . Why is methane important?. Clean fuel source It is a powerful greenhouse gas It removes Hydroxyls from the troposphere

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Methane l.jpg
Methane

  • CH4

  • Colorless gas

  • Component of the atmosphere

  • Main component of natural gas

  • CH4 + 2O2→CO2 + 2H20


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Why is methane important?

  • Clean fuel source

  • It is a powerful greenhouse gas

  • It removes Hydroxyls from the troposphere

  • Affects concentrations of water vapor and ozone in the stratosphere

  • Used in the industrial production of hydrogen, methanol, acetic acid, and acetic anhydride


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Methane as a fuel source

  • Used as a fuel source for electricity generation, heating, and cooking

  • Methanol is used in vehicles

  • Heat of combustion is about 802 kJ/mol

  • Highest heat to weight ratio

  • Produces the least CO2 per unit of heat


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How much methane is there?

  • ~350 ppb 18,000 years ago

  • Today atmospheric levels are ~1800 ppb and rising

  • The most abundant greenhouse gas after CO2 and H2O vapor

  • Most abundant trace gas in the atmosphere


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Where is methane from?

  • It is produced by organisms and geological processes

  • Livestock produces 37% of all human-induced methane

  • Most is released from wetlands in the northern hemisphere and tropics


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Methane is cycled

  • Certain organisms and geological processes produce methane

  • Other organisms and processes use methane and release new products

  • Several organisms and processes are involved

  • The methane cycle is part of the larger carbon cycle



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Sources: Natural

Wetlands

Termites

Oceans

Hydrates & Clathrates


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Wetlands

  • Produces 225 Tg per year.

  • 50% of wetlands are peat rich.

  • Produced by methanogenic bacteria decomposing organic materials in anaerobic environments.

  • CH4 produced in sediments are diffused to the surface via the water column, gas bubbles, or plants.

  • C6H12O6 -> 3CO2 + 3CH4

  • Massive amounts of CH4 trapped in permafrost could be released if temperatures rise.


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Termites

  • Produces 20 Tg per year

  • Concentrated mostly in grasslands or forests

  • Produced in the breakdown of cellulose by CH4 oxidizing bacteria


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Oceans

  • Produces 15 Tg per year

  • Sources are poorly known.

  • Coastal areas have higher but more variable concentrations.

  • Produced by seepage areas in seabed with organic rich nutrients.


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Hydrates

  • Produces 10 Tg per year

  • CH4 Hydrates are ridgid water cages surrounding CH4 molecules.

  • Mostly on the continental shelf of all latitudes.

  • Hydrate stability requires high pressures and cold temperatures. Most hydrates occur at depths and regions insulated from climate change.

  • Climate warming would lead to destabilization of hydrates and release CH4 molecules.


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Sources: Anthropogenic

  • Rice Cultivation

  • Fossil Fuels

  • Biomass Burning

  • Landfills


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Rice Cultivation

  • Produces 100 Tg per year

  • Production has increased over 40% since the 1980’s.

  • Produced by anaerobic consumption of organic material by methanogenic bacteria


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Domestic Animals

  • Produces 115 Tg per year

  • 75% are produced from cows

  • Produced from fermentation of carbohydrates in the rumen. Microbes in the rumen are capable of breaking down cellulose.

  • Quanity & Quality of feed, plus weight, age, activity level, and species affect how much CH4 is produced.


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Fossil Fuels

  • Produces 110 Tg per year

  • Coal Gas and Natural Gas consist almost entirely of methane.

  • Fossil Fuel sources are coal mining as well as exploration, production, transmissions.


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Biomass Burning

  • Produces 40 Tg per year

  • CH4 is released when vegetation is burned.

  • Amount produced depends on the burning technology, temperature, moisture and carbon content in the vegetation.


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Landfills

  • Produces 40 Tg per year

  • Decomposition of biodegradable organic material in landfills produces CO2 and CH4.


