The process by which plants and algae acquire sulfur—converting sulfate into sulfide—requires a lot of energy and produces harmful intermediates and byproducts that need to be immediately transformed. Because of this, it has been hypothesized that methanogens, which are usually short on energy, are unable to convert sulfate into sulfide and must rely on other forms of sulfur. However, the (decades-old) discovery that the methanogen Methanothermococcus thermolithotrophicus grows on sulfate as the only sulfur source has called this into question.

Now, new research uncovers how M. thermolithotrophicus does this, considering the energetic costs and toxic intermediates, and why it is the only known methanogen that has this capability.

This research is published in Nature Microbiology in the article, “Assimilatory sulfate-reduction in the marine methanogen Methanothermococcus thermolithotrophicus.”

“When I started my PhD, I really had to convince M. thermolithotrophicus to eat sulfate instead of sulfide,” said Marion Jespersen, a graduate student at the Max Planck Institute for marine microbiology. “But after optimizing the medium, Methanothermococcus became a pro at growing on sulfate, with cell densities comparable to those when growing on sulfide.”

To understand the molecular mechanisms of sulfate assimilation, the scientists identified five genes in the bacterium’s genome that had the potential to encode sulfate-reduction-associated enzymes.

methanogen microbe
The cascade of a chemical reaction starting from sulfate (SO42-) to sulfide (H2S). [Marion Jespersen/Max Planck Institute for Marine Microbiology]

By characterizing the enzymes, 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 subsequent enzymes were new.

“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,” said 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 explained.

The last two enzymes of the pathway are made to cope with two poisons that are generated during the assimilation of sulfate. The first one, 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.

“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,” said Tristan Wagner, PhD, head of the Max Planck Research Group Microbial Metabolism.

Hydrogenotrophic methanogens, such as M. thermolithotrophicus, have the ability to convert dihydrogen and carbon dioxide into methane. In other words, they can convert the greenhouse gas CO2 into the biofuel CH4, which can be used, for example, to heat 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,” concluded Wagner.

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