Extremophile Microbes: Submarine Methanotrophs and Sub-Surface Thermophilic Sulfate Reducers

20 October 2006

Separate papers in the scientific journals Science and Nature this week describe two novel extremophile microbial communities: one that lives on methane (methanotrophs) and so functions as a methane sink, the other that uses the decomposition of water (H₂O) by radioactive decay of uranium, thorium, and potassium as a source of the hydrogen (H₂) needed for their respiration.

Submarine mud volcanoes are important sources of the greenhouse gas methane, and some recent research has shown active submarine mud volcanoes may be more numerous than earlier thought.

The Nature study, by a team of German and French researchers, examined the waters around a mud volcano in the Barents Sea and identified three key methanotrophic communities: aerobic methanotrophic bacteria (Methylococcales), anaerobic methanotrophic archaea (ANME-2) thriving below siboglinid tubeworms, and a previously undescribed clade of archaea (ANME-3) associated with bacterial mats.

By consuming the methane, the microbes reduce the amount that potentially ends up in the upper ocean and atmosphere. However, the researchers also found that there is a natural cap on the capacity of the microbial methane filter of less than 40% of the total flux. (The upward flow of sulphate- and oxygen-free mud volcano fluids restricts the availability of these electron acceptors for methane oxidation, and hence the habitat range of methanotrophs.

It seems unlikely, according to the research team, that the methanotrophs will produce a quick way to reduce methane in the atmosphere. They can’t be grown in the lab yet, and they are extremely slow growing.

In the Science study, researchers from Indiana University Bloomington and eight collaborating institutions report on a self-sustaining community of bacteria that live in rocks 2.8 kilometers below Earth’s surface and are sustained by geologically produced sulfate and hydrogen with no apparent reliance on photosynthetically derived substrates.

Coauthors of the paper studied a new water-filled fracture inside a South African gold mine, and found that the fracture water contained hydrocarbons and hydrogen not likely to have been created through biological processes, but rather from decomposition of water exposed to radiation from uranium-bearing rocks.

High density DNA microarray analysis revealed a vast number of bacterial species present, but the samples were dominated by a single new species related to hydrothermal vent bacteria from the division Firmicutes. The ancient age of the fracture water and comparative DNA analysis of the bacterial genes suggests subsurface Firmicutes were removed from contact with their surface cousins anywhere from 3 million to 25 million years ago.

Radiation emanating from uranium minerals in or near the fracture allows for the formation of hydrogen gas from decomposition of water and formation of sulfate from decomposition of sulfur minerals. Firmicutes are able to harvest energy from the reaction of hydrogen and sulfate, allowing other microbes in the fracture community to use the chemical waste from the Firmicutes as food.

(A hat-tip to Allen!)

Resources:

• “Novel microbial communities of the Haakon Mosby mud volcano and their role as a methane sink”; Helge Niemann, Tina Lösekann, Dirk de Beer, Marcus Elvert, Thierry Nadalig, Katrin Knittel, Rudolf Amann, Eberhard J. Sauter, Michael Schlüter, Michael Klages, Jean Paul Foucher and Antje Boetius; Nature 443, 854-858(19 October 2006) | doi:10.1038/nature05227

• “Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome”; Li-Hung Lin, Pei-Ling Wang, Douglas Rumble, Johanna Lippmann-Pipke, Erik Boice, Lisa M. Pratt, Barbara Sherwood Lollar, Eoin L. Brodie, Terry C. Hazen, Gary L. Andersen, Todd Z. DeSantis, Duane P. Moser, Dave Kershaw, T. C. Onstott; Science 20 October 2006:Vol. 314. no. 5798, pp. 479 - 482 DOI: 10.1126/science.1127376