The decline in oxygen content of aquatic systems is one of the most alarming consequences of anthropogenic global change. Recent decades have seen tremendous shifts in the location and strength of oxyclines globally, impacting biogeochemical pathways and shifting ecosystems. Increasing observational efforts have shown discrepancies with global modelling efforts. These discrepancies demonstrate the need to better understand the physical processes governing these interfaces, the biogeochemical processes and microbes living across them, to improve associated models, which would allow improved predictions in a changing hydrosphere. The transition from oxic to anoxic conditions induces a physiological change; the shift from aerobic to anaerobic metabolisms. These transitions are naturally present in all aquatic environments, whether in the water column or within sediments of marine and freshwater environments and determine the cycling and fate of relevant elements such as C, N, P and S, as well as trace metals. In marine and freshwater environments, recent discoveries of anaerobic processes being active in oxic environments and aerobic processes found in anoxic ones are redefining these transitions. Thus, modifying our understanding of the fate of relevant elements in aquatic environments, and to re-evaluate the consumption and production of inorganic nutrients, potent greenhouse gases (N 2 O and CH 4 ) and toxic gases (H 2 S). The importance of recently discovered metabolisms is also coming to light, largely as a result of technological advances in sequencing, novel single cell techniques and geochemical approaches. This session seeks to bring together researchers from marine and freshwater to reveal and understand the complex interplay between chemical, biological and physical processes at the oxic-anoxic transition in water columns and sediments.
Lead Organizer: Emilio Garcia-Robledo, University of Cadiz (emilio.garcia@uca.es)
Co-organizers:
Laura Bristow, University of Gothenburg (laura.bristow@gu.se)
Bastien Queste, University of Gothenburg (bastien.queste@gu.se)
Presentations
03:00 PM
ORGANIC-NOX DRIVEN NITROGEN LOSS AT FRESHWATER SEDIMENT INTERFACES (5512)
Tutorial/Invited: Invited
Primary Presenter: Cameron Callbeck, University of Basel (cameron.callbeck@unibas.ch)
The conventional view of the aquatic nitrogen (N) cycle is centered around organic nitrogen (R-NH2) remineralization to ammonium. Remineralized ammonium is subsequently oxidized (via nitrification) to inorganic NOx providing the oxidized nitrogen needed to support nitrogen removal by denitrification and/or anammox. However, organic N can also exist in an oxidized state (R-NOx), whereby its remineralization can directly generate inorganic NOx – bypassing the nitrification branch of the nitrogen cycle. While turnover of organic NOx is thought to be widespread in nature, the extent to which this oxidized organic N pool is utilized by microorganisms involved in the nitrogen cycle remains unconstrained. We report that denitrifying microbes in freshwater sediments are capable of converting organic NOx in the form of simple nitro-substituted alkanes to N2 gas, contributing to the removal of fixed N. Our estimates based on the turnover of laboratory-synthesized 15N-labelled organic NOx indicated that organic NOx driven denitrification (ONDD) can match rates of canonical denitrification, fueled by the sediment nitrate flux. In the studied sediments, uncultured microbes within the order Burkholderiales likely contributed to ONDD. These bacteria carry not only the conventional denitrification enzymes, but also the nitronate monooxygenase (nmo) – a key enzyme that catalyzes the release of nitrate/nitrite from organic NOx using oxygen. We propose that organic NOx and its transformation represent a new facet of the lacustrine nitrogen cycle.
