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Dr. Claudia Bergen

Department of Environmental Sciences
Profiles & Affiliations

aquatic biogeochemist

I am an isotope biogeochemist and microbial ecologist fascinated by how tiny microbes drive global biogeochemical cycles under natural and anthropogenic challenges. My research spans from the freshwater to the marine environments and integrates biogeochemical field studies, laboratory experiments, isotope models, and molecular analysis to quantitatively characterize and predict microbial processes in a changing environment with focus on the aquatic nitrogen cycle.

Selected Publications

Frey, C., Tang, W., Ward, B. B., & Lehmann, M. F. (2024). Sample preservation methods for nitrous oxide concentration and isotope ratio measurements in aquatic environments [Journal-article]. Limnology and Oceanography: Methods, 22(10), 771–788. https://doi.org/10.1002/lom3.10638

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Frey, Claudia, Sun, Xin, Szemberski, Laura, Casciotti, Karen L., Garcia-Robledo, Emilio, Jayakumar, Amal, Kelly, Colette, Lehmann, Moritz F., & Ward, Bess B. (2023). Kinetics of nitrous oxide production from ammonia oxidation in the Eastern Tropical North Pacific. Limnology and Oceanography, 68(2), 424–438. https://doi.org/10.1002/lno.12283

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Bourbonnais, Annie, Frey, Claudia, Sun, Xin, Bristow, Laura A., Jayakumar, Amal, Ostrom, Nataniel E., Casciotti, Karen L., & Ward, Bess B. (2021). Protocols for assessing transformation rates of nitrous oxide in the water column. Frontiers in Marine Science, 8, 293. https://doi.org/10.3389/fmars.2021.611937

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Frey, Claudia, Bange, Hermann W., Achterberg, Eric P., Jayakumar, Amal, Löscher, Carolin R., Arévalo-Martínez, Damian L., León-Palmero, Elizabeth, Sun, Mingshuan, Sun, Xin, Xie, Ruifang C., Oleynik, Sergey, & Ward, Bess B. (2020). Regulation of nitrous oxide production in low-oxygen waters off the coast of Peru. Biogeosciences, 17(8), 2263–2287. https://doi.org/10.5194/bg-17-2263-2020

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Selected Projects & Collaborations

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NoLaMa "No laughing matter - N2O cycling in lacustrine environments"

Research Project  | 3 Project Members

Nitrous oxide (N 2 O, also known as laughing gas) has become the third most important anthropogenic greenhouse gas, after CO 2 and methane. Oceanic N 2 O emissions to the atmosphere represent up to 35 % of the global natural sources. Freshwater N 2 O emission are less well constrained, due to high spatial and temporal variability of aquatic N 2 O fluxes from inland waters. The exact biogeochemical controls on N 2 O cycling are still poorly constrained. In order to understand changes in the magnitude of N 2 O fluxes from aquatic ecosystems in response to fluctuating biogeochemical conditions (i.e., redox state, dissolved nitrogen, and organic substrates), it is imperative to determine the individual contributions of the microbial (ammonium oxidation/nitrification, nitrifier-denitrification, and denitrification) and abiotic N 2 O production pathways and their sensitivity to changing environmental conditions. The potential niche overlap of denitrifiers and nitrifiers along oxygen gradients, or between ammonium oxidizing bacteria and archaea in freshwater, make it difficult to distinguish between the different N 2 O sources and their process-specific controls. For example, an increasing number of studies suggest that denitrification, mostly known as N 2 O sink in anoxic waters, is a largely overlooked N 2 O source in the suboxic water masses overlaying open ocean oxygen minimum zones. At the same time, despite the canonical view that N 2 O reduction is an anaerobic process, high abundances of N 2 O reduction genes and transcripts have been found in marine oxic waters indicating a potential unknown N 2 O sink in surface waters. Whether such an aerobic N 2 O sink exists also in lakes remained unaddressed. As for nitrification in surface/subsurface lake waters, the role of dissolved organic N compounds (i.e. urea and cyanate) as potential substrate and precursor in N 2 O production is uncertain. The aim is to identify and quantify specific N 2 O production and consumption pathways in lacustrine environments, and to provide insight into the key microbial players, pathways, dynamics and environmental controls on N 2 O cycling. We will shed light on the blurring redox boundaries and environmental controls (e.g., nutrient availability) that modulate net N 2 O production in a lacustrine environment, where autotrophic denitrification is the dominating N loss pathway (Lake Lugano, Switzerland). The main objectives of the proposed project are to: 1) Identify the seasonal and vertical variability of N 2 O consumption, on the relative importance of N 2 O production processes in the water column, the abundance of process marker genes/transcripts, and the N 2 O producing/consuming microbial community composition in the studied lake. 2) Assess the sensitivity of N 2 O production and reduction processes to changes in dissolved nitrogen substrate and oxygen availability. 3) Determine the importance of underappreciated organic N-sources such as urea and cyanate for biotic and abiotic N 2 O production in the lake water column. We will use 15 N/ 18 O-tracer incubation experiments for potential-rate estimates along with natural abundance isotope measurements of N 2 O, NO 3 - , NO 2 - , and NH 4 + , which will provide an integrated signal of overlapping processes that affect the stable isotope pools of these molecules over short and longer time periods. We will combine isotope-biogeochemical results with data on abundance and community composition of N 2 O producers and consumers. The results that we expect to come out of the proposed project will provide essential knowledge about the mechanistic regulation of different N 2 O production pathways in aquatic environments, and will thus help to improve and validate existing and future global models predicting lacustrine N 2 O emissions.

