Researchers from the Max Planck Institute for Marine Microbiology have discovered how a methane-producing microbe survives poisoning with toxic sulfide. Methanogens are tiny organisms that produce methane in the absence of oxygen. For example, the production of methane in the digestive tract of ruminants plays an important role in the global carbon cycle, as methane is a very potent greenhouse gas. However, methane can also serve as an energy source for heating homes.

A toxic basis for growth

Purpose of the currently published study Nature Chemical Biology, There are two marine thermophilic methanogens: Methanothermococcus thermolithotrophic (live in geothermally heated sediments at a temperature of about 65 °C) and Methanocaldococcus jannaschii (prefers deep sea volcanoes with a temperature of about 85°C). They derive their cellular energy by producing methane and obtain sulfur in the form of sulphur in their environment to grow.

Although sulfur is toxic to most organisms, it is essential for methanogens and can tolerate even high concentrations. However, the Achilles heel is sulfide, the toxic and reactive sulfur compound that destroys the enzyme needed to produce methane. Both test organisms are occasionally exposed to sulfite in their environment, for example when oxygen enters and reacts with reduced sulfide. Its partial oxidation leads to the formation of sulfites, and therefore the methanogens must protect themselves. But how can they do this?

Molecular snapshot of the process

Marion Jespersen and Tristan Wagner of the Max Planck Institute for Marine Microbiology in Bremen, Germany, together with Antonio Pierik of the University of Kaiserslautern, now present a snapshot of the enzyme that detoxifies sulfide. This butterfly-shaped enzyme is known as F420-linked sulfide reductase, or Fsr. It can convert sulfide into sulfite, a safe source of sulfur needed by methanogens for growth.

In the current study, Jespersen and colleagues describe how the enzyme works. “The enzyme captures the sulfide and directly reduces it to sulfide, which can be incorporated, for example, into the composition of amino acids,” explains Jespersen. “As a result, the methanogen is not poisoned and even uses the product as a source of sulfur.” . They turn poison into food!”

Sounds simple. But in fact, Jespersen and his colleagues realized they were dealing with a fascinating and complex overlap. “There are two ways to reduce sulfites: ingestion and assimilation,” explains Jespersen. “The organism under study uses an enzyme designed to be digestive, but uses an assimilative mechanism. At least in terms of living conditions, we can say it combines the best of both worlds.”

The enzymes of both the dissimilation and assimilation pathways are assumed to have evolved from a common ancestor. “Sulfite reductases are ancient enzymes that have a significant impact on global sulfur and carbon cycles,” adds Tristan Wagner, head of the Max Planck Microbial Metabolism Research Group at the Max Planck Institute in Bremen. “Our enzyme, Fsr, is probably a snapshot of this ancient primitive enzyme, a fascinating look at evolution.”

Biotechnological applications

Fsr not only uncovers evolutionary implications, but also helps us better understand the fascinating world of marine microbes. Methanogens, which can only grow on sulfide, bypass the need to use dangerous sulfites, which are the usual sulfur substrates.

“This opens up opportunities for safer biotechnological applications to study these important microorganisms. The optimal solution would be to find a methanogen that reduces sulfate, which is an inexpensive, common and completely safe source of sulfur,” says Wagner.

Actually, this methanogen already exists, Methanothermococcus thermolithotrophic. The researchers hypothesized that Fsr regulates the final reaction of this sulfate reduction pathway, as one of its intermediates will be sulfide.

“Our next challenge is to understand how these miracle microbes can convert sulfate to sulfite to get a complete picture of their abilities.”