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
05:00 PM
Dynamic coexistence of aerobic and anaerobic metabolisms in anoxic zones (7052)
Tutorial/Invited: Invited
Primary Presenter: Emily Zakem, Carnegie Institution for Science (ezakem@carnegiescience.edu)
The activities of microorganisms in and around anoxic zones control the losses of bioavailable nitrogen and emissions of nitrous oxide. Accurate quantification