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A dif­fer­ent take on P: Bac­teria use or­ganic phos­phorus and re­lease meth­ane in the pro­cess

Nov 8, 2023
Off Bar­ba­dos, re­search­ers from Bre­men have in­vest­ig­ated how bac­teria in­ad­vert­ently re­lease meth­ane in or­der to ob­tain phos­phorus – with sig­ni­fic­ant ef­fects on at­mo­spheric green­house gases.

Some bacteria are able to tap into unusual sources of nutrients in the surface water of the oceans. This enables them to increase their primary production and extract more carbon dioxide from the atmosphere. In doing so, however, they release the potent greenhouse gas methane.

Atlantic
Infinite vastness: Gases are constantly exchanged between the ocean and the atmosphere. The study presented here shows how tiny marine organisms contribute significantly to the release of the greenhouse gas methane in the tropical Atlantic. © Jan von Arx/Max Planck Institute for Marine Microbiology

Meth­ane as a waste product of phos­phorus cap­ture

The po­tent green­house gas meth­ane con­stantly es­capes from the sea into the at­mo­sphere and con­trib­utes sig­ni­fic­antly to global warm­ing. Meth­ane is mainly pro­duced by mi­croor­gan­isms and mostly in places where there is no oxy­gen. A few years ago, however, re­search­ers demon­strated that bac­teria are cap­able of so-called aer­obic meth­ane pro­duc­tion. These bac­teria pro­duce meth­ane as a waste product dur­ing the ac­quis­i­tion of phos­phorus, a nu­tri­ent that is cru­cial for sur­vival and ex­tremely rare in the sea. With the help of spe­cial en­zymes, the bac­teria can re­lease phos­phorus from or­ganic com­pounds, such as methyl­phos­phon­ate. Im­port­antly, these en­zymes also func­tion in the pres­ence of oxy­gen, for ex­ample in the sur­face wa­ters.

Study in the trop­ical At­lantic: Spread far be­low the wa­ter sur­face

The dis­tri­bu­tion and sig­ni­fic­ance of these bac­teria and their abil­it­ies are still poorly re­searched and un­der­stood. Re­search­ers from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men now present a study in which they in­vest­ig­ate bac­terial meth­ane pro­duc­tion in the sur­face wa­ter off the Carib­bean is­land of Bar­ba­dos. There is plenty of oxy­gen and little phos­phorus in the wa­ter there. “So far, this pro­cess has only been stud­ied in a few re­gions, mainly in the Pa­cific,” ex­plains first au­thor Jan von Arx. “We have now ex­amined it for the first time in the west­ern trop­ical North At­lantic.” The re­search­ers show that meth­ane pro­duc­tion is highest near the wa­ter sur­face. “But we were also able to de­tect meth­ane at depths of up to 200 metres, even though at these depths there is ac­tu­ally enough phos­phate and the bac­teria would­n't need to use methyl­phos­phon­ate,” says senior au­thor Jana Milucka, head of the Green­house Gases Re­search Group at the Max Planck In­sti­tute in Bre­men. In ad­di­tion, the types of bac­teria that pro­duce meth­ane des­pite oxy­gen be­ing present also change with depth: While the cy­anobac­terium Trichodesmium, a well-known and wide­spread mar­ine primary pro­du­cer, dom­in­ated meth­ane pro­duc­tion at the sur­face, so-called Alphaproteobacteria were pre­dom­in­ant at greater depths.

Car­bon from the at­mo­sphere: give and take

By us­ing the oth­er­wise un­avail­able phos­phorus source methyl­phos­phon­ate, it is pos­sible for the bac­teria to fix more car­bon in the sur­face wa­ter than if they were de­pend­ent on phos­phate alone. “Ac­cord­ing to our cal­cu­la­tions, the bac­teria can cover around a tenth of their phos­phorus re­quire­ments from methyl­phos­phon­ate,” says von Arx. “This al­lows them to re­move sig­ni­fic­ant amounts of car­bon di­ox­ide from the at­mo­sphere in this re­gion. This clearly un­der­lines the eco­lo­gical im­port­ance of phos­phon­ates in the car­bon cycle of nu­tri­ent-poor ocean re­gions.”

