Controls on iron and sulfate reduction in Arctic fjord sediments under climate change

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The primary objective of this project is to understand how the two most important processes for anaerobic oxidation of organic carbon are controlled in marine sediments, namely the microbial reduction of iron and of sulfate. Microbial mineralization of organic matter is a driving force behind many element cycles in marine sediments. The dominant terminal oxidant affects not only the geochemistry of the sediment but also the release of nutrients, such as iron, manganese, or phosphate. The terminal electron-accepting processes change with depth in the sediment, following a sequential order, which is thought to be determined by thermodynamics. Yet, a substantial spatial overlap between iron and sulfate reduction is frequently found. It appears that the competition between iron- and sulfate-reducing microorganisms is affected by many other factors, like the availability of the electron donor or the crystallinity of the iron, what kinetically constrains iron reduction. However, it is not understood how exactly these different factors influence the balance between iron and sulfate reduction in marine sediments.I will investigate how the competition between iron- and sulfate- reducers is controlled in the arctic seabed, which is strongly affected by climate change. The ongoing and predicted warming of the Arctic Ocean will change the availability of iron, due to enhanced glacial runoff, and of organic carbon, due to reduced sea ice cover, what leads to increased algal productivity and sedimentation. It is not understood how these changes will influence the balance between iron and sulfate reduction and consequently the biogeochemistry in Arctic sediments. The fjords on the west coast of Svalbard, which are severely impacted by climate change, provide ideal field sites where warm Atlantic water meets cold Arctic water. In the sediments of the different fjords contrasting geochemical conditions can be found, including conditions favoring either iron or sulfate reduction. I will combine geochemical, microbiological and molecular biological analyses of sediments that have high or low loading of iron or organic carbon with carefully conceived experiments with these sediments as well as pure culture experiments. The study will relieve how sensitively Arctic sediments will respond to altered depositional influx. The Center for Geomicrobiology at Aarhus University is the perfect host because of their great expertise in biogeochemical processes and microbial communities in marine sediments and their experience and engagement in field work in the Arctic. The preliminary work that I already performed for this project showed how sensitive the balance between iron and sulfate reduction is to changed iron and organic carbon content and how I can further progress with this study.