ANAMMARKS: ANaerobic AMmonium oxidiation bioMARKers in paleoenvironmentS

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In modern marine environment, 30-50% of nitrogen lost from the ocean is due to anaerobic ammonium oxidation (anammox). This bacterial process removes an important nutrient, nitrogen, from the marine phytoplankton system. Thus, anammox has a direct consequence on global marine primary production, the uptake of carbon dioxide, and the carbon cycle. Anammox bacteria performing this process are only active in low-oxygen to anoxic settings, included oxygen minimum zones (OMZs) in the water column. OMZs are expanding in our current changing climate and it is important to understand how this expansion will affect anammox activity and in turn the carbon cycle. Reconstructing paleoclimate in analogs for modern and future climate allows us to study how future changes will affect elements like the anammox processes. There are several instances in Earth's climate history when expanding OMZ has led to full-scale oceanic anoxia. Anammox bacteria are members of a deep-branching phylum, and the process has been hypothesised to have played an important role in creating and maintaining oceanic anoxia during crucial periods of Earth's history (e.g. Jurassic and Cretaceous Oceanic Anoxic Events (OAEs)). Determining how anammox was involved in these past scenarios will help better predict what likely outcomes we can expect in our future.Organic geochemistry uses molecular fossils, called biomarkers, to study the impact microbial processes have had on the environment. Currently, tracing anammox bacteria using biomarkers is done using ladderane lipids. However, the applicability of a biomarker has temporal limitations. For example, the inability to withstand degradative processes, which occur during and after deposition, restricts how far back in time these biomarkers can be applied. Although ladderane lipids are excellent biomarkers for modern environments, they are highly labile and not well suited for tracing past anammox activity. Thus, in order to clarify the role anammox has played during these past extreme climate events, lipids must first be identified that can be used as biomarkers in more mature sediments.Two distinct lipid classes have shown potential as biomarkers for past anammox, and will be assessed in this project. These lipids will be evaluated and will be implemented to trace anammox in past oceanic settings. The first class (bacteriohopanepolyols, specifically BHT isomer) seem suitable for sediments deposited within the last 50 Ma, and that have not been exposed to thermal stresses after burial. For example, we will apply these biomarkers to a 2 Myr sediment record underlying the Peru OMZ to explore the hypothesis that anammox influences the expansion of OMZs by contributing to nitrogen removal during increased OMZ. The second class (unusual cyclic and branched long-chain alkanes) extends the time window of detection into thermally mature sediments. These biomarkers will be investigated in OAE events to determine how anammox influenced a shift towards nitrogen-fixation being the dominate pathway of nutrient uptake during OAEs. Additionally, these alkanes will be economically benefit project partners in the petroleum industry, where biomarkers for anoxia would indirectly indicate preservation potential of organic matter and petroleum. We will create a simplified method for anammox detection that we will disseminate to other geochemistry laboratories for their studies of the anammox process. Combined, these findings and those specifically from our system studies will help understand past nitrogen cycling by using our established biomarkers to trace past anammox activity. Finally, the results of our studies of paleo-anammox will be incorporated into the biogeochemical model GENIE. This will improve our understanding of the role anammox played in past nitrogen cycling. Subsequently, model results will help to better predict the implications of anammox on future nitrogen and carbon cycling under our changing climate.