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Sinks

Losses of CH4 occur in lithosphere and the atmosphere

Lithosphere: Dry soil oxidation

Atmosphere: Trophospheric reactions with OH and losses to the stratosphere


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Dry Soil Oxidation

  • A loss of 30 Tg per year

  • Can act as an sink for both atmospheric CH4 and CH4 produced in deeper soil layers.

  • Methanotrophs use CH4 as a source of carbon in a process called methane oxidation

  • Methanotrophs exist in two forms:

    'high capacity - low affinity' methanotrophs

    ’low capacity - high affinity' methanotrophs

    Changes in land use practices, better fertilizer application, and land conversion may help prevent the loss of methane sinks.


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Tropospheric Reactions With OH

  • A loss of 510 Tg per year

  • The most dominant form of CH4 loss.

  • OH + CH4 -> CH3 + H2O

  • CH4 is removed when it reacts with the hydroxyl radical OH. This happens when cosmic rays strike a water vapor molecule.

  • Hydroxyl Radicals are considered the “Detergent” of the atmosphere because they react with many pollutants.


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Losses To The Stratosphere

  • A loss of 40 Tg per year

  • Plays a minor role in removing CH4 from the atmosphere

  • CH4 is loss to the stratosphere when it reacts with OH, Cl and O(1D)

  • The stratosphere is very dry and any water vapor produced from methane oxidation becomes a greenhouse gas itself.


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General Facts about Methane

  • Known as a ‘well-mixed greenhouse gas’ because of long lifetime so it is able to evenly distribute in atmosphere

  • More potent than CO2, but concentration of CO2 is higher so CO2 has greater effect on global warming

    -Control of methane emissions turns out to be a more powerful lever to control global warming than anticipated


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Effects on the Environment

  • Methane is a greenhouse gas ->absorbs heat in the atmosphere->Increase in temperature

  • Since Industrial Revolution, atmospheric methane concentration has doubled

  • Contributes about 20% towards the greenhouse effect, second to CO2



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Controversy

  • Levels of Greenhouse gases can be determined by:

    -measuring level of gases after mixing with other gases in the atmosphere (official IPCC)

    OR

    -measuring level of gases before entering atmosphere.


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Prevention for the Future

  • Methane to Market (United States, the UK, China, Russia, Brazil, India, Italy, Japan, Australia and Nigeria)

    Goal: To recover Methane and use it as clean energy.


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  • Federal Clean Air Act

    Collection of methane gas from landfills and treated

  • Processing manure using anaerobic digesters making methane available for conversion to useful energy

  • New rice harvesting method, reduce methane gas, increase yield



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Methanogenesis - The Big Picture

Why is this process interesting?

  • Terminus of anaerobic food chain.

  • Prevents sequestering of large amts or organic material in anaerobic ecosystems.

  • Potential natural gas source.

  • Methanogens are the most important CO2- reducing prokaryotes, but are also a source of greenhouse gas (CH4).



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Methanogenesis: The Process

Overall Reaction:

4 H2 + H+ + HCO3- CH4 + 3H2O


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And who are these?

  • METHANOGENS

  • Archaea, ubiquitous in highly reducing habitats, also found in some “oxic” habitats

  • Obligate anaerobes

  • H2 + CO2 --> methane (CH4)

  • Autotrophic (fix CO2), chemolithotrophic (use inorganic H2 as e- donor)

  • Some use endproducts of fermentation (acetate, etc.)

  • Some important genera: Methanococcus, Methanosarcina

  • Capable of using elemental Sulfur as a terminal e- acceptor (anaerobic respiration - sulfur reduction)

  • METHANOTROPHS

  • Bacteria

  • Aerobes

  • Utilize methane:

  • CH4 + O2 --> CO2 + H2O

  • Rxn happens at interface btwn anoxic and oxic layers of soil (as CH4 diffuses up).

