Methane (CH4) and nitrous oxide (N2O) are important greenhouse gases with a warming potential 34 and 298 times higher, respectively, than that of CO 2 on a 100-year timescale. Both gases increased rapidly in atmospheric abundance during the last decade, with a significant portion of the CH4 and N2O coming from freshwaters and marine ecosystems. In aquatic environments, the production and consumption of CH 4 and N 2 O is tightly controlled by microbial activity. Microbes performing these reactions inhabit both water column and sediments, attach to particles and live in symbiosis with animals. Their activity occurs under both oxic (e.g., methane oxidation, ammonia oxidation) as well as anoxic conditions (e.g., methanogenesis, anaerobic methane oxidation, denitrification). However, recent findings challenge many traditional ideas about CH 4 and N 2 O cycling. For example, CH 4 is also produced in oxic environments by various bacteria and phytoplankton, while N 2 O is consumed in oxic conditions by oxygen tolerant denitrifying or dinitrogen fixing microbes. At the same time, not only microbes, but abiotic reactions may be also responsible for greenhouse gas production. Aquatic ecosystems are currently under tremendous pressure due to globally increasing temperatures and nutrient loads, with a concomitant decrease in oxygen levels. Therefore, the response to these changing conditions may affect the production and consumption of CH 4 and N 2 O in these ecosystems. In this session, we aim to combine our knowledge of microbial greenhouse gas metabolism, including the production, consumption, and fluxes of methane and nitrous oxide in aquatic environments, as well as the underlying microbial communities and interactions among them, in order to assess the controlling factors for greenhouse gas fluxes in the light of climate change. We therefore encourage contributions that address the biogeochemistry and microbiology to all aspects of ongoing experimental, field and modeling work, including molecular-based or isotope labelling studies, as well as flux quantification.
Lead Organizer: Elizabeth Leon-Palmero, University of Southern Denmark & Princeton University (eleonpalmero@protonmail.com)
Co-organizers:
Sina Schorn, Max Planck Institute for Marine Microbiology (sschorn@mpi-bremen.de)
Bess B. Ward, Princeton University (bbw@princeton.edu)
Jana Milucka, Max Planck Institute for Marine Microbiology (jmilucka@mpi-bremen.de)
Carsten J. Schubert, Swiss Federal Institute of Aquatic Science and Technology (EAWAG) (carsten.schubert@eawag.ch)
Presentations
03:00 PM
IMPLEMENTING ISOTOPOMERS IN PROCESS-BASED MODELS REVEALS NITROUS OXIDE SOURCES IN OXYGEN MINIMUM ZONES (6181)
Primary Presenter: Colette Kelly, Stanford University (clkelly@stanford.edu)
Marine oxygen deficient zones are hotspots of nitrous oxide (N2O) production, but the drivers and mechanisms of this production remain poorly understood. Here we use the stable nitrogen and oxygen isotopes of nitrate, nitrite, and N2O to constrain nitrogen cycle processes and their rates. N2O is particularly rich in isotopic information since we can measure the isotopic content of each nitrogen atom individually (referred to as “isotopomers”) and their difference (“site preference”). We used these measurements to constrain the rates of N2O cycle processes in a regional, process-based model (Nitrogen cycling in Oxygen Minimum Zones, or NitrOMZ) of the eastern tropical North Pacific oxygen deficient zone, which includes N2O production via nitrification, denitrification, and hybrid N2O production. We found that the model can reproduce the observed high near-surface concentrations of N2O — and corresponding site preference minimum — with a combination of N2O production via nitrification, denitrification, and N2O production from nitrate via denitrification, with a nitrite “shunt” within the cell. In permanently anoxic waters, the model was only able to reproduce isotopic observations when denitrification is allowed to produce N2O with a novel, non-zero site preference. In oxygen-deficient waters, isotopomer profiles also indicated a shift from N2O production from nitrite to N2O production from nitrate — and thus a potential shift between denitrifying communities. Sensitivity tests with increasing bottom boundary dissolved oxygen and decreasing top boundary particulate organic carbon flux indicate that the N2O cycle is highly sensitive to both, with contrasting responses that are recorded in the isotopomer profiles.
