Glacier bacteria’s contribution to carbon cycling

Glaciers might seem rather inhospitable environments. However, they are home to a diverse and vibrant microbial community. It’s becoming increasingly clear that they play a bigger role in the carbon cycle than previously thought.

Cotton Glacier stream in Antarctica

A new study pub­lished in the journal Nature Geoscience shows how mi­cro­bial com­munit­ies in melt­ing gla­ciers con­trib­ute to the Earth’s car­bon cycle, a find­ing that has global im­plic­a­tions as the bulk of the Earth’s gla­ciers are shrinking in re­sponse to a warm­ing cli­mate.

The pa­per chal­lenges the pre­vail­ing the­ory that mi­croor­gan­isms found in gla­cial melt­water primar­ily con­sume an­cient or­ganic car­bon that was once de­pos­ited on gla­cial sur­faces and in­cor­por­ated into ice as gla­ciers formed.

The study was con­duc­ted by Heidi Smith and Christine Fore­man of the Cen­ter for Biofilm En­gin­eer­ing in Montana State Uni­versity’s Col­lege of En­gin­eer­ing, USA; Mar­cel Kuypers and Sten Littmann of the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many; and re­search­ers at the Uni­versity of Col­or­ado at Boulder, the U.S. Geo­lo­gical Sur­vey, and Stock­holm Uni­versity in Sweden.

Activated photosynthetic bacteria

“We felt that there was an­other side to the story,” said Smith. “What we showed for the first time is that a large pro­por­tion of the or­ganic car­bon is in­stead com­ing from pho­to­syn­thetic bac­teria that are also found in the ice and that be­come act­ive as the ice melts,” Smith said. Like plants, those bac­teria ab­sorb car­bon di­ox­ide and in turn provide a source of or­ganic mat­ter.

The re­search team made the dis­cov­ery after sampling melt­water from a large stream flow­ing over the sur­face of a gla­cier in the McMurdo Dry Val­leys re­gion of Ant­arc­tica in 2012. Af­ter­ward, Smith spent two months at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, where she worked with col­leagues to track how dif­fer­ent car­bon iso­topes moved through the melt­water’s eco­sys­tem, al­low­ing the team to de­term­ine the car­bon’s ori­gin and activ­ity.

Bacterial carbon preferred

The re­search­ers ul­ti­mately found that the gla­cial mi­crobes util­ised the car­bon pro­duced by the pho­to­syn­thetic bac­teria at a greater rate than the older, more com­plex car­bon mo­lecules de­pos­ited in the ice, be­cause the bac­terial car­bon is more “labile,” or eas­ily broken down. The labile car­bon “is kind of like a Snick­ers bar,” Smith said, mean­ing that it’s a quick, en­er­gis­ing food source that’s most avail­able to the mi­crobes.

Moreover, the re­search­ers found that the pho­to­syn­thetic bac­teria pro­duced roughly four times more car­bon than was taken up by the mi­crobes, res­ult­ing in an ex­cess of or­ganic car­bon be­ing flushed down­stream. “The eco­lo­gical im­pact of this bio­lo­gic­ally pro­duced or­ganic car­bon on down­stream eco­sys­tems will be amp­li­fied due to its highly labile nature,” Fore­man said.

Al­though in­di­vidual gla­cial streams ex­port re­l­at­ively small amounts of or­ganic car­bon, the large mass of gla­ciers, which cover more than 10 per­cent of the Earth’s sur­face, means that total gla­cial run­off is an im­port­ant source of the ma­ter­ial. Mar­ine or­ganic car­bon un­der­pins wide-ran­ging eco­lo­gical pro­cesses such as the pro­duc­tion of phyto­plank­ton, the found­a­tion of the oceans’ food­web.

Marine microbial communities most impacted

As gla­ciers in­creas­ingly melt and re­lease the or­gan­ic­ally pro­duced, labile car­bon, “we think that mar­ine mi­cro­bial com­munit­ies will be most im­pacted,” Smith said. “We hope this gen­er­ates more dis­cus­sion.”

Eliza­beth Ku­jaw­in­ski, a ten­ured sci­ent­ist at Woods Hole Ocean­o­graphic In­sti­tu­tion, called the team’s work “an el­eg­ant com­bin­a­tion” of re­search meth­ods. Taken to­gether with an­other study pub­lished in the same is­sue of Nature Geoscience about mi­cro­bial car­bon cyc­ling in Green­land, Smith’s pa­per “de­flates the no­tion that gla­cier sur­faces are poor hosts for mi­cro­bial meta­bol­ism,” ac­cord­ing to Ku­jaw­in­ski. The two stud­ies “have es­tab­lished that mi­cro­bial car­bon cyc­ling on gla­cier sur­faces can­not be ig­nored,” she ad­ded.

Original publications:
H. J. Smith, R. A. Foster, D. M. McK­night, J. T. Lisle, S. Littmann , M. M. M. Kuypers und C. M. Fore­man: Microbial formation of labile organic carbon in Antarctic glacial environments. Nature Geoscience. 
DOI: 10.1038/ngeo2925
E. Ku­jaw­in­ski: The power of glacial microbes. Nature Geoscience. DOI: 10.1038/ngeo2933

Image credit: C. Foreman

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