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Expression of benthic isotope effects associated with nitrogen elimination and regeneration in lacustrine sediments

Research Project  | 6 Project Members

In many ocean regions, as a limiting nutrient, bioavailable nitrogen (N) controls marine primary productivity and thus the ocean's capacity to fix and sequester atmospheric CO 2 in its interior. In many lakes, N from both natural and anthropogenic sources is an important driver of eutrophication. Therefore, both in the ocean and in lakes, it is crucial to understand the sources and sinks of fixed N. Denitrification, the microbial reduction of nitrate to dinitrogen (N 2 ), and other modes of suboxic N 2 production (e.g., the anaerobic oxidation of ammonium, or anammox), are the most important sinks of fixed N in aquatic environments, but particularly with regards to the N cycle in the ocean, there is a persistent debate regarding the overall size of sinks and sources. Isotope ratios of nitrogenous species (e.g., 15 N/ 14 N) can provide important constraints on the natural N cycle. In order to use stable isotope measurements as a means to trace fluxes of N in aquatic systems, however, it is imperative to understand the isotope effects associated with these fluxes. While denitrification at the organism-level is known to be associated with a marked N isotope fractionation, the expression of the N isotope effect of benthic (i.e., sedimentary) denitrification in the water column above is only poorly constrained, and likely varies with the environmental conditions. Moreover, the possible impacts of other benthic N cycling processes on the N isotope exchange between the sediments and the water column (e.g., anammox, nitrate reduction to ammonium (DNRA), nitrate uptake, and/or nitrate regeneration) remain uncertain. Understanding the overall N isotope effect of net benthic N loss is a prerequisite for using N isotope measurements to infer its relative importance in the N cycle, in the global ocean or in a specific environment. Here we propose an in-depth investigation of the isotope effects of benthic fixed N elimination and nitrate regeneration in aquatic sediments. The prime goal of the proposed research is to build a thorough understanding of the modulating controls on the nitrate and nitrite N (and O) isotope signatures of denitrification and anammox (and the interacting effects from other benthic N cycling reactions), and the N isotopic composition of gaseous N (i.e., N 2 and N 2 O) that is ultimately lost from the sediments. We predict that the expression of the biological isotope effect of benthic N elimination at the level of sediment-water exchange will vary across different environments, and will strongly depend on the reactivity of the sediments, the O 2 penetration, the physical boundary conditions (i.e., diffusive transport), and on the extent to which other processes than denitrification contribute to the overall N cycling (nitrification, anammox, DNRA). Combining 1.) laboratory experiments with natural and artificial sediments, 2.) field investigations into the sediment porewater (N and O) isotope dynamics of distinct lacustrine and marine denitrifying benthic environments, and 3.) mathematical modeling , and making use of innovative multi-isotope techniques (natural abundance isotope analysis of NO 3 - / NO 2 - , ammonium, dissolved organic N, and N 2 O, as well as 15 N tracer experiments), we attempt to gain complementary information on how the combined isotope effects of benthic nitrate reduction and nitrate regeneration are expressed in the water column of lakes and the ocean. With the diagenetic porewater N isotope model that this project will deliver we will establish a quantitative framework for assessing benthic isotope fluxes and for verifying our hypotheses. The research proposed will result in the first comprehensive characterization of sediment pore-water N (and O) isotope dynamics in lacustrine settings, and will allow experimental constraints on the variability of N isotope effects during benthic nitrate reduction across different biogrochemical regimes. While the field component of the project focuses on lake sediments, the results expected will be directly pertinent to understanding of fixed-N elimination isotope effects in the ocean. It will thus be highly relevant for the use of N isotope measurements for local, regional, and even global N budgets, and will provide the basis for both paleolimnological and -oceanographic extrapolation.