03:15 PM
Deoxygenation and warming enhance estuarine N2O production (4988)
Primary Presenter: Weiyi Tang, Princeton University (weiyit@princeton.edu)
Global estuaries are large but highly uncertain sources of nitrous oxide (N2O) to the atmosphere. Estuaries have been severely perturbed by excess nitrogen input, deoxygenation and warming, with potential impacts on N2O emissions. Here we use the Chesapeake Bay – the largest estuary in the United States - to investigate the response of N2O production to deoxygenation and warming. Using four 15N tracers (15NH4+, 15N-urea, 15NO2-, 15NO3-), we measured N2O production from nitrification and denitrification under manipulated oxygen and temperature conditions. At >10 µM O2, nitrification was the major N2O production pathway, while denitrification’s contribution was negligible. When O2 was experimentally reduced to <5 µM, N2O production from nitrification increased from 0.1 to 0.4 nmol N2O L-1 d-1, due to a combination of increased N2O yield and a decreased nitrification rate. In contrast, N2O production from denitrification increased exponentially from ~0 to 15 nmol N2O L-1 d-1 when O2 was lowered from 25 to near 0 µM; the maximum N2O yield occurred at ~5 µM O2. When the temperature was raised from 15ºC to 35ºC (ambient temperature ~26ºC), N2O production from nitrification and denitrification rose by 2-10 fold, due to an increase in both rates but with relatively constant N2O yields. Due to the projected expansion of hypoxia and warming in future estuaries, N2O production and emission may therefore increase, in turn enhancing greenhouse warming and ozone depletion. These new observations provide a benchmark for modeling and predicting N2O production under future climate change.
03:30 PM
CABLE BACTERIA STIMULATE NITROUS OXIDE PRODUCTION IN MARINE SEDIMENT VIA (CHEMO)DENITRIFICATION (4890)
Primary Presenter: Ugo Marzocchi, Aarhus University (ugomar@bio.au.dk)
Chemodenitrification is the abiotic reaction of nitrite and ferrous iron that leads to nitrous oxide production. Cable bacteria can increase the availability of both reactants in sediment porewater by mediating nitrate reduction to nitrite and by promoting the dissolution of FeS minerals. We thereby hypothesized that chemodenitrification is stimulated in sediment with cable bacteria. Sediment cores from Aarhus harbour (Denmark) were incubated under aerated, nitrate-amended water. Additional sediment cores were provided with filters at 0.3 cm depth to minimize cable bacteria growth. Electric fields diagnostic of cable bacteria activity developed to intensities up to three fold higher in the treatment cores compared to the filter controls over a two-week period. Nitrous oxide was sporadically detected in the filter-controls (max conc. 5 µM), whereas it was consistently present in the treatment cores reaching concentrations up to 20 µM in the sediment subsurface. Nitrous oxide production occurred below the oxygen penetration depth excluding possible contribution from nitrification. Notably, nitrous oxide production rates and electric field intensities were positively correlated both in treatment (Pearson, r=0.96, p=0.001) and filter-control cores (Pearson, r=0.80, p=0.057). These results clearly link cable bacteria activity to nitrous oxide production. Abiotic tests are needed to quantify the chemical vs. biological contribution to the production of nitrous oxide.
