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© HYDRA Marine Sciences GmbH | Seagrass meadows are widespread and cover a total of close to 600,000 square kilometers worldwide, which is roughly equivalent to the area of France.

Seagrasses con­tinue to re­lease meth­ane after their die-off

Meth­ane is formed and re­leased from seagrass mead­ows, even dec­ades after the plants died off.

Seagrass meadows play an important role in the marine carbon cycle and our climate. On the one hand, they sequester carbon dioxide from the atmosphere and store it underground, on the other hand, they emit the potent greenhouse gas methane. Researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, have now investigated what controls methane production and release from seagrass meadows. They now present their results in the journal Proceedings of the National Academy of Sciences (PNAS).

Seagrasses cover shal­low coastal re­gions of tem­per­ate and trop­ical seas world­wide. Seagrass mead­ows form the basis of an es­sen­tial eco­sys­tem that is home to nu­mer­ous an­im­als, in­clud­ing en­dangered spe­cies of sea turtles, seahorses, and fishes. They also pro­tect the coasts from erosion and se­quester mil­lions of tons of car­bon di­ox­ide from the at­mo­sphere every year. But seagrass mead­ows also emit green­house gases, es­pe­cially meth­ane, which has a much stronger ef­fect on our cli­mate than car­bon di­ox­ide.

Where does the meth­ane come from?

Sina Schorn and her col­leagues from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy and Hy­dra Mar­ine Sci­ences first in­vest­ig­ated what the meth­ane in the seagrass mead­ows is formed from. Seagrasses, like many ter­restrial plants, form large peat de­pos­its un­der the sed­i­ment sur­face. Ter­restrial peats are known to re­lease large amounts of meth­ane from the de­com­pos­i­tion of the or­ganic ma­ter­ial. Thus, the re­search­ers ex­pec­ted that the mech­an­isms be­hind meth­ane pro­duc­tion are sim­ilar in seagrass mead­ows. However: The op­pos­ite was the case.  “Here we ex­per­i­enced our first sur­prise,” ex­plains Schorn, the lead au­thor of the study.

“In seagrass sed­i­ments, meth­ane is formed solely from one class of or­ganic com­pounds,” Schorn says. “These so-called methyl­ated com­pounds are pro­duced by the seagrass plant it­self. Spe­cial­ized mi­croor­gan­isms, the meth­ano­genic ar­chaea, then con­vert these com­pounds into meth­ane.” The com­pounds in­clude, amongst oth­ers, be­taine: a com­pound that helps seagrasses cope with changes in sea­wa­ter sa­lin­ity. As meth­ano­genic ar­chaea can use these com­pounds dir­ectly, meth­ane pro­duc­tion in seagrass mead­ows is highly ef­fi­cient and ro­bust against en­vir­on­mental stresses.

And something else is dif­fer­ent in seagrass mead­ows than on land: The re­lease of meth­ane into the wa­ter column is very fast. First of all, the plant tis­sue acts as a straw, help­ing the gas to es­cape from the seabed into the wa­ter. Be­cause seagrasses only grow in shal­low wa­ter, pela­gic mi­croor­gan­isms have little op­por­tun­ity to con­sume the meth­ane be­fore it ends up in the at­mo­sphere. Ad­di­tion­ally, the sea­wa­ter flow­ing through the sands on which these seagrasses grow, quickly ‘washes out’ meth­ane from the sed­i­ment.

Also dead seagrass mead­ows are sources of meth­ane

As part of their study, the Bre­men re­search­ers sampled a dead seagrass meadow. “Here we en­countered an­other sur­prise,” re­ports Jana Milucka, senior au­thor of the study and head of the Green­house Gases Re­search Group at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy. “The rates of meth­ane pro­duc­tion were sim­ilar to those in the in­tact seagrass meadow.” Ob­vi­ously, meth­ane is still formed in dead seagrass sed­i­ments. “We be­lieve that the reason be­hind this con­tinu­ing meth­ane pro­duc­tion is that methyl­ated com­pounds per­sist in the plant tis­sue for a very long time,” Milucka says. They could even be de­tec­ted in plant tis­sue that had died more than two dec­ades ago.

Meth­ane emis­sions par­tially off­set the blue car­bon ef­fect

“Cur­rently, we are see­ing a die-off of seagrass mead­ows world­wide which has a dev­ast­at­ing ef­fect on the coastal eco­sys­tems. Our res­ults cau­tion that whereas upon the death of the plant car­bon di­ox­ide from the at­mo­sphere will no longer be se­questered and stored in the sed­i­ment as ‘blue car­bon’, meth­ane may still con­tinue to be re­leased”, Milucka ex­plains.

The work re­in­forces the im­port­ance of seagrass mead­ows for our cli­mate and high­lights the need to bet­ter un­der­stand and con­serve these eco­sys­tems. Seagrass mead­ows are nearshore hab­it­ats, and coastal re­gions are most dra­mat­ic­ally af­fected by an­thro­po­genic changes. “We need to un­der­stand how the seagrass meadow eco­sys­tem func­tions in or­der to de­term­ine the im­pact of the on­go­ing global change on it,” Schorn em­phas­izes.

Next, the re­search­ers at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy plan to ex­tend their meas­ure­ments to other re­gions and other seagrass spe­cies. They also plan to study the mi­croor­gan­isms in­volved in meth­ane pro­duc­tion in greater de­tail, as they are sur­pris­ingly di­verse and largely un­der­stud­ied.

  • Methane is a chemical compound with the molecular formula CH4. It is the simplest hydrocarbon and a potent greenhouse gas.
  • Methanogenesis is the formation of methane by microorganisms called methanogens.
  • Methanogens are microorganisms (archaea) that produce methane in the absence of oxygen.

  • “Di­verse methylo­trophic meth­ano­genic ar­chaea cause high meth­ane emis­sions from seagrass mead­ows” PNAS (2022) | DOI: 10.1073/pnas.2106628119

Max Planck Institute for Marine Microbiology 2022

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