03:15 PM
Impact of oxygen depletion on the biogeochemistry of benthic habitats in Mediterranean lagoons with contrasted eutrophication status (6451)
Primary Presenter: Julie Régis, Université de Nîmes (julie.regis@unimes.fr)
Mediterranean lagoons have been historically impacted by anthropogenic pressures. After more than two decades of environmental restoration efforts, most of these Mediterranean ecosystems showed an uneven recovery dynamic with respect to eutrophication, water chemical quality and are still impacted by seasonal or permanent deoxygenation. These observations are often attributed to the sediment that is acting as a source of legacy nutrients and pollutants, especially during oxygen depletion events, which can sustain eutrophication and water quality degradation. Depending on their eutrophication status, benthic habitats of Mediterranean lagoons can evolve from bare non-vegetated sediment to ecosystems dominated by opportunistic macroalgae, seagrass beds and perennial macroalgae. Their presence and resilience to deoxygenation events are expected to alter theoretical biogeochemical cycles, which are currently poorly studied. In a context of increasing deoxygenation favored by climate change, they are suspected to play an emerging and increasing role in the release of nutrients and pollutants from sediments that could delay the restoration of lagoon ecosystems. Four typical benthic habitats representing contrasted eutrophication status in Mediterranean lagoons (bare sediment, sediment with Chaetomorpha sp., sediment with Halopythis sp., and sediment with Zostera noltei) were exposed to long-term confinement using benthic chambers. Biogeochemical responses of these habitats were monitored for O2 demand, nutrients (NH4+, PO43-), indicators of early diagenesis processes (Mn, Fe, sulfides) and trace elements, using water samples and sediment cores analysis. Benthic fluxes of oxygen and nutrients were the highest in habitats with Chaetomorpha sp. and Halopythis sp., macrophytes representing early stages in the recovery trajectory. When the oxygen demand exceeded its production, anoxia occurred within 1-2 days. This led to higher nutrient remobilization, their concentration in the water column then exceeded the threshold of "Bad" eutrophication status within a few days. Metal-oxide reduction in the surface sediment occurred under hypoxic conditions while sulfate reduction occurred under anoxic conditions. Significant releases of As, Sb, and Co were also observed under conditions of strong reduction and low pH in the presence of sulfides whereas precipitation and co-precipitation of sulfide with Fe, Co and Mo occurred under anoxic conditions. In bare sediments, punctual hypoxia was measured during nights with benthic fluxes of nutrients and trace elements from 2 to 100 times lower. In Zostera meadow, the O2 consumption was not strong enough with respect to its production to induce deoxygenation events, indicating the high resilience of such habitats with respect to deoxygenation in these Mediterranean lagoons. This work documents how the eutrophication status in Mediterranean lagoons and their corresponding benthic habitats may modify the fate of oxygen, nutrients and trace elements in these coastal areas.
03:30 PM
A niche for cable bacteria in anoxic sediments along the continental margin of Mexico (Soledad Basin) (6613)
Primary Presenter: Caroline Slomp, Utrecht University (c.p.slomp@uu.nl)
Since the discovery of cable bacteria more than a decade ago, these sulfur-oxidizers have been found in sediments in a range of marine environments. However, we are still far from understanding the environmental relevance of cable bacteria in marine settings. In this study, we assess the abundance and activity of cable bacteria in biogeochemically well-characterized sediments from five hypoxic and anoxic basins along the continental margin of California and Mexico. Results of Fluorescence in-situ hybridization (FISH) reveal low numbers of cable bacteria in sediments of the hypoxic and anoxic San Clemente, Catalina, San Pedro and San Blas basins. In the anoxic Soledad Basin, in contrast, we find abundant cable bacteria near the sediment surface. Microprofiling of oxygen, sulfide, pH and electric potential reveal a lack of cable bacteria activity at the sites at the time of sampling. In our presentation, we will discuss the environmental factors that may contribute to the more abundant presence of cable bacteria in Soledad Basin when compared to the other basins. We also will specifically highlight the potential role of temporal variations in the supply of electron donors and acceptors near the sediment-water interface in creating a niche for cable bacteria in Soledad Basin.
03:45 PM
THE COMPLEX INTERPLAY BETWEEN NITROGEN FORMS, ORGANIC MATTER, AND BACTERIAL ABUNDANCES DURING HYPOXIA IN AN URBAN ESTUARY (5436)
Primary Presenter: Georgie Humphries, Advanced Science Research Center, NY (georgia.humphries42@qmail.cuny.edu)
Nitrogen (N) inputs to developed coastlines are linked with multiple ecosystem and socio-economic impacts worldwide such as algal blooms, habitat/resource deterioration, and hypoxia. This study investigated the microbial and biogeochemical processes associated with recurrent, seasonal bottom-water hypoxia in an urban estuary, western Long Island Sound, that receives high N inputs. A field study spanned two hypoxia events, entailing surface and bottom depth water sampling for dissolved inorganic and organic forms of N and phosphorus, organic carbon (DOC), as well as chlorophyll a, bacterial abundances, and physical water quality. Results showed that dissolved organic matter was highest at the most hypoxic locations, with DOC significantly and negatively correlated with bottom-water dissolved oxygen, indicating an increase in bacterial respiration. Ammonia-N was the dominant DIN form pre-hypoxia and declined throughout hypoxia while nitrate + nitrite depicted the reverse, being minimal pre- then increasing during and post-hypoxia. This indicates that ammonia oxidation, a key pathway in water column nitrification, contributed to the switch in dominant DIN forms. At the most hypoxic location, bottom depth bacterial concentrations peaked pre-hypoxia, declined throughout, and were significantly and positively correlated with ammonia-N, confirming that hypoxia influences bacterially induced N-cycling within LIS. These findings provide novel insight to feedbacks between major biogeochemical (N and C) cycles and has implications for N management in hypoxic urban estuaries.
