Can iron-metabolizing bacteria control the fate of carbon during permafrost thaw?
Final Report Abstract
Increasing global temperatures are causing permafrost thaw, which releases organic carbon (OC) in the form of greenhouse gases (GHG), amplifying global climate change. However, accurate predictions of the total extent of GHG release are not yet possible, since the degradation of OC depends largely on poorly constrained microbial degradation rates. One factor influencing microbial degradation is the bioavailability of OC, which is commonly decreased due to adsorption to, or coprecipitation with reactive iron minerals under oxic conditions, compared to unbound dissolved OC under both oxic and anoxic conditions. We have shown recently that ongoing permafrost thaw causes waterlogging and anoxia, leading to microbial reduction and dissolution of Fe(III) minerals which likely releases the associated OC, influencing net GHG emissions and microbial community composition. In this project, we therefore aimed to understand (i) the potential (maximum) quantity and chemical composition of Fe-bound OC, (ii) the impact on GHG emissions upon microbial reduction of Fe-OC associations and (iii) the key microbial players involved in the formation and dissolution of Fe-OC associations. We studied these phenomena across a permafrost thaw gradient from intact permafrost (“palsa”) to partially thawed (“bog”) and fully thawed (“fen”) soils. Our main findings are as follows: (i) The potential maximum quantity of bound OC via adsorption decreased by more than 50% from fen soils compared to palsa and bog soils. This was due to increasing pH as well as a shift in initial OC composition. Additionally, (formed) Fe minerals selectively bound more aromatic and higher molecular weight OC, leaving more bioavailable OC in solution. (ii) Fe-OC associations were dissolved due to microbial reduction in palsa and bog soils upon the establishment of anoxic conditions, but are only partially dissolved in fen soils. The impact on resulting changes of GHG was dependent on thaw stage, with an increase in CO2 emissions in palsa and fen soils due to coupled respiration of OC. Methane (CH4) emissions in bog and fen soils were suppressed due to microbial Fe(III) reduction. In conclusion, Fe minerals are not an effective OC sink under future anoxic conditions in thawing permafrost soils. (iii) Both Fe(III)-reducing and Fe(II)-oxidizing microorganisms have been enriched from the different thaw stages, amongst them the first phototrophic Fe(II)-oxidizers from a soil environment. The fact that it was possible to enrich those microorganisms from multiple locations within the thawing permafrost system, adapted to different light conditions, might indicate their importance in these systems and their potential role in iron and carbon cycling.
Publications
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Emerging investigator series: preferential adsorption and coprecipitation of permafrost organic matter with poorly crystalline iron minerals. Environmental Science: Processes & Impacts, 26(8), 1322-1335.
Voggenreiter, Eva; Schmitt-Kopplin, Philippe; ThomasArrigo, Laurel; Bryce, Casey; Kappler, Andreas & Joshi, Prachi
