Strange Isotopes: Scientists Explain a Mysterious Methane Isotope Paradox of the Seafloor

Sampling with the ROV in the home of the investigated microbes, the Guaymas-Beckens.
Credit: Woods Hole Oceanographic Institution

Why methane carbon isotopes in the deep sea behave so differently than expected.

Deep down in the seafloor anaerobic microbes consume large amounts of methane, a potent greenhouse gas when it enters atmosphere. Even though this process is a crucial element of the global carbon cycle, it is still poorly understood. Gunter Wegener from the Max Planck Institute for Marine Microbiology and the MARUM, Center for Marine Environmental Sciences, Bremen, Germany, and Jonathan Gropp from the Weizmann Institute of Science in Rehovot, Israel, now found the solution to a long-standing enigma in this process: why methane carbon isotopes behave so differently than expected. In a joint effort with their colleagues Heidi Taubner, Itay Halevy, and Marcus Elvert they present the answer in the journal Sci­ence Ad­vances.

Meth­ane, a chem­ical com­pound with the mo­lecu­lar for­mula CH4, is not only a power­ful green­house gas, but also an im­port­ant en­ergy source. It heats our homes, and even sea­floor mi­crobes make a liv­ing of it. The mi­crobes use a pro­cess called an­aer­obic ox­id­a­tion of meth­ane (AOM), which hap­pens com­monly in the sea­floor in so-called sulfate-meth­ane trans­ition zones – lay­ers in the sea­floor where sulfate from the sea­wa­ter meets meth­ane from the deeper sed­i­ment. Here, spe­cial­ized mi­croor­gan­isms, the AN­aer­obic­ally MEth­ane-ox­id­iz­ing (ANME) ar­chaea, con­sume the meth­ane. They live in close as­so­ci­ation with bac­teria, which use elec­trons re­leased dur­ing meth­ane ox­id­a­tion for sulfate re­duc­tion. For this pur­pose, these or­gan­isms form char­ac­ter­istic con­sor­tia.

This pro­cess takes place glob­ally in the sea­floor and hence is an im­port­ant part of the car­bon cycle. However, study­ing the AOM pro­cess is chal­len­ging be­cause the re­ac­tion is very slow. For its in­vest­ig­a­tion, re­search­ers of­ten use a chem­ical knack: the stable iso­tope ra­tios in meth­ane. But un­for­tu­nately, these iso­topes do not al­ways be­have as ex­pec­ted, which led to ser­i­ous con­fu­sion on the role and func­tion of the mi­crobes in­volved. Now re­search­ers from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy and the MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences in Ger­many to­gether with col­leagues from the Weiz­mann In­sti­tute of Sci­ence in Is­rael have solved this iso­tope en­igma and pub­lished their res­ults in the journal Science Advances. This paves the way for a bet­ter un­der­stand­ing of the im­port­ant pro­cess of an­aer­obic meth­ane ox­id­a­tion.

Iso­topes re­veal re­ac­tion path­ways

The puzzle and its solu­tion in de­tail: Iso­topes are dif­fer­ent “ver­sions” of an ele­ment with dif­fer­ent masses. The iso­topes of an ele­ment have the same num­ber of pro­tons (pos­it­ively charged particles) in the nuc­leus and there­fore the same po­s­i­tion in the peri­odic table (iso topos = Greek, same place). However, they dif­fer in the num­ber of neut­rons (neut­ral particles) in the nuc­leus. For ex­ample, car­bon has two stable iso­topes, the lighter 12C and the heav­ier 13C. Ad­di­tion­ally, there is the fa­mil­iar ra­dio­act­ive iso­tope 14C, a very rare car­bon spe­cies that is used to de­term­ine the age of car­bon-bear­ing ma­ter­i­als. Al­though the chem­ical prop­er­ties of the two stable iso­topes are identical, the dif­fer­ence in mass res­ults in dif­fer­ent re­ac­tion rates. When chem­ical com­pounds re­act, the ones with the lighter iso­topes are usu­ally con­ver­ted faster, leav­ing the heav­ier vari­ant in the ini­tial re­act­ant. This change in iso­topic com­pos­i­tion is known as iso­topic frac­tion­a­tion, and has been used for dec­ades to track chem­ical re­ac­tions. In the case of meth­ane ox­id­a­tion, this means that 12C-meth­ane is primar­ily con­sumed, lead­ing to an en­rich­ment of 13C in the re­main­ing meth­ane. Con­versely, a mi­cro­bial pro­duc­tion of meth­ane (meth­ano­gen­esis) would res­ult in par­tic­u­larly light meth­ane. “Real­ity, however, is sur­pris­ingly dif­fer­ent,” Gunter We­gener re­ports. “Con­trary to the lo­gic de­scribed above, we of­ten find very light meth­ane in sulfate-meth­ane trans­ition zones.”

Nature does­n’t fol­low the text­book: Light meth­ane in sulfate-meth­ane trans­ition zones

This para­dox raises ques­tions, such as: Is meth­ane not con­sumed there, but rather pro­duced? And who, if not the nu­mer­ous ANME ar­chaea, should be re­spons­ible for this? “In my lab, we have the world’s largest col­lec­tion of ANME cul­tures. There we could try to find out if and how the meth­ane ox­id­izers them­selves could be re­spons­ible for the form­a­tion of light meth­ane,” We­gener con­tin­ues. “The first res­ults were de­flat­ing: At the high sulfate con­cen­tra­tions we nor­mally find in sea­wa­ter, the cul­tured mi­croor­gan­isms be­haved ac­cord­ing to the text­book. The re­main­ing meth­ane was en­riched in the heav­ier iso­topes.” However, if the same ex­per­i­ments were car­ried out with little sulfate, meth­ane got en­riched in 12C, it be­came lighter. And this happened even though meth­ane con­tin­ued to be con­sumed at the same time – an ef­fect that at first glance had little lo­gic.

The avail­ab­il­ity of sulfate gov­erns the iso­topes ef­fects in AOM

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