Methane a natural gas - PDF by exg15922

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									                                                                 Methane:                                                          a natural gas

                                                                 The methane produced by
                                                                 microbes is a big contributor to
                                                                 global warming, but as Setareh
                                                                 and James Chong describe,
                                                                 it also has great potential as a
                                                                 source of green energy.




S
            ome believe that methanogens may be among            years ago, before the ‘great oxygenation event’ that was          of methane is even worse over a short-       Computer models of methane molecules. Prof. K. Seddon & J. Van Den Berg / Queen’s
                                                                                                                                                                                 University, Belfast / Science Photo Library
            the most ancient forms of life. What is the          triggered by the evolution of microbial photosynthesis.           er time scale – it has a GWP of 68
            evidence for this? These single-celled prokaryotic   Methanogens have evolved a unique metabolism that allows          over 20 years – but it is a relatively       Natural gas burning. Photos.com / Jupiter Images

            organisms belong to the domain Archaea, itself a     them to survive on the energy generated by the reduction of       unstable molecule once in the upper
            lineage of cells that appear to be deeply rooted     CO2 and other small carbon compounds by hydrogen. This is         atmosphere. Contemplating the num-          Biofuel potential                                natural gas/methane as a fuel source
            between the eubacteria (‘true’ bacteria) and the     not a particularly energetically favourable process and results   bers is a rather frightening prospect: if   Of course, not only methanotrophs                are the production of CO2 and the fact
eukaryotes. Archaea were originally called archaebacteria        in the biological generation of methane as a waste product        all the methane produced by methano-        can use methane for energy. The major            that as a fossil fuel, natural gas is a
until the ‘Archaea-ologists’ felt a name that more obviously     (hence the name). The ‘methanogens-as-one-of-the-earliest-        gens each year reached the atmosphere,      flammable component of natural gas                finite source of energy. Methane from
demonstrated the differences between these organisms and         forms-of-life’ theory suggests that methane excreted by           it would be the equivalent of releasing     is methane. Natural gas is currently             methanogens does not have the same
eubacteria was required. While the archaea are best known for    methanogens could have helped to warm primordial Earth.           23 billion tonnes of CO2. Current glo-      the fossil fuel of choice due to its             problems. Methanogenic methane is re-
their love of extremes – the most thermophilic, acidophilic,                                                                       bal CO2 emissions are about 8 billion       relatively clean burning properties and          newable, and formed from recently fixed
halophilic (salt-loving) and barophilic (pressure-loving)        Biological methane                                                tonnes per year; thus, methanogens          it provides more than 40 % of current            atmospheric CO2, so burning methane
organisms all belong to this domain – most methanogens           Methanogens are the main source of biological methane on          have the potential to provide three times   energy needs in the UK. Methane is               from a methanogenic source is effect-
show distinctly mesophilic tendencies. However, methano-         the planet, producing in the order of a billion tonnes per        the heating effect of anthropogenic         an odourless gas (if you work with               ively carbon neutral.
gens require an environment containing less than 10 p.p.m.       annum globally. Methane is a serious greenhouse gas with          carbon emissions! Fortunately, a large      methanogens you always need to get
O2 (i.e. <0.001 %) to grow! For most oxygen-requiring life,      23 times the global warming potential (GWP) of CO2 over           proportion of methanogen-produced           this into the conversation early!) that          Anaerobic digestion
this could be considered to be rather extreme. This very         100 years. That is, a single molecule of methane released         methane is captured by other, methano-      is relatively easy to handle, and burns          In the microbial world, methanogens
low oxygen concentration coincides with the probable com-        into the atmosphere has the same thermal retention capacity       trophic, bacteria that can in turn use      with oxygen to produce only water and            are terminal electron acceptors, tak-
position of the atmosphere of anoxic Earth some 2.5 billion      as 23 molecules of CO2 over 100 years. The warming effect         methane as an energy source.                CO2. Two of the main problems with               ing the breakdown products from


