How microbes trigger greenhouse gas emissions before complete permafrost thaw

Microbial reduction of oxidized iron (Fe(III)) and the associated dissolution of reactive iron(III) minerals mobilizes mineral-bound organic carbon, which contributes to carbon dioxide production and promotes methanogenesis and methane emission even before complete permafrost thaw.
Published in Earth & Environment
How microbes trigger greenhouse gas emissions before complete permafrost thaw

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In June, as the temperatures rise and mosquitos awake, our team once again arrives at Stordalen Mire, a thawing permafrost peatland in northern Sweden. We tread across wooden boardwalks, past birch trees and cotton grass closer to raised mounds, known as palsas, underlain by intact permafrost insulated by dry peat.

Figure 1. Thawing permafrost peatland with collapsing palsas in the North of Sweden (Stordalen mire, Abisko). ©Monique Patzner

 As we already showed in a former study that iron minerals cannot preserve organic carbon along a thaw gradient, we now want to answer the question what happens to the mobilized organic carbon after being released into the surrounding porewater following mineral dissolution during palsa collapse. For this, we install automatic Eosense eosFD gas flux chambers, as well as static flux chambers. We have to take metal cores into squishy mud, pull out peat and porewater samples to bring them back to the laboratory. There, we characterize iron-organic carbon associations in the solid phase, study microorganisms forming and dissolving iron minerals and microorganisms producing greenhouse gases such as carbon dioxide and methane. “Although there is extensive research going on to study the microbes that dwell in permafrost, iron-cycling microorganisms and their interplay with others e.g. methane-producers have been rather unstudied”, explains Casey Bryce, corresponding author of the study. Here, the Tuebinger scientists worked together with colleagues from Bristol (UK), Fort Collins (USA), Copenhagen (Denmark) and Freising (Germany) creating an interdisciplinary team to work on microbial iron cycling in thawing permafrost environments.

Figure 2. Gas and porewater sampling along collapsing palsa hillslopes. © Monique Patzner

In the past, Stordalen Mire was covered in permafrost. Due to rising global temperatures, most of it has degraded into a heterogenous patch of semiwet bogs and grassy wetlands. For most of human history, permafrost has been one of the largest carbon sinks, trapping plant and animal material in its frozen underground for centuries. But now climate change has enormous consequences for permafrost environments, causing rapid changes in soil conditions with direct consequences for organic carbon destabilization. As the recent IPCC report states: “[…] thawing terrestrial permafrost will lead to carbon release under a warmer world. However, there is low confidence on the timing, magnitude and linearity of the permafrost climate feedback owing to the wide range of published estimates and the incomplete knowledge and representation in models of drivers and relationships. [...]". “It is therefore crucial to pierce together all carbon sources and sinks with permafrost thaw”, says Andreas Kappler, head of the Geomicrobiology group at the University Tuebingen.

Merritt Logan, PhD candidate at the Colorado State University, pulls out the first lysimeters along one of the sampled palsa hillslopes. “Dark, brown and a little bit of red”, he smiles, “must be full of iron and carbon”. Later, the analyses reveal that reactive iron minerals binding and protecting organic carbon in palsa soils, are dissolved by iron-reducing bacteria during the permafrost collapse towards semiwet bogs, releasing aqueous Fe2+ and organic carbon.

Figure 3. Already collapsed palsas, causing enlargement of semisweet bog areas. © Monique Patzner

The porewater sample is stored anoxically in a serum bottle, wrapped in aluminum foil, and stored in a cooling box. After a long day in the field with no sunset, the samples are brought back to the Arctic Research Station in Abisko.

The mineral bound and porewater organic carbon is further analyzed at the National High Magnetic Field Laboratory at Florida State University with our collaborators of the Colorado State University. Our analyses show that aliphatic organic carbon is released following iron mineral dissolution, rendering previously stable carbon vulnerable to microbial decomposition and subsequent release as greenhouse gases to the atmosphere.

Microbial community analysis and carbon emission measurements at the University Tuebingen further indicate that the mobilization of organic carbon following iron mineral dissolution is accompanied by an increase in the abundance of hydrogenotrophic methanogenic microorganisms and methane emissions at the collapsing front following the palsa hillslopes towards the semiwet bogs.

Our findings suggest that dissolution of reactive iron minerals along collapsing palsa hillslopes contributes to carbon dioxide and methane production and emission, even before complete permafrost thaw.

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