On the one hand, the mi­croor­gan­isms cap­ture car­bon di­ox­ide, but on the other hand they re­lease the much more po­tent green­house gas meth­ane. “Our study shows high meth­ane pro­duc­tion in the oxy­gen-sat­ur­ated wa­ter column – something that was long con­sidered im­possible, but is now in­creas­ingly be­ing ob­served,” says Milucka. “Since the bac­teria in­volved are found in all the world's oceans, the meth­ane pro­duced from methyl­phos­phon­ate prob­ably con­trib­utes sig­ni­fic­antly to the re­lease of this green­house gas from the sea, es­pe­cially in phos­phate-poor en­vir­on­ments.”

The marine methane paradox

In 2008, researchers in the USA reported a remarkable discovery: They showed how methane can be formed in the presence of oxygen - the so-called aerobic methane production. It may sound unspectacular, but this study solved one of the longest-standing mysteries in the world of methane biogeochemistry: the so-called marine methane paradox. The methane paradox refers to the supersaturation of methane in oxygen-rich surface waters - a place where methane production should not occur because oxygen is traditionally harmful to methane-producing microorganisms (archaea). The newly discovered process of methane production in oxic waters is carried out by bacteria that utilise an enzymatic pathway that is insensitive to oxygen. In contrast to the classical methane-producing archaea, the bacteria do not produce methane to obtain energy from this process. In their case, methane is a by-product of a reaction that primarily serves to obtain phosphorus. Since inorganic forms of phosphorus (such as phosphate) are only available in small quantities, many marine bacteria are forced to resort to organic phosphorus (such as phosphate esters and phosphonates). The latter group of compounds, more specifically methylphosphonate (MPn), has been shown to serve as a precursor for aerobic methane production in the sea.

In­creased meth­ane re­lease due to cli­mate change?

How much meth­ane is re­leased into the en­vir­on­ment de­pends on the ra­tio of its pro­duc­tion and ox­id­a­tion. “However, we still don't have a clear over­view of where the meth­ane in the ocean comes from and how it dis­ap­pears. We also don't know how these so-called sources and sinks of meth­ane in the ocean will re­act to the on­go­ing cli­mate change,” ex­plains Milucka. “We sus­pect that aer­obic meth­ane pro­duc­tion will in­crease in the fu­ture, as phos­phate will be­come even scarcer due to the warm­ing of the ocean and the res­ult­ing stronger strat­i­fic­a­tion of the wa­ter column. This is prob­lem­atic be­cause this pro­cess takes place in sur­face wa­ters and there­fore the meth­ane pro­duced in this way can im­me­di­ately es­cape into the at­mo­sphere,” adds von Arx.

In or­der to be able to pre­dict fu­ture changes in the re­lease of cli­mate-rel­ev­ant gases, the pro­cesses in­volved and the de­term­in­ing factors must be re­searched fur­ther. “If we un­der­stand how a pro­cess works, we have a bet­ter chance of pre­dict­ing and/​or coun­ter­act­ing its neg­at­ive ef­fects," con­cludes von Arx.

Ori­ginal pub­lic­a­tion

Jan N. von Arx, Abiel T. Kid­ane, Miriam Phil­ippi, Wiebke Mohr, Gaute Lavik, Sina Schorn, Mar­cel M. M. Kuypers, Jana Milucka (2023): Methyl­phos­phon­ate-driven meth­ane form­a­tion and its link to primary pro­duc­tion in the oli­go­trophic North At­lantic. Nature Com­mu­nic­a­tions, Oc­to­ber 16 2023.

DOI: 10.1038/s41467-023-42304-4

Par­ti­cip­at­ing in­sti­tu­tions

The M161 ex­ped­i­tion was part of the EUREC4A pro­ject, in which the Max Planck In­sti­tute (MPI) for Mar­ine Mi­cro­bi­o­logy, the MPI for Met­eor­o­logy, the MPI for the Struc­ture and Dy­nam­ics of Mat­ter and nu­mer­ous other in­sti­tu­tions and uni­versit­ies were in­volved. More in­form­a­tion on EUREC4A can be found here: https://eurec4a.eu/

Please dir­ect your quer­ies to:

Scientist

Greenhouse Gases Research Group

Jan von Arx

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

3133

Phone: 

+49 421 2028-6440

Jan von Arx

Group Leader

Greenhouse Gases Research Group

Dr. Jana Milucka

MPI für Marine Mikrobiologie
Celsiusstr. 1
D-28359 Bremen
Deutschland

Room: 

3128

Phone: 

+49 421 2028-6340

Dr. Jana Milucka

Head of Press & Communications

Dr. Fanni Aspetsberger

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

1345

Phone: 

+49 421 2028-9470

Dr. Fanni Aspetsberger
 
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