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
10:30 AM
CIRCADIAN VARIABILITY OF GREENHOUSE GAS FLUXES IN A EUTROPHIC RESERVOIR OVER SEASONS (7398)
Primary Presenter: Eva Rodriguez-Velasco, University of Granada (erodvel@ugr.es)
Reservoirs are recognized as significant sources of greenhouse gases (GHGs), yet the global estimations of these emissions are highly uncertain due to the limited measurements taken under optimal conditions. The diurnal variability in GHG fluxes is known to be strongly influenced by various physical and biogeochemical factors, including water temperature, wind speed, primary production, and photochemical reactions. However, our understanding of how diurnal variability in gas emission processes changes over seasons is very limited. In this study, we conducted a year-long analysis of daily changes in GHG emissions from a eutrophic reservoir in Southern Spain. Our findings revealed similar daily patterns for CO2, CH4, and N2O fluxes, with higher emissions during the day than during the night, particularly during the summer months. Additionally, the stratification period led to nighttime peaks in CH4 and N2O fluxes. Furthermore, our study also identified significant dependences between GHG emissions and various environmental factors, such as radiation, wind speed, and water temperature. These findings have significant implications for our understanding of the complex mechanisms underlying GHG emissions from reservoirs, and the challenges in accurately estimating these emissions at a global scale. In addition, our study emphasizes the significance of incorporating various time scales to evaluate the variability in future GHG measurements and models and provides important insights for developing effective mitigation strategies for these emissions.
10:45 AM
METHANE OXIDATION IN SURFACE WATER REDUCES EMISSIONS TO THE ATMOSPHERE IN MANGROVES (5801)
Primary Presenter: Yvonne Yau, Gothenburg (yvonne.yau@gu.se)
Mangroves store significant amounts of organic carbon in sediments. During carbon burial, methane (CH4) is produced in anoxic, organic-rich sediments and released to the surface waters via porewater exchange and ebullition. Yet, highly variable CH4 emissions have been reported in mangroves because of high uncertainty of methane production and oxidation rates. Combining the stable isotopic composition of methane (δ13C-CH4) in porewater and surface water can reveal the fraction of CH4 oxidized or emitted to the atmosphere. Here, we report high-temporal resolution CH4 concentrations from creek waters and porewater at two mangrove creeks in Brazil along with measurements of δ13C of CH4. Enriched δ13C-CH4 in top surface layer sediments indicates CH4 oxidation in the surface sediment before porewaters reach the water column. Tidal pumping in mangrove facilitates CH4 oxidation in the water column. Surface waters at low tide exhibited a lighter δ13C-CH4 than at high tide. A similar δ13C-CH4 signature between low tide surface water (-70 ± 2 ‰) and porewater (-74 ± 4 ‰) imply that sediment is the source of CH4. A stable isotope mass balance showed that 30 – 66% of was oxidized within water column, with the rate of 19-121 µmol m-2 d-1. Air-sea CH4 emissions were estimated at both mangroves (68 ± 65 in a pristine and 179 ± 205 µmol m-2 d-1 in an urbanized mangrove), on the same order of magnitude as the CH4 oxidation rate. Overall, our results suggested that CH4 oxidation in mangrove surface water and sediments partially reduce CH4 emissions to the atmosphere.
11:00 AM
MANGANESE-DRIVEN ANAEROBIC OXIDATION OF METHANE IN COASTAL MARINE SEDIMENTS (6653)
Primary Presenter: Robin Klomp, Radboud University (robin.klomp@ru.nl)
In marine sediments, methane (CH4) is formed when organic matter is degraded via methanogenesis. A large fraction of the formed CH4 is typically removed via microbially mediated anaerobic oxidation of methane (AOM). During AOM, oxidation of CH4 can be coupled to reduction of, for example, sulfate, nitrate and iron and manganese (Mn) oxides. Increased eutrophication in coastal environments can lead to a higher production of CH4 and a shallowing of the sulfate-methane transition zone (SMTZ). This may result in an increased importance of metal oxides in AOM. To date, it is not known what microbes perform Mn-AOM in marine sediments and which types of Mn oxides are being used. In this study, we combine field data with batch incubations to explore novel pathways of AOM coupled to reduction of Mn oxides in sediments of a seasonally euxinic coastal marine basin (Lake Grevelingen, The Netherlands). Results of sediment and pore water analyses reveal that Mn oxides are buried below the SMTZ in layers that remain free of sulfide. Batch incubations in which sediment is supplied with isotopically labelled 13CH4 and Mn oxides show production of 13CO2 and the reduction of different forms of Mn oxides, including birnessite and pyrolusite. Analysis of the 16S rRNA genes reveal an increase of ANME-2a/b and ANME-3 when Mn oxides are reduced. Our results show that burial of Mn oxides in CH4-rich sediments of a seasonally euxinic coastal marine basin may lead to Mn-AOM, performed by anaerobic archaeal methanotrophs.
