Scientists at the Max Planck Institute for Marine Microbiology have succeeded in improving the cultivation of a microorganism that can fix nitrogen (N2) in the production of methane (CH4) and ammonia (NH3) and examined exciting details of its metabolism.
Carbon and nitrogen are essential elements of life. Some organisms occupy key positions in the cycle of both – including Methanothermococcus thermolithotrophicus. Behind the complicated name lies a complicated microbe. M. thermolithotrophicus is a marine thermophilic methanogen.
It lives in ocean sediments, from sandy coasts and salt marshes to the deep sea, preferring temperatures around 65°C. It is able to absorb nitrogen (N2) and carbon dioxide (CO2) in ammonia (NH3) and methane (CH4) using hydrogen (H2). Both products, ammonia and methane, are of great interest for biotechnological applications in fertilizer and biofuel production.
Tristan Wagner and Nevena Maslać from the Max Planck Institute for Marine Microbiology have now succeeded in growing this microbe in a ferment – a challenging undertaking.
“It is very complicated to create the perfect conditions for this microbe to thrive while N2– high temperatures, no oxygen and an eye on hydrogen and carbon dioxide levels,” says Maslać, who conducted the research as part of her PhD project. “But with some ingenuity and perseverance, we’ve managed to get them to thrive in our lab, reaching the highest cell densities reported to date.”
Once the cultures were up and running, the scientists were able to study the microbe’s physiology in detail and later deepen their study by examining how the microbe’s metabolism adapts to the N2-fixation. “Working closely with our colleagues Chandni Sidhu and Hanno Teeling, we combined physiological assays and differential transcriptomics, which allowed us to delve deeper into the metabolism of M. thermolithotrophicus,” explains Maslać.
As improbable as a bumblebee
The metabolic abilities of M. thermolithotrophicus are enigmatic: these microbes use methanogenesis, a metabolism that originated on early anoxic Earth, to obtain their cellular energy. Compared to humans, who use oxygen to convert glucose into carbon dioxide, methanogens derive a very limited amount of energy from methanogenesis. Paradoxically, the fixation of nitrogen requires gigantic amounts of energy that would deplete it.
“They’re a bit like bumblebees, which are theoretically too heavy to fly, but obviously fly anyway,” says senior author Tristan Wagner, group leader of the Max Planck Research Group Microbial Metabolism. “Despite this energy limitation, these fascinating microbes have even turned out to be the best nitrogen fixers in some environments.”
A robust nitrogenase
The enzyme that organisms use to fix nitrogen is called nitrogenase. Most common nitrogenases require molybdenum to perform the reaction. Molybdenum nitrogenase is well studied in bacteria living as symbionts in plant roots. Your nitrogenase can be inhibited by tungstate.
Surprisingly, the Bremen scientists found that M. thermolithotrophicus is not disturbed by tungstate when growing on N2. “Our microbe only relied on molybdenum to fix N2 and is not disturbed by tungstate, which implies an adaptation of metal detection systems, making it even more robust for various potential applications,” says Maslać.
Ammonia production rethought
Nitrogen fixation, i.e. extraction of nitrogen from N2, is the main process to introduce nitrogen into the biological cycle. For industrial fertilizer production, this process takes place via the Haber-Bosch process, in which nitrogen is artificially fixed with hydrogen to form ammonia at high temperatures and pressures. It is used to produce most of the world’s ammonia, an essential fertilizer for sustaining global agriculture.
The Haber-Bosch process is extremely energy-intensive: it consumes 2% of the energy generated worldwide and at the same time releases up to 1.4% of global CO2 emissions. As a result, people are looking for more sustainable alternatives to produce ammonia.
“The process used by M. thermolithotrophicus shows that there are still solutions in the microbial world that could enable more efficient production of ammonia, and that they can even be combined with biofuel production from methane,” says Wagner.
“With this study, we understood that among N2-fixation conditions, the methanogen sacrifices its production of proteins in favor of nitrogen fixation, a particularly intelligent energy redistribution strategy,” summarizes Wagner, as well as looking into other parts of the organism’s metabolism.”
The study was published in mBio.
Nevena Maslać et al, Comparative Transcriptomics Sheds Light on Remodeling of Gene Expression during Diazotrophy in the Thermophilic Methanogen Methanothermococcus thermolithotrophicus, mBio (2022). DOI: 10.1128/mbio.02443-22
Provided by the Max Planck Society
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