03:45 PM
NITROUS OXIDE DISTRIBUTION IN THE WATERS AROUND DISKO ISLAND, GREENLAND. (5806)
Primary Presenter: Annabell Moser, University of Southern Denmark (annabellm@biology.sdu.dk)
Nitrous oxide (N2O) is the strongest natural greenhouse gas, and a major stratospheric ozone-depleting agent. The global ocean is considered a significant N2O source, however, some high-latitude regions, e. g. the Arctic Ocean or the Baltic Sea, can also act as a sink. In those regions, the surface water can seasonally be undersaturated in N2O indicating N2O consumption or removal. The pathway leading to the undersaturation of N2O in oxygen-rich waters is not fully understood yet. To investigate this enigma, we carried out a shipboard survey to the Greenland Sea around Disko Island. We determined N2O distribution in the water column along vertical profiles and metagenomic analyses. Here the first results are presented, shedding light on the potential sink or source of N2O in oxic waters in high-latitude waters. These results will help to better understand the processes leading to the under- or oversaturation of N2O. Further, it will provide invaluable data for the global budget of N2O emissions as well as for climate change prediction models.
04:00 PM
PROCESSES AND MICROORGANISMS DRIVING NITROUS OXIDE PRODUCTION IN THE BENGUELA UPWELLING SYSTEM (5364)
Primary Presenter: Gabriela Dangl, Leibniz Insitute for Baltic Sea Research Warnemuende (gabriela.dangl@gmail.com)
Upwelling systems and their associated oxygen minimum zones (OMZs) represent hotspots of N2O production in the ocean. The Benguela Upwelling System (BUS) is one of the most productive regions worldwide, and an important, yet variable, source of N2O to the atmosphere. The exact environmental controls on microbial N2O production pathways are still uncertain, as is our understanding of how marine N2O production changes in relation to global change. Questions remain, in particular, regarding the relative importance of nitrification versus denitrification as key N2O production processes, and their regulation by different environmental factors. To identify key metabolic pathways of N2O production in the BUS, and to assess the microbial players involved, we combined N2O production rate measurements, and analysis of natural abundance isotope composition of N2O with microbial diversity analysis of nitrifying and denitrifying microbial communities. We demonstrate that nitrification and denitrification contribute equally to the accumulation of N2O in low-O2 waters. Yet, in the same waters we also verified a high potential for N2O reduction to N2. We identified Thioglobus ponitus and Nitrosopumilus sp. as the potentially major microbial drivers of N2O production. In light of expected changes in upwelling intensities and the expansion of the OMZ, understanding the dynamics that likely modulate the balance between reductive and oxidative N2O production will be a prerequisite for estimating future changes in N2O emissions from the BUS.
04:15 PM
Photochemodenitrification: a novel reaction producing nitrous oxide in fresh and marine waters (6393)
Primary Presenter: Elizabeth Leon Palmero, Princeton University & University of Southern Denmark (ell@sdu.dk)
Nitrous oxide (N2O) is the main stratospheric ozone depleting agent, and one of the strongest greenhouse gases, about 300 times more potent than carbon dioxide. Ammonia oxidizers and denitrifiers are microbial groups that are supposed to control the N2O budget in aquatic systems. However, recent studies suggest that abiotic reactions such as chemodenitrification may also contribute to the N2O production. Here, we describe a novel process contributing to the production of N2O, which we refer to as photochemodenitrification. We detected a significant and consistent production of N2O induced by sunlight under abiotic conditions. This production varied from 3.3 to 317.4 nmol m-2 d-1 (i.e., from 0.4 to 31.0 nmol L-1 d-1) in two freshwater reservoirs in different years. Using 15N-labelled tracers, we demonstrated that nitrite and nitrate acted as substrates for this process in a freshwater reservoir and a coastal area. Unexpectedly, the N2O production by photochemodenitrification exceeded the biological production of N2O by ammonia oxidation in surface waters in the study reservoirs and may explain the higher N2O emissions detected at daytime in these reservoirs. Our results demonstrate that photochemodenitrification may be an essential and overlooked process occurring in both, fresh and marine waters, globally, and responsible for a significant fraction of the N2O emission of aquatic systems.
SS020C New Insights on The Methane and Nitrous Oxide Cycles from Freshwater and Marine Ecosystems Under Changing Climate
Description
Time: 3:00 PM
Date: 5/6/2023
Room: Auditorium Mallorca