The Global Methane Cycle

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The Global Methane Cycle
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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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Carbon cycle diagram
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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 -gt 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
  1980s.
• 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 -gt 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 -gtabsorbs heat in the
  atmosphere-gtIncrease in temperature
• Since Industrial Revolution, atmospheric methane
  concentration has doubled
• Contributes about 20 towards the greenhouse
  effect, second to CO2

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Concentration of Methane over time
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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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Methane Cycling Microbes
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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 within the Carbon Cycle.
Water column
sediments
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Methanogenesis The Process
4 H2 H HCO3- CH4 3H2O
Overall Reaction
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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 --gt 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 --gt CO2 H2O
• Rxn happens at interface btwn anoxic and oxic
  layers of soil (as CH4 diffuses up).
• Genus Methylomonas

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Phylogeny of Methanogens
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The Methanotrophs
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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 ---gt 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
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  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, 440918-921.
• Price, P.B. 2007. Microbial life in glacial ice
  and implications for a cold origin of life. FEMS
  Microbiol. Ecol. 59217-231.
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References

• http//www.scienceagogo.com/news/20050625005443dat
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• http//www.ace.mmu.ac.uk/eae/Global_Warming/Older/
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