Certain Bacteria Play a Greater Role Than Previously Thought in Preventing the Release of Methane From Lakes

Methane-oxidizing bacteria could play a greater role than previously thought in preventing the release of climate-damaging methane from lakes, researchers from Bremen report. They also show who is behind the process and how it works.

Meth­ane is a po­tent green­house gas fre­quently pro­duced in the sea and in fresh wa­ter. Lakes in par­tic­u­lar re­lease large quant­it­ies of this cli­mate-killer. For­tu­nately, however, there are mi­croor­gan­isms that coun­ter­act this: They are able to util­ize meth­ane to grow and gen­er­ate en­ergy, thus pre­vent­ing it from be­ing re­leased into the at­mo­sphere. These mi­croor­gan­isms, known as meth­an­o­trophs, are there­fore re­garded as an im­port­ant "bio­lo­gical meth­ane fil­ter".

Meth­an­o­trophs com­prise vari­ous groups of mi­croor­gan­isms, and many ques­tions about their way of life have yet to be answered. A study by re­search­ers from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, and the Swiss Eawag, which has now been pub­lished in the journal Nature Com­mu­nic­a­tions, shows the as­ton­ish­ing abil­it­ies of some of these or­gan­isms and their pre­vi­ously over­looked role for our cli­mate.

Aer­obic mi­croor­gan­isms in oxy­gen-free wa­ters

For their study, the re­search­ers around Sina Schorn and Jana Milucka from the Max Planck In­sti­tute in Bre­men traveled to Lake Zug in Switzer­land. This lake is al­most 200 meters deep and per­man­ently oxy­gen-free from a depth of around 120 meters. Nev­er­the­less, the oxy­gen-free wa­ter con­tains so-called aer­obic meth­ane-ox­id­iz­ing bac­teria (MOB for short). These, as their name im­plies, are es­sen­tially de­pend­ent on oxy­gen. Whether and how they can break down meth­ane in the oxy­gen-free wa­ter was un­clear un­til now.

Milucka and Schorn's team there­fore de­cided to take a closer look at the activ­ity of these mi­croor­gan­isms. For their study, they used meth­ane mo­lecules (CH4) that were labeled with “heavy” car­bon atoms (13C in­stead of 12C). These were ad­ded to nat­ural lake wa­ter samples con­tain­ing the in­hab­it­ing mi­croor­gan­isms. Sub­sequently, the sci­ent­ists fol­lowed the path of the heavy car­bon in in­di­vidual cells us­ing spe­cial in­stru­ments (known as NanoSIMS). This al­lowed them to ob­serve how the bac­teria con­vert the meth­ane into car­bon di­ox­ide, which is also a po­tent green­house gas but less cli­mate-dam­aging than meth­ane. Part of the car­bon was also in­cor­por­ated dir­ectly into the bac­terial cells. This re­vealed which cells in the bac­terial com­munity were act­ive and which were not. Us­ing mod­ern meth­ods such as meta­ge­n­om­ics and meta­tran­scrip­tom­ics, they also in­vest­ig­ated which meta­bolic path­ways the bac­teria used.

Only one bac­terial group is act­ive without oxy­gen

“Our res­ults show that aer­obic MOB re­main act­ive also in oxy­gen-free wa­ter,” says Sina Schorn, who is now a re­searcher at the Uni­versity of Gothen­burg. “However, this only ap­plies to a cer­tain group of MOB, eas­ily re­cog­niz­able by their dis­tinct­ive rod-shaped cells. To our sur­prise, these cells were equally act­ive un­der oxic and an­oxic con­di­tions, i.e. with and without oxy­gen. Thus, if we meas­ure lower rates of meth­ane ox­id­a­tion in an­oxic wa­ters, it is prob­ably be­cause there are fewer of these spe­cial rod-shaped cells and not be­cause the bac­teria are less act­ive.”

Meta­bolic ver­sat­il­ity against meth­ane re­lease

The Max Planck re­search­ers en­countered an­other sur­prise when they took a closer look at the meta­bolic cap­ab­il­it­ies of this group of bac­teria. "Based on the genes present, we were able to de­term­ine how the bac­teria re­spond when oxy­gen be­comes scarce," ex­plains Jana Milucka, head of the Green­house Gases Re­search Group at the Max Planck In­sti­tute in Bre­men. "We found genes that are used for a spe­cial type of meth­ane-based fer­ment­a­tion." While this pro­cess had already been demon­strated for MOB cul­tures in the labor­at­ory, it had not yet been stud­ied in the en­vir­on­ment. The re­search­ers also dis­covered sev­eral genes for de­ni­tri­fic­a­tion, which likely al­low the bac­teria to use ni­trate in­stead of oxy­gen to gen­er­ate en­ergy.

The fer­ment­a­tion pro­cess, in par­tic­u­lar, is in­ter­est­ing. "If the MOB per­form fer­ment­a­tion, they likely re­lease sub­stances that other bac­teria can use for growth. This means the car­bon con­tained in the meth­ane is re­tained in the lake for a longer period of time and does not reach the at­mo­sphere. This rep­res­ents a sink for meth­ane car­bon in an­oxic en­vir­on­ments that is typ­ic­ally not ac­coun­ted for, which we will need to in­clude in our fu­ture cal­cu­la­tions," says Milucka.

Sig­ni­fic­ant re­duc­tion of present and fu­ture meth­ane emis­sions

In this study, the Bre­men re­search­ers ex­plain who breaks down meth­ane in oxy­gen-free hab­it­ats and how this de­grad­a­tion takes place. They show that meth­ane-ox­id­iz­ing bac­teria are sur­pris­ingly im­port­ant to keep the re­lease of meth­ane from these hab­it­ats to the at­mo­sphere in check.

“Meth­ane is a po­tent green­house gas that is re­spons­ible for about a third of the cur­rent global rise in tem­per­at­ure,” says Schorn, ex­plain­ing the sig­ni­fic­ance of the res­ults now pub­lished. “Meth­ane ox­id­a­tion by mi­croor­gan­isms is the only bio­lo­gical sink for meth­ane. Their activ­ity is there­fore cru­cial for con­trolling meth­ane emis­sions into the at­mo­sphere and thus for reg­u­lat­ing the global cli­mate. Given the cur­rent and pre­dicted in­crease in an­oxic con­di­tions in tem­per­ate lakes, the im­port­ance of MOB for meth­ane de­grad­a­tion in lakes is ex­pec­ted to grow. Our res­ults sug­gest that MOB will make a sig­ni­fic­ant con­tri­bu­tion to green­house gas mit­ig­a­tion and car­bon stor­age in the fu­ture.”

Reference: Schorn S, Graf JS, Littmann S, et al. Persistent activity of aerobic methane-oxidizing bacteria in anoxic lake waters due to metabolic versatility. Nat Commun. 2024;15(1):5293. doi: 10.1038/s41467-024-49602-5

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