Scientists revealed biological methane formation from saturated long-chain hydrocarbons
Methane, an inflammable gas, is an important end product of natural decomposition processes that take place under exclusion of air, for instance in swamps, lake and ditch sediments, as well as in sewage digestors. It often forms gas bubbles ascending to the water surface and is also referred to as biogas or swamp gas. The process is due to the cooperative action of microorganisms that belong to the bacteria and the archaea, the two major forms of microbial life. They make their living by converting the components of sedimented, dead biomass, such as cellulose, starch or proteins via several steps to the simplest carbon compounds, methane and carbon dioxide.
Aquatic sites with microbial methane production are wide-spread, and methane, similar to carbon dioxide, influences the heat budget and chemistry of our atmosphere. Because of this global relevance and the unique underlying enzymatic processes running without any oxygen, the bacteria and archaea involved in methane formation have fascinated microbiologists and been under investigation since years, especially in laboratories in the Untited States and Europe.
Until now, little was known about the compounds that serve for methane formation in old, deep parts of aquatic sediments where cellulose or other easily decomposable biomolecules have been consumed by microbes already a long time ago. Here, microbial life is expected to be sustained by relatively long-lived and sluggish substances. Investigation of such processes in natural sediments is of increasing interest, but is also tedious due to the slowness of the reactions and sensitivity to sampling procedures. One wide-spread class of long-lived compounds in deep, oxygen-free sediments are saturated hydrocarbons. They originate from plants, animals and microbes (as part of their fat-like cell components), from pollution with petroleum products, and from long-term chemical transformation reactions of buried biomass; the latter process has given rise to petroleum.
By selective cultivation of microbes from deep ditch sediment, the researchers in Bremen and Hamburg were able to grow a natural association of bacteria and archaea with a saturated hydrocarbon, hexadecane, as the sole nutrient under strict exclusion of air. Hexadecane, a hydrocarbon consisting of 16 carbon atoms and 34 hydrogen atoms, was converted (with participation of water in the reaction) to methane and carbon dioxide.
Methane is the simplest hydrocarbon, consisting of one carbon atom and four hydrogen atoms. Hence, the process represents, figuratively speaking, a microbial hydrocarbon Ñcrackingì. Conversion of hexadecane to methane was verified by analytical measurements including the use of non-radioactive isotopes. The process under the optimized laboratory conditions is, for sure, faster than in the natural sediment, but still slow in comparison to growth of many other microorganisms. For demonstration of substantial hexadecane consumption, the culture had to be grown for more than one year. Involved bacteria and archaea were detected microscopically and via characteristic DNA-sequences. It is still unknown by means of which special enzymes the involved microbes make use of the chemically very sluggish saturated hydrocarbon, and in addition obtain sufficient energy for their life. Hydrocarbons in the absence of oxyen represent a low-energy diet for microbes.
The investigation contributes to our understanding of slow yet globally relevant microbial processes in deep subsurface environments. It is concluded that saturated hydrocarbons are part of the hitherto largely unknown compounds that sustain microbial life in deep, old sediments. Hence, the process as observed in the studied culture could, in principle, lead to methane formation in such old sediments and maybe even in petroleum reservoirs.
Contact: Friedrich Widdel
Max Planck Institute for Marine Microbiology,
Bremen/Germany
Phone: (+49 421) 20 28 - 7 02
Fax: (+49 421) 20 28 - 790 or - 5 80
Journal
Nature