A New DIET for the World’s Most Prodigious Methanogen


New results published in Energy and Environmental Science reveal the surprising finding that Methanosaeta, the microorganism considered to be responsible for most of the methane production on Earth, can convert carbon dioxide to methane.  Many methanogens can reduce carbon dioxide to methane by oxidizing H2, but Methanosatea were found to reduce carbon dioxide with electrons they receive through biological electrical connections with Geobacter species.  This direct interspecies electron transfer (DIET) was documented in defined co-cultures of Geobacter metallireducens and Methanosaeta harundinacea.  Furthermore, metagenomic and metatrancriptomic analysis suggested that DIET between Geobacter and Methanosaeta species was the predominant mechanism for the conversion of carbon dioxide to methane in an anaerobic digester converting simulated brewery waste to methane.

DIET offers an alternative to the nearly 50 year-old concept that methanogenic microbial communities share electrons via interspecies H2 transfer. Conversion of wastes to methane is a proven and growing bioenergy strategy.  Promoting DIET may accelerate and stabilize this process.  Microbial activity in wetlands and rice paddies is an important source of atmospheric methane, which has strong greenhouse properties. Methanosaeta species are ubiquitous in such environments suggesting that a substantial portion of global methane production could be derived from DIET.

Basic Science with an Applied Product

Geobacterspecies are of interest because of their novel electron transfer capabilities, the ability to transfer electrons outside the cell and transport these electrons over long distances via conductive filaments known as microbial nanowires.  Geobacters have a major impact on the natural environment and have practical application in the fields of bioenergy, bioremediation, and bioelectronics.

Geobacter-Fe © 2005 eye of science

The first Geobacter species (initially designated strain GS-15) was isolated from sediments in the Potomac River, just down stream from Washington D.C. in 1987. This organism, which is known as Geobacter metallireducens, was the first organism found to oxidize organic compounds to carbon dioxide with iron oxide as the electron acceptor. In other words, Geobacter metallireducens gains its energy by using iron oxide (an abundant rust-like mineral in soils and sediments) in the same way that humans use oxygen. As outlined in the publication links, Geobacter metallireducens and other Geobacter species that have subsequently been isolated from a wide diversity of environments provide a model for important iron transformations on modern earth and may explain geological phenomena, such as the massive accumulation of magnetite in ancient iron formations.

Bioremediation Geobacter species are also of interest because of their role in environmental restoration. For example, Geobacter species can destroy petroleum contaminants in polluted groundwater by oxidizing these compounds to harmless carbon dioxide and can remove radioactive metal contaminants from groundwater.  As understanding of the functioning of Geobacter species has improved it has been possible to use this information to modify environmental conditions in order to accelerate the rate of bioremediation.

Bioenergy  Geobacter species play an important role in some anaerobic wastewater digesters degrading organic contaminants with electron transfer to microorganisms that produce methane, an important biofuel.  Recent results suggest that this electron transfer proceeds through Geobacter’s conductive microbial nanowires.  The ability of Geobacter species to oxidize organic compounds with electron transfer to electrodes shows promise as a strategy for producing bioelectricity, especially in remote environments.

Microbial Electrosynthesis This is a process for converting the greenhouse gas carbon dioxide to transportation fuels and other useful organic products.  When driven with solar technology microbial electrosynthesis is an artificial form of photosynthesis that offers the possibility of converting sunlight and carbon dioxide to desirable organic compounds much more efficiently and more sustainably than biomass-based processes.

Bioelectronics Geobacter species have novel electronic properties that may have practical applications.  For example, they can form highly cohesive conductive films that have conductivities that rival those of synthetic conductive polymers. The conductivity of the Geobacter films results from a network of microbial nanowires, thin (ca. 3 nm) protein filaments that conduct electrons along their length with metallic-like conductivity.  Thus, Geobacter offers the possibility of making electronic sensors and other devices,that work under water and can readily couple biological and abiological interfaces, from inexpensive feedstocks, like acetic acid (i.e. vinegar).

Systems Approach to Environmental MicrobiologyGeobacter species have proven to be an excellent model for the development of genome-scale analysis of natural environments, bioremediation, and bioenergy applications.  This approach has included sophisticated diagnosis of the physiological status of the subsurface microbial community during bioremediation to guide bioremediation supplements and predictive computer modeling of groundwater bioremediation coupling genome-scale metabolic models with geohydrological models.

Life in Extreme Environments - Some Like it Hot

Recent Publications

Dawn E. Holmes, Ludovic Giloteaux, Roberto Orellana, Kenneth H. Williams, Mark J. Robbins and Derek R. Lovley.  2014.  Methane production from protozoan endosymbionts following stimulation of microbial metabolism within subsurface sediments.  Frontiers in Microbiology.  doi: 10.3389/fmicb.2014.00366. 

Dawn E Holmes, Ludovic Giloteaux, Akhilesh K Chaurasia, Kenneth H Williams, Birgit Luef, Michael J Wilkins, Kelly C Wrighton, Courtney A Thompson, Luis R Comolli and Derek R Lovley.  2014.  Evidence of Geobacter-associated phage in a uranium-contaminated aquifer.  The ISME Journal.  doi:10.1038/ismej.2014.128

Nikhil S Malvankar, Gary M King and Derek R Lovley.  2014.  Centimeter-long electron transport in marine sediments via conductive minerals.  The ISME Journal.  doi:10.1038/ismej.2014.131

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