Methanogens
Manish Patel
S
What are they?
S Taxonomy
S Classified as archaebacteria
S Over 50 species, in 5 orders and 9 families
S Typically appear coccoid or bacilli
S Some have cell wall made of pseudopeptidoglycan or a layer of
proteins
S Obligate Anaerobes – O2 is poisonous
Where are they on Earth?
S Found in environments depleted of electron acceptors (O2, NO3,
Fe3+, SO4):
S Swamps and marshes
S Largest natural methane source – 250 Tg
S Sediments at the bottom of rivers and lakes/oceans
S Digestive tracts of animals
S Cattle livestock by humans - ~150 Tg
S Extremophiles
S Geothermal environments
S Found in ice cores from 3 km below Greenland and 4-5 km below the
surface in South African Gold Mines
S Found in sediments from “White Smoker” chimneys, 2600m deep in
the East Pacific Rise (Temp: 48-95˚C; Pressure: 200 atm)
S Can reproduce in temperatures ranges of 15-100˚C
How do they survive?
S
Methanogenesis:
S Formation of methane by
microbes.
S Use carbon dioxide, formate,
methanol, methylamines,
methylthiols and acetate for
carbon source.
S H2 is their electron source.
S Not to be confused with
methanotrophs
General
Overview
for CO2

• Actual ∆G’ is ~ +40kJ/mol because of low H2 (electron source) partial pressure.
• Catalyzed by formylmethanofuran dehydrogenase
• Reduction driven by transmembrane electrochemical Na+ potential
• Formyl transfer catalyzed by formyl transferase
• Tetrahydromethanopterin (H4MPT) is
analogous to tetrahydrofuran (H4F) as a methylcarrier
• Reversible hydrolysis of N5,Nl0-methenylH4MPT to N5-formyl-H4MPT, is catalyzed by
N5,N10-methenyl-H4MPT cyclohydrolase
• Catalyzed by methylene H4MPT dehydrogenase
twice.
• Utilizes a reduced coenzyme F420, a deazaflavine
derivative and hydride acceptor (from H2).
• Alternative enzymes use H2 directly.
• F420 is unique to methanogens.
• F420H2 is regenerated by an F420-dependent hydrogenase.
• All of these F420-dependent hydrogenases contain flavin, Ni,
and Fe-S clusters.
• Catalyzed by a transmembrane methyltransferase, which
also acts as a sodium ion pump.
• Na+ gradient could be used as mentioned before in MF
formation or in a Na+-driven ATP synthase.
• Coenzyme M is also exclusive to methanogenic microbes.
• Catalyzed by methyl-coenzyme M reductase
• Utilizes 2 coenzymes:
• 7-mercaptoheptanoylthreonine
phosphate (HS-HTP) reduces
Coenzyme M and displaces methane
• F430, a nickel porphinoid, coordinates
the displacement.
•
•
•
A F420-nonreactive hydrogenase activates H2
and channels the electrons into the electron
transport chain, and the electrons are
transferred to CoM-S-S-HTE
In some methanogens, this is coupled with a
Na+ pump instead of a H+ pump.
In one species, this complex contains nickel,
FAD, non-heme iron, and acid-labile sulfur.
Also, b-type cytochromes are present for use
in an electron transport chain.
Alternative Carbon Sources
How does
Methanogenesis
play into the
whole
metabolism?
We know how
they work. How
can we find them
in space?
We know how
they work. How
can we find them
in space?
We know how
they work. How
can we find them
in space?
Not what we
imagined.
Looking for Methanogens
S When looking for potential extraterrestrial methanogenic life, the
presence of methane alone does not indicate life.
S Considering the flux of methane helps to determine whether methane
biogenic or abiogenic.
S Focus on atmospheric methane at the surface.
S The observed non-uniformity in methane could be due to localized
microbial colonies or localized hydrogeochemical sources.
S Also determine whether to look for past life or current life. One factor
could be where liquid water was or is currently.
Methane Sources and Sinks
S
Methane on Mars could be
produced from any one of the four
potential sources:
S 1. Internal, such as volcanoes
or hydrothermal sources
S 2. Exogenous, such as
meteorites, comets, or
interplanetary dust particles.
S 3. Internal, such as a
hydrogeochemical process
involving serpentinization.
S 4. Biological.
On Earth:
Methane Sources and Sinks
S
Loss of methane on Mars
S
Primarily from photolysis,
giving Mars’ methane a
photochemical lifetime of
about 300 years.
S
Reacting with other trace
chemcials/oxidants (peroxide)
S
Diffusion into the regiolith
S
Significant sinks could be
responsible for non-uniform
CH4 distribution.
Summing Up Methane Flux
Techniques
S Spectroscopic studies
S Taking into account doppler shifts
and to find CH4 signatures
S
Fourier Transform Spectrometer at
the Canada–France–Hawaii
Telescope
S
Infrared Telescop Facility (IRTF)
S
Gemini Telescope
S Robots
S Take soil samples for
spectroscopy
Life Tests:
S Carbon Isotopes:
S Less energy to form a bond between 12C and 12C elements than
between 12C and 13C elements.
S It is a good indicator of life, as we know it on Earth.
S PDB (Pee Dee Belemnite) Inorganic Standard value,
Rstd = 13C/12C = 0.01123722
S Requires a measureable quantity in the atmosphere. Mars
requires instruments capable of measuring 0.01ppbv
Life Tests:
S Chirality
S Organic molecules are mostly left-handed on Earth, but amino
acids from meteorites have been found to also be left-handed.
S Overtime, organic molecules will tend to form a racemic mixture,
so the presence of chiral organic molecules could indicate present
life.
S Fluorescence
S F420 detected with excitation-emission wavelengths of 400-468nm
S Potential test if core samples can be obtained from deep enough,
where methanogens would reside, past the permafrost.
References:
S Blaut, M. “Metabolism of Methanogens” Antonie van Leeuwenhoek.
1994. 66. 187-208.
S Thauer, R. K. “Biochemistry of Methanogenesis” Microbiology. 1998.
144. 2377-2406.
S Atreya, S. K.; Mahaffy, P. R.; Wong, A-S. “Methane and related trace
species on Mars: Origin, loss, implications for life, and habitability.”
Planetary and Space Science. 2007. 55. 358-369.
S http://microbewiki.kenyon.edu/index.php/Methanococcus_jannasc
hii
S Methanogens Wiki