11:15 AM
BIOGEOCHEMICAL CONTROLS ON NITROUS OXIDE PRODUCTION AND CONSUMPTION IN THE SOUTH BASIN OF LAKE LUGANO, SWITZERLAND (5116)
Primary Presenter: Teresa Einzmann, University of Basel (teresa.einzmann@unibas.ch)
Nitrous oxide (N2O) is a strong greenhouse gas and ozone-destroying agent, with increasing atmospheric mixing ratios over the last few decades. The contribution of lakes to global N2O emissions is uncertain, in parts due to the lack of a better understanding of the environmental controls on N2O production and consumption processes. In this study, we investigated N2O dynamics in the seasonally stratified South Basin of eutrophic Lake Lugano (Switzerland) over one complete annual cycle by analysing the concentrations and stable isotopic composition of N2O, including N2O site preference (SP), and performing tracer incubation experiments with <sup>15</sup>N-labelled nitrogen compounds. Strong accumulation of N2O (up to 100 nmol/L) was observed in the anoxic benthic nepheloid layer in near-bottom waters during summer stratification. It was accompanied by a significant decline in δ<sup>15</sup>N<sup>bulk</sup> to -12 ‰ and an increase in SP to +42 %permil;. Incomplete heterotrophic denitrification is usually the main production mechanism for N2O in low-oxygen environments. However, N2O produced through denitrification is expected to yield a low SP, whereas high SP values rather point to an oxidative N2O production pathway. The combined isotopic constraints on fractional N2O reduction remained ambivalent. N2O in the bottom water is potentially produced by an alternative denitrification pathway, i.e. chemolithotrophic denitrification, or its production may be coupled to an oxidative process, which will be investigated in future experiments.
11:30 AM
WINTER NITROGEN CYCLING IN SEDIMENTS OF LARGE BOREAL LAKES AFFECTED BY BROWNING AND MINING (5326)
Primary Presenter: Carlos Palacin-Lizarbe, University of Eastern Finland (carlos.palacin@uef.fi)
The ice-covered period of boreal lakes has contrasting environmental conditions respect to the ice-free, with cold temperatures, absence of light, and minor gas exchange between water and atmosphere. Focusing on the N cycle, winter seems a suitable period for N-transforming prokaryotes with a high availability of reactive N due to minor assimilation by photoautotrophs. However, there is limited data about winter N cycling rates and the microbes involved on, and about the role of organic matter quality on N cycling. We studied 2 oligotrophic large boreal lakes in North Karelia, Finland, with clear-water and brown-water sides, and an additional side affected by mining resulting in higher nitrate and sulphate levels in the hypolimnion. During winter of 2021 we sampled at the beginning and at the end of the ice-cover. Using the IPT we incubated sediment cores with 15NO3- and quantified the products of 1) complete denitrification (N2), 2) truncated denitrification (N2O), and 3) (DNRA, NH4+) to infer the process rates. Also, to see the role of organic matter, we did anoxic slurry incubations with 15NO3- and 1) lake water, 2) miliQ water, 3) algal dissolved organic matter (DOM) extract, or 4) peatland DOM extract. We characterized the DOM using FT-ICR MS. We also explore the genetic potential (DNA) of the sediment microbiome by using several sequencing techniques: 1) 16S rRNA, 2) targeted (N and CH4 functional genes), and 3) shotgun. Preliminary results identify the N-transforming microbes and point to changing nitrate consuming activities and genetic potentials between sites.
11:45 AM
DIRECT BIOLOGICAL FIXATION PROVIDES A FRESHWATER SINK FOR NITROUS OXIDE (6895)
Primary Presenter: Yueyue Si, Queen Mary University of London (yueyuesi00@gmail.com)
Nitrous oxide (N2O) is a potent climate gas, with its strong warming potential and ozone-depleting properties both focusing research on N2O sources. While N2O sinks have been reported in many oxic waters, these remain poorly understood and are often dismissed simply as the reduction of N2O to N2 through denitrification. Although a sink through biological N2O fixation has been observed in the Pacific, the regulation of N2O compared to canonical N2 fixation is unknown. Here we show that both N2O and N2 can be fixed by freshwater communities but with distinct seasonalities and temperature dependencies. While N2 fixation is stronger in spring and summer, N2O fixation appears to be independent of temperature, driving a strong N2O sink in winter. Moreover, by quantifying both N2O and N2 fixation we show that N2O fixation is direct, rather than N2O being first reduced to N2 through denitrification, and direct N2O fixation may explain N2O sinks widely reported in natural waters. With weaker N2O sinks at higher temperatures, rising temperatures could result in an erosion of the natural sinks for N2O.
SS020B New Insights on The Methane and Nitrous Oxide Cycles from Freshwater and Marine Ecosystems Under Changing Climate
Description
Time: 10:30 AM
Date: 5/6/2023
Room: Auditorium Mallorca