124                                                                                                microbiology today aug 08       microbiology today aug 08                                                                                                         125
 consortia of (mainly) constitutively        insignificantly low. However, by 2007
 anaerobic bacteria at the end of the        agricultural AD from a relatively
 organic degradation process. Anaerobic      small number of facilities in the USA
 digestion (AD) has long been used as        was responsible for the production
 a low-tech, environmentally friendly,       of more than 215,000,000 kWh of
 way to process organic waste into           electricity, emphasizing how rapidly
‘biogas’. Biogas is a mixture of CO2 and     and effectively this technology can be
 methane that can be burned in much          deployed.
 the same way as natural gas to provide         In the UK, AD is still a very under-
 both heat and power. Anaerobic              used technology despite Defra recently
 digesters tend to be more popular in        making it the preferred method for            great ancientness, or perhaps due to the   microbes, which he observed by            that their presence is good for your gut          of microbiology can be smelly! However,
 countries with warmer environments          disposing of food waste. AD is per-           pervasiveness of oxygen forcing them       igniting ‘combustible air’ collected      flora.                                             growing methanogens alone is nothing
 where the natural processes involved        ceived as an unreliable and difficult tech-    to hide, have been found in all of the     from marshes in northern Italy in 1776.                                                     compared with trying to grow a stably-
 in organic matter breakdown are per-        nology to use in the UK – something           anaerobic ecological niches explored to    As well as being hugely abundant in       Growing pains                                     interacting mixed population of an-
 ceived as faster. However, a number         that works better at higher ambient           date. There are thermophilic methano-      rice paddies, swamps and landfill          Despite their abundance, growing                  aerobes. But in order to understand the
 of facilities exist in the UK (mainly for   temperatures. There is no scientific           gens (the first archaeal genome to be       sites, methanogens are endemic in         methanogens in the laboratory poses a             dynamics that occur in these consortia,
 processing wastewater) that also make       basis for these notions. The Arctic           sequenced was of the hyperthermo-          ruminants (responsible for as much        significant challenge; gases need to be            these issues must be resolved. Con-
 use of methanogens. AD has also seen a      tundra has been estimated to con-             philic Methanocaldococcus jannaschii,      as 15 % of global methane emissions),     scrubbed of all traces of oxygen, and             sortial dynamics need to be measured,
 recent rise in popularity in continental    tain as much as 32,000 Gt (1 Gt = 109         which grows at 85 °C), halophilic          termites and other insects (5 % of        hydrogen must be handled in a non-                understood and modelled so that the
 Europe, where incentives for green          tonnes) of methane generated by the           methanogens and psychrophilic meth-        global methane emissions are due to       explosive manner. Large quantities of             robust but rather inefficient process of
 energy production have made electricity     slow action of methanogenic degra-            anogens (isolated from Antarctica).        methanogens in termite guts). The         strong reducing agents (mainly sulfur-            anaerobic digestion can be improved
 generated from biogas a cost-effective      dation of organic matter trapped under                                                   limited studies that exist concerning     containing compounds) do lend a                   and added to the range of green energy/
 method of disposing of plant and other      the permafrost. So methanogens and            Methanogen habitats                        the presence of methanogens in human      certain pall to the atmosphere that               biofuel options currently being exam-
 agricultural waste. In the USA in 2000,     their associated consortia of anaerobes       Alessandro Volta (the scientist who        guts vary wildly, but it seems likely     perhaps justifies the initial reaction             ined as alternatives to mankind’s de-
 the amount of electricity produced          can certainly tolerate the British climate.   first described the Volt) was the first      that more than 30% of the population      many people have to the thought of                pendence on fossil fuels. This is a
 by AD from agricultural sources was         Methanogens, perhaps due to their             person to describe methane from            contain methanogens in their guts, and    working with methanogens – this kind              microbiological challenge that should
                                                                                                                                                                                                                                  no longer be neglected.
                                                                                                                                                                                 Top left. Microscale methane generation.
                                                                                                                                                                                  Termites are equipped with portable
                                                                                                                                                                                  anaerobic digesters that contain a huge           Setareh Chong & James Chong
                                                                                                                                                                                  diversity of specialized microbes for the         Department of Biology (Area 5),
                                                                                                                                                                                  breakdown of lignocellulose. Photos.com /
                                                                                                                                                                                  Jupiter Images
                                                                                                                                                                                                                                    PO Box 373, University of York,
                                                                                                                                                                                                                                    York YO10 5YW, UK (t 01904
                                                                                                                                                                                 Top right. A scanning electron
                                                                                                                                                                                                                                    328628; e jpjc1@york.ac.uk)
                                                                                                                                                                                  micrograph of Methanothermobacter
                                                                                                                                                                                  thermautotrophicus. This organism was
                                                                                                                                                                                  originally isolated from sewage sludge          Further reading
                                                                                                                                                                                  and grows optimally at 60 °C. Bar, 5 µm.          United States Environmental Protection
                                                                                                                                                                                  Meg Stark & Paul McDermott
                                                                                                                                                                                                                                    Agency (2006). Global anthropogenic
                                                                                                                                                                                 Far left. Methanogens growing in                  non-CO2 greenhouse gas emissions:
                                                                                                                                                                                  captivity. A 2 litre culture of Methanococcus     1990–2020. www.epa.gov/nonco2/
                                                                                                                                                                                  maripaludis growing on an 80 : 20 mix of H2       econ-inv/international.html
                                                                                                                                                                                  and CO2. Methane is produced as a waste
                                                                                                                                                                                  product by all methanogens. James Chong           Defra (2008). UK Greenhouse
                                                                                                                                                                                                                                    Gas Inventory, 1990 to 2006. ISBN
                                                                                                                                                                                 Sushi and burgers make methane. Paddy
                                                                                                                                                                                  fields are one of the major anthropogenic
                                                                                                                                                                                                                                    0-95548-234-2.
                                                                                                                                                                                  methanogen niches that are expanding.             www.defra.gov.uk/corporate/ministers/
                                                                                                                                                                                  In 2005, global rice cultivation was              speeches/joan-ruddock/jr071016.htm
                                                                                                                                                                                  responsible for the emission of ~32 million
                                                                                                                                                                                  tonnes of methane (the equivalent of 672          Semiletov, I., Shakhova, N.,
                                                                                                                                                                                  million tonnes of CO2) – more than the            Romanovsky, V. & Pipko, I. (2004).
                                                                                                                                                                                  UK’s total greenhouse gas emissions for           Methane climate forcing and methane
                                                                                                                                                                                  the same year. Global enteric fermentation
                                                                                                                                                                                                                                    observations in the Siberian Arctic
                                                                                                                                                                                  produced nearly three times this amount
                                                                                                                                                                                  of methane (92 million tonnes) in 2005.           land-shelf system. World Resour Rev 16,
                                                                                                                                                                                  James Chong                                       503–542.




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