Mix and Match: How a Methanogen creates its own sulfate reduction machinery

To understand the molecular mechanisms of sulfate assimilation, the scientists analyzed the genome of M. thermolithotrophicus. They found five genes that had the potential to encode sulfate-reduction-associated enzymes. “We managed to characterize every one of those enzymes and therefore explored the complete pathway. A true tour de force when you think about its complexity,” says Tristan Wagner, head of the Max Planck Research Group Microbial Metabolism.

By characterizing the enzymes one-by-one, the scientists assembled the first sulfate assimilation pathway from a methanogen. While the first two enzymes of the pathway are well known and occur in many microbes and plants, the next enzymes were of a new kind. “We were stunned to see that it appears as if M. thermolithotrophicus has hijacked one enzyme from a dissimilatory sulfate-reducing organism and slightly modified it to serve its own needs,” says Jespersen. While some microbes assimilate sulfate as a cellular building block, others use it to obtain energy in a dissimilatory process – as humans do when respiring oxygen. The microbes that perform dissimilatory sulfate-reduction employ a different set of enzymes to do so. The methanogen studied here converted one of these dissimilatory enzymes into an assimilatory one. “A simple, yet highly effective strategy and most likely the reason why this methanogen is able to grow on sulfate. So far, this particular enzyme has only been found in M. thermolithotrophicus and no other methanogens,” Jespersen explains.

However, M. thermolithotrophicus also needs to cope with two poisons that are generated during the assimilation of sulfate. That´s what the last two enzymes of the pathway are made for: The first one, again similar to a dissimilatory enzyme, generates sulfide from sulfite. The second one is a new type of phosphatase with robust efficiency to hydrolyze the other poison, shortly known as PAP.

“It seems that M. thermolithotrophicus collected genetic information from its microbial environment that enabled it to grow on sulfate. By mixing and matching assimilatory and dissimilatory enzymes, it created its own functional sulfate reduction machinery,” says Wagner.

Hydrogenotrophic methanogens, such as M. thermolithotrophicus, have the amazing ability to convert dihydrogen (H₂, for example artificially produced from renewable energy) and carbon dioxide (CO₂) into methane (CH₄). In other words, they can convert the greenhouse gas CO₂ into the biofuel CH₄, which can be used, for example, to heat our homes. To do this, methanogens are grown in large bioreactors. A current bottleneck in the cultivation of methanogens is their need for the highly hazardous and explosive hydrogen sulfide gas as a sulfur source. With the discovery of the sulfate-assimilation pathway in _{M. thermolithotrophicus}, it is possible to genetically engineer methanogens that are already used in biotechnology to use this pathway instead – leading to safer and more cost-effective biogas production.

“An unresolved burning question is why M. thermolithotrophicus would assimilate sulfate in nature. For this, we will have to go out into the field and see if the enzymes required for this pathway are also expressed in the natural environment of the microbe”, concludes Wagner.