04:00 PM
AEROBIC RESPIRATION KINETICS IN THE OXYCLINE OF THE STRATIFIED MARIAGER FJORD (7006)
Primary Presenter: Irene Ramirez Hernandez, University of Cadiz (irene.ramirez@gm.uca.es)
The loss of oxygen in the oceans is considered one of the most important threats currently occurring in marine ecosystems. The occurrence of hypoxic and anoxic events in coastal waters are becoming increasingly frequent. Some basins and fjords have permanently or periodically anoxic waters, making them excellent locations for studies of pelagic oxic anoxic interfaces. The study of respiratory kinetics provides insight into the adaptation of the microbial community to decreasing levels of O2. Prokaryotes can use different types of terminal oxidases with a broad range of affinities for O2 during aerobic respiration thus modulating their affinity for O2. Here we explore the respiratory kinetics in natural communities in the seasonally anoxic water column of the Mariager Fjord, Denmark. We measured O2 consumption and CO2 production rates of water samples collected at different depths in the oxycline while exposing them to a wide range of O2 concentrations from full saturation to nanomolar levels. Maximum respiration rates ranged from 254 to 355 nM O2/h, being dependent of the temperature (from 14 to 17 deg). The half saturation constant (Km) decreased from values of 139 nM in oxic waters to 76 nM in the hypoxic layer, suggesting a dominant contribution of low affinity terminal oxidases in fully oxic waters and the adaptation of the community in low O2 waters with increasing use of high affinity terminal oxidases. Our data show the adaptation of the microbial community in water layers with low O2 levels, decreasing the apparent Km for increasing the respiration efficiency.
04:15 PM
THE COUPLING OF NITROGEN AND SULFUR CYCLING IN A SEASONALLY ANOXIC COASTAL BASIN (6539)
Primary Presenter: Bo Thamdrup, University of Southern Denmark (bot@biology.sdu.dk)
As an extreme consequence of declining oxygen concentrations in marine systems, coastal anoxic events characterized by an accumulation of toxic hydrogen sulfide (H2S) have increased in occurrence over the past decades. H2S is thought to mainly originate from the sediment and is generally contained within the anoxic water mass by microbial sulfide oxidation processes at the oxic-anoxic interface. While oxygen is an important oxidant here, recent studies point to the additional involvement of nitrate and nitrite. Aiming at a deeper understanding of pathways, interactions, and controls, we studied nitrogen and sulfur cycling in the seasonally anoxic Mariager Fjord, Denmark. H2S accumulated from the oxic-anoxic interface at ~14 m to reach ~400 µM near the bottom at 26 m depth. Sulfate reduction was detected at rates of 0.02 – 0.12 µM d-1 through the anoxic water column, corresponding to a depth-integrated rate of 0.6 mmol m-2 d-1 or approx. 25% of the estimated flux of H2S to the oxic-anoxic interface. Thus, while rarely quantified, pelagic sulfate reduction appears to contribute substantially to H2S accumulation in this system. Anoxic incubations with 15N-labeled nitrate demonstrated a high potential for nitrogen dependent sulfide oxidation with ~1st-order dependence of nitrate reduction rates on H2S concentrations up to 50 µM. N2 was the main product of nitrite reduction, thus emphasizing H2S as the primary driver of fixed nitrogen removal at the oxic-anoxic interface. Indeed, pelagic nitrogen removal rates may exceed benthic rates in the sediments surrounding the basin.
SS107A Oxic-Anoxic Interfaces: Pathways, Dynamics and Exchanges
Description
Time: 3:00 PM
Date: 5/6/2023
Room: Sala Portixol 2