  • Genus: Methylomonas




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Cool and current studies

A new biogeochemical cycling pathway couples anaerobic methane oxidation to denitrification. (Nature; vol. 440, 04/06).

  • Freshwater sediments receive high loads of anthropogenic NO3- and CH4.

  • Theoretically microbes should be able to use NO3-/NO2- to anaerobically oxidize CH4, but prior to this report it has not been shown.

  • This study showed direct, anaerobic oxidation of methane coupled to denitrification in the complete absence of O2.

    CH4 + NO3-/NO2- + H+ ---> CO2 + N2 + H2O

    • consortium consisted of two microbes: an uncultured bacteria and an archaea related to marine methanotrophic Archaea.

      Relevance:

    • new pathways fill in holes in our understanding of global biogeochemical cycling.


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Methanogens on Mars? FEMS Micro. Ecol. (2007)

Greenland Ice

Sheet

  • 10 ppbv CH4 on mars, estimated 300 tons lost/year - 270 tons/year must be generated to offset loss.

  • Is this biogenic or abiogenic methane?

  • Metabolic rates of methanogens in extreme cold on Earth can be used to estimate metabolic rates and cell densities of martian methanogens.


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Conclusions

  • Methane is the simplest hydrocarbon

  • Methane is a clean fuel source

  • Methane is a powerful greenhouse gas

  • Methane is part of the carbon cycle


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Conclusions

  • The major reservoirs of CH4 are fossil fuel reservoirs, hydrates and Clathrates, and the atmosphere

  • Wetlands are the dominant natural source of CH4

  • Domestic animals are the largest anthropogenic source of CH4

  • The Hydroxyl Radical OH, is the dominant form of CH4 loss.

  • There has been an increase in terrestrial sources and a decrease in sinks, leading to excess CH4 in the atmosphere


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Conclusions

  • Reducing Methane will have a stronger influence in reduce Greenhouse effect

  • ‘Methane To Market’ and ‘Federal Clean Air Act’ hopes to reduce Methane output

  • New agriculture and use of anaerobic digesters also reduce Methane output


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Conclusions

  • The methane cycle is composed of two groups: methanogens (produce methane) and methanotrophs (consume methane).

  • Methanogenesis is carried out anaerobically by Archaea, methanotrophy is carried out aerobically by bacteria.

  • Methanogens are ubiquitous in anoxic environments (wetlands, sediments, etc.) as well as some oxic ones.


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REFERENCES

  • Madigan, Martinko, and Parker. 1997. Brock Biology of Microorganisms. 8th ed. Prentice-Hall, New Jersey.

  • Stanier, R.Y., Ingraham, J.L., Wheelis, M.L., and Painter, P.R. 1986. The Microbial World. 5th ed. Prentice-Hall, New Jersey.

  • Strous, M. et al. 2006. A microbial consortium couples anaerobic methane oxidation to denitrification. Nature, 440:918-921.

  • Price, P.B. 2007. Microbial life in glacial ice and implications for a cold origin of life. FEMS Microbiol. Ecol. 59:217-231.

  • http://www.newscientist.com/article.ns?id=mg18725124.500

  • http://www.meteor.iastate.edu/gcp/studentpapers/1996/atmoschem/brockberg.html

  • http://www.epa.gov/methane/sources.html

  • http://www.ghgonline.org/methanesinksoil.htm

  • http://www.igac.noaa.gov/newsletter/21/methane_sink.php

  • http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16827

  • http://www.grida.no/climate/ipcc_tar/wg1/134.htm#4211


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References

  • http://www.scienceagogo.com/news/20050625005443data_trunc_sys.shtml

  • http://www.ace.mmu.ac.uk/eae/Global_Warming/Older/Methane.html

  • http://egov.oregon.gov/ENERGY/RENEW/Biomass/Environment.shtml

  • http://www.ens-newswire.com/ens/jul2005/2005-07-19-01.asp


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