Los científicos del Instituto Max Planck de Microbiología Marina han descubierto que Metanotermococo termolitotrófico, un metanógeno que anteriormente se creía incapaz de convertir el sulfato en sulfuro debido a los altos costos de energía del proceso y los subproductos dañinos, de hecho puede crecer en el sulfato. Los investigadores descubrieron cinco genes que codifican enzimas asociadas a la reducción de sulfato en el genoma del metanógeno y, al caracterizar estas enzimas, ensamblaron la primera vía de asimilación de sulfato a partir de un metanógeno.
Cómo un microbio metanogénico vuelve a ensamblar una vía metabólica pieza por pieza para transformar el sulfato en un bloque de construcción celular.
Los investigadores han descubierto que el metanógeno Metanotermococo termolitotrófico puede convertir el sulfato en sulfuro, desafiando las suposiciones anteriores. Al identificar una vía única de asimilación de sulfato en este metanógeno, los hallazgos abren la posibilidad de una producción de biogás más segura y rentable a través de la ingeniería genética.
El azufre, un componente esencial de la vida
El azufre es un elemento fundamental para la vida y todos los organismos lo necesitan para sintetizar materiales celulares. Los autótrofos, como las plantas y las algas, adquieren azufre al convertir el sulfato en sulfuro, que se puede incorporar a la biomasa. Sin embargo, este proceso requiere mucha energía y produce productos intermedios y subproductos nocivos que deben transformarse de inmediato. Como resultado, anteriormente se creía que los microbios conocidos como metanógenos, que generalmente tienen poca energía, no podrían convertir el sulfato en sulfuro. Por lo tanto, se asumió que estos microbios, que producen la mitad del metano del mundo, dependen de otras formas de azufre, como el sulfuro.
¿Un sulfato asimilador de metanógeno?
Este dogma se rompió en 1986 con el descubrimiento del metanógeno. Methanothermococcus thermolithotrophicus, creciendo en sulfato como la única fuente de azufre. ¿Cómo es esto posible, considerando los costos energéticos y los intermediarios tóxicos? ¿Por qué es el único metanógeno que parece ser capaz de crecer en este azufre?
» data-gt-translate-attributes=»[{» attribute=»»>species? Does this organism use chemical tricks or a yet unknown strategy to allow sulfate assimilation? Marion Jespersen and Tristan Wagner at the Max Planck Institute for Marine Microbiology have now found answers to these questions and published them in the journal Nature Microbiology.
PhD student Marion Jespersen works on a fermenter in which M. thermolithotrophicus grows exclusively on sulfate as sulfur source. Credit: Tristan Wagner / Max Planck Institute for Marine Microbiology
The first challenge the researchers met was to get the microbe to grow on the new sulfur source. “When I started my PhD, I really had to convince M. thermolithotrophicus to eat sulfate instead of sulfide,” says Marion Jespersen. “But after optimizing the medium, Methanothermococcus became a pro at growing on sulfate, with cell densities comparable to those when growing on sulfide.”
“Things got really exciting when we measured the disappearance of sulfate as the organism grew. This was when we could really prove that the methanogen converts this substrate.” This allowed the researchers to safely cultivate M. thermolithotrophicus in bioreactors in large scales, as they were no longer dependent on the toxic and explosive hydrogen sulfide gas for growth. “It provided us with enough biomass to study this fascinating organism,” explains Jespersen. Now the researchers were ready to dig into the details of the underlying processes.
The first molecular dissection of the sulfate assimilation pathway
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.
The cascade of chemical reaction starting from sulfate (SO42-) to sulfide (H2S). Credit: Marion Jespersen / Max Planck Institute for Marine Microbiology
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.
New avenues for biotechnological application
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.
Reference: “Assimilatory sulfate-reduction in the marine methanogen Methanothermococcus thermolithotrophicus” 5 June 2023, Nature Microbiology.
DOI: 10.1038/s41564-023-01398-8
