The marine methane-producing microorganism Methanothermococcus thermolithotrophicus is naturally excellent at converting CO2 into methane. Researchers have now discovered how this unique organism is able to grow on sulfate, since other methane-producing microbes cannot.
Sulfur is a basic building block for all life and is used in numerous essential molecules from proteins to vitamins.
Most organisms obtain sulfur from sulfate, converting it into sulfide, which can be used to build biomolecules.
This was thought not applicable to microorganisms called methanogens (methane-producing), for many particular reasons, one being the energy required to convert sulfate into sulfide and the production of intermediate toxic molecules.
However, one methanogenic archaeon, M. thermolithotrophicus, is surprisingly able to use sulfate as its sole sulfur source.
M. thermolithotrophicus is also promising because it can produce methane, which can be used as natural gas to heat our homes.
Other methanogens can do this too, but they require sulfide as a source of sulfur, and sulfide not only smells like rotten eggs but is also expensive, explosive and toxic. Conversely, M. thermolithotrophicus only requires sulfate as a source of sulfur, and this property has enormous biotechnological potential.
In a new study, researchers characterised how M. thermolithotrophicus achieves this.
“Many companies are working to build bioreactors with methanogens that can produce methane as a biofuel from CO2. This may reduce the extraction of natural gas. We wanted to establish how the biotransformation from sulfate to sulfide works in a methanogen,” explains a researcher behind the study, Tristan Wagner, Head, Microbial Metabolism Research Group, Max Planck Institute for Marine Biology, Bremen, Germany.
The research has been published in Nature Microbiology.
Converting sulfate produces toxic compounds
Methanogens produce half of the methane released into the atmosphere. Methane is 28 times more potent as a greenhouse gas than CO2 but is promising as a biofuel.
First, the researchers established that M. thermolithotrophicus can actually grow on sulfate by placing it in pressurised glass flasks without access to any form of sulfur other than sulfate. M. thermolithotrophicus grew and the sulfate disappeared.
“This is very interesting because the chemical transformation from sulfate to sulfide produces toxic molecules that can kill the microorganism. M. thermolithotrophicus somehow circumvents this problem,” says Tristan Wagner.
Five enzymes are required to convert sulfate to sulfide
In the next part of the research, the researchers examined M. thermolithotrophicus in depth and identified five genes that encode for five enzymes involved in the conversion of sulfate to sulfide.
Researchers would normally map one of these enzymes at a time, but the researchers mapped the function of all of them.
Together, the enzymes enable M. thermolithotrophicus to use sulfate as a source of sulfur, something other methanogens cannot.
Two of the enzymes were already known: ATP-sulfurylase, which generates adenosine 5′-phosphosulfate (APS); and APS-kinase, which phosphorylates APS to 3′-phosphoadenosine 5′-phosphosulfate (PAPS).
The next enzyme is PAPS reductase, which converts PAPS into sulfite and PAP. Both molecules are highly toxic and must be converted into something else as soon as possible.
Tristan Wagner says that the researchers were very surprised to discover the novel PAPS reductase in M. thermolithotrophicus.
“Plants and microorganisms use another PAPS reductase that differs from the one in M. thermolithotrophicus, which has most probably captured the gene for an APS reductase from another organism and converted it to be able to reduce PAPS to PAP and sulfite. This is extraordinary,” explains Tristan Wagner.
Because PAP and sulfite are so toxic, M. thermolithotrophicus needs two additional enzymes to convert these two substances into something non-toxic.
A PAP phosphatase hydrolyses PAP into AMP, which is reused in forming adenosine triphosphate (ATP), and a sulfite reductase turns sulfite into sulfide, which M. thermolithotrophicus can use to make new biomolecules.
Major industrial potential
According to Tristan Wagner, the discovery of the enzymes that enable M. thermolithotrophicus to grow on sulfate is promising for several reasons.
From a scientific perspective, the researchers would like to know where the genes coding for the sulfate assimilation pathway of M. thermolithotrophicus are coming from or from which other microbe(s) they have been derived.
Second, the discovery has biotechnological potential for converting CO2 into methane, which can be used as biofuel.
Current production methods for making methane using methanogens require hydrogen sulfide, and this is not ideal.
However, the discovery of the five enzymes opens up the possibility for researchers to genetically engineer methanogens, which are already used in natural gas power plants, by inserting the genes for the five enzymes in M. thermolithotrophicus into the DNA of the other methanogens, thereby enabling them to turn sulfate into sulfide. This will make the process cheaper and create safer conditions in the bioreactors.
“Further, the enzymes are resistant to high temperatures, which makes them more robust, so they should be able to function with sufficient efficiency in the current generation of bioreactors,” concludes Tristan Wagner.