24-Nov-2022 - Max-Planck-Institut für marine Mikrobiologie

Break­ing ni­tro­gen while gen­er­at­ing meth­ane

In­sights into a “hot” mi­crobe that can grow on ni­tro­gen while pro­du­cing meth­ane

Scientists at the Max Planck Institute for Marine Microbiology have successfully enhanced cultivation of a microorganism that can fix nitrogen (N2) while producing methane (CH4) and ammonia (NH3) and investigated exciting details of its metabolism.

Car­bon and ni­tro­gen are es­sen­tial ele­ments of life. Some or­gan­isms take up key po­s­i­tions for the cyc­ling of both of them – among them Methanothermococcus thermolithotrophicus. Be­hind the com­plic­ated name hides a com­plic­ated mi­crobe. M. thermolithotrophicus is a mar­ine heat-lov­ing meth­ano­gen. It lives in ocean sed­i­ments, from sandy coasts and salty marshes to the deep-sea, prefer­ably at tem­per­at­ures around 65 °C. It is able to turn ni­tro­gen (N2) and car­bon di­ox­ide (CO2) into am­mo­nia (NH3) and meth­ane (CH4) by us­ing hy­dro­gen (H2). Both products, am­mo­nia and meth­ane, are very in­ter­est­ing for bi­o­tech­no­lo­gical ap­plic­a­tions in fer­til­izer and bio­fuels pro­duc­tion.

Tristan Wag­ner and Nevena Maslać from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy have now man­aged to grow this mi­crobe in a fer­menter – a chal­len­ging en­deav­our. “It is very com­plic­ated to provide the per­fect con­di­tions for this mi­crobe to thrive while fix­ing N2 – high tem­per­at­ures, no oxy­gen and keep­ing an eye on hy­dro­gen and car­bon di­ox­ide levels”, says Maslać, who car­ried out the re­search as part of her PhD pro­ject. “But with some in­genu­ity and per­sever­ance, we man­aged to make them thrive in our lab and reach the highest cell dens­it­ies re­por­ted so far.” Once the cul­tures were up and run­ning, the sci­ent­ists were able to in­vest­ig­ate the physiology of the mi­crobe in de­tail, and later on deepen their study by look­ing how the meta­bol­ism from the mi­crobe ad­apts to the N2-fix­a­tion. “In close col­lab­or­a­tion with our col­leagues Chandni Sidhu and Hanno Teel­ing, we com­bined physiolo­gical tests and dif­fer­en­tial tran­scrip­tom­ics, which al­lowed us to dig deeper into the meta­bol­ism of M. thermolithotrophicus”, Maslać ex­plains.

As im­prob­able as a bumble­bee

The meta­bolic abil­it­ies of M. thermolithotrophicus are puzz­ling: These mi­crobes use meth­ano­gen­esis, a meta­bol­ism that ori­gin­ated on the early an­oxic Earth, to ac­quire their cel­lu­lar en­ergy. Com­pared to hu­mans that use oxy­gen to trans­form gluc­ose into car­bon di­ox­ide, meth­ano­gens ob­tain only a very lim­ited amount of en­ergy from meth­ano­gen­esis. Para­dox­ic­ally, fix­ing ni­tro­gen re­quires gi­gantic amounts of en­ergy, which would ex­haust them. “They are a bit like bumble­bees, which are the­or­et­ic­ally too heavy to fly but ob­vi­ously do so, nev­er­the­less”, says senior au­thor Tristan Wag­ner, group leader of the Max Planck Re­search Group Mi­cro­bial Meta­bol­ism. “Des­pite such en­ergy lim­it­a­tion, these fas­cin­at­ing mi­crobes have even been found to be the prime 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. Their ni­tro­genase can be in­hib­ited by tung­state. Sur­pris­ingly, the Bre­men sci­ent­ists found that M. ther­mo­li­tho­trophi­cus is not dis­turbed by tung­state while grow­ing on N2. “Our mi­crobe was only de­pend­ent on mo­lyb­denum to fix N2 and not bothered by tung­state, which im­plies an ad­apt­a­tion of metal-ac­quis­i­tion sys­tems, mak­ing it even more ro­bust for dif­fer­ent po­ten­tial ap­plic­a­tions”, says Maslać.

Re­think­ing am­mo­nia pro­duc­tion

Ni­tro­gen fix­a­tion, i.e., gain­ing ni­tro­gen from N2, is the ma­jor pro­cess to in­sert ni­tro­gen into the bio­lo­gical cycle. For in­dus­trial fer­til­izer pro­duc­tion this pro­cess is car­ried out via the Haber-Bosch pro­cess, which ar­ti­fi­cially fixes ni­tro­gen to pro­duce am­mo­nia with hy­dro­gen un­der high tem­per­at­ures and pres­sures. It is used to pro­duce most of the world’s am­mo­nia, an es­sen­tial fer­til­izer to sus­tain global ag­ri­cul­ture. The Haber-Bosch pro­cess is ex­tremely en­ergy-de­mand­ing: It con­sumes 2% of the world’s en­ergy out­put, and re­leas­ing at the same time up to 1.4% of global car­bon emis­sions. Thus, people are look­ing for more sus­tain­able al­tern­at­ives to pro­duce am­mo­nia. “The pro­cess used by M. thermolithotrophicus shows that out there in the mi­cro­bial world there are still solu­tions that might al­low for a more ef­fi­cient pro­duc­tion of am­mo­nia, and that they can even be com­bined with bio­fuel pro­duc­tion through meth­ane”, says Wag­ner. “With this study, we un­der­stood that un­der N2-fix­ing con­di­tions, the meth­ano­gen sac­ri­fices its pro­duc­tion of pro­teins to fa­vor ni­tro­gen cap­ture, a par­tic­u­larly smart strategy of en­ergy real­loc­a­tion”, Wag­ner sums up. “Our next step will be to move into the mo­lecu­lar de­tails of the pro­cess and the en­zymes in­volved, as well as to look into other parts of the or­gan­is­m’s meta­bol­ism.”

Facts, background information, dossiers
  • methane
  • ammonia
  • Methanothermococcus…
  • microorganism cultivation
  • meth­ano­gens
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