Rudolf Thauer finds energy insights at the bottom of the Black Sea.
It wriggles up through fissures in Earth’s crust. It emanates from landfills. It is the primary component of cow burps and natural gas. This colorless, odorless gas is methane, and it is one of Earth’s most abundant—and, in some ways, most elusive—energy resources.
Produced primarily by living creatures breaking down organic matter, methane is the largest hydrocarbon source on Earth, making it the most common member of the chemical family that includes such fuels as petroleum and propane. Despite its abundance, however, methane remains difficult to use as a fuel because it is a gas under normal conditions, making it notoriously hard to store or transport.
Enter Rudolf Thauer, Ph.D., and some remarkable organisms capable of manipulating methane in ways once thought to be impossible.
In the murky depths of the Black Sea, scientists discovered an ecosystem powered by the production and consumption of methane. There, bacteria and archaea thrive near underwater methane seeps, catalyzing chemical reactions and swapping electrons in a bustling microscopic economy.
Deciphering the biochemistry of these unique creatures could inspire new technologies to capture and store methane. Insights from this research may also inform the development and application of hydrogen fuels.
Thauer heads the Emeritus group at the Max Planck Institute for Terrestrial Microbiology and was the founding director of the institute when it was established in 1991.
Diving for Answers
To get to the epicenter of Thauer’s research, you must board a boat, venture into Eastern Europe’s Black Sea, climb into a submersible, and dive more than 700 feet beneath the surface.
Winding your way through the darkness, you’ll come across a field of chimney-like reefs. It is here—in a deep marine environment lacking oxygen but replete with methane and sulfate—that a thriving community of bacteria and archaea have evolved the ability to accomplish amazing chemical feats.
Of the creatures Thauer has analyzed from the bottom of the Black Sea, perhaps the most fascinating are the archaea that metabolize methane to harness its energy. For their metabolic process, these microbes must oxidize—or remove electrons from—methane under anaerobic (no oxygen) conditions. Scientists previously thought biological systems weren’t capable of oxidizing methane in anaerobic environments because of the strength of the CH bond in methane—cleaving it requires 440 kilojoules per mole of energy—and usually oxygen would be required as a terminal electron acceptor in the metabolic process.
Thauer’s Black Sea super-microbes manage this feat by using a nickel (Ni(III))-containing protein to catalyze the dehydrogenation of methane to create a one-carbon unit at the same oxidation level as methanol. Since oxygen is not available, the Black Sea microbes typically use sulfate as the terminal electron acceptor to grab the extra electron that is expelled during this reaction. The resulting oxidized methane species is then used in the microbe’s metabolic process.
Other organisms have been found that use MnIV, FeIII or nitrite as terminal electron acceptors in anaerobic methane oxidation, but the organisms that do this are not related to those that mediate the anaerobic oxidation of methane with sulfate. Scientists do not yet understand the process organisms use to oxidize methane with Fe(III) and Mn(IV), though they have gleaned some insights about how the process happens when nitrite is the electron acceptor.
For chemists and engineers seeking better ways to store and transport methane for human energy needs, Thauer’s findings offer intriguing glimpses of how we might learn to oxidize methane to methanol, which can be easily stored and transported.
Furthermore, Thauer said, nickel enzymes with amino acid sequences similar to those of the nickel enzyme he observed in the Black Sea microbes are used by organisms of at least three relatively distantly related branches of the taxonomic tree, indicating the ability to oxidize methane with sulfate was evolved early and then developed further.
In addition to the methane-oxidizing archaea, Thauer studies archaea that are involved in creating methane by degrading biomass with hydrogen as an intermediate. Insights from ecosystems in which these microorganisms thrive could offer clues to help scientists create and use hydrogen as a fuel. “In this anaerobic world, they have a hydrogen economy…they produce hydrogen and they utilize hydrogen,” said Thauer. If humans want to use hydrogen as fuel, we may turn to nature for ideas on creating catalysts that produce hydrogen and catalysts that utilize it.
Microbes form and use hydrogen using catalysts called hydrogenases, of which there are three types: hydrogenases containing nickel and iron, hydrogenases containing two iron atoms, and hydrogenases containing one iron atom. Despite their similar functions, these proteins are not related and show no similarity in their genetic sequences, suggesting they evolved independently. “If, in nature, a principle has evolved independently several times, then it’s a good sign that that is the solution to the problem,” said Thauer.
“If, in nature, a principle has evolved independently several times, then it’s a good sign that that is the solution to the problem.” –Rudolf Thauer
From the chemistry of microbes, Thauer says, “we can learn a lot of principles. Maybe the solution will be completely different, but what we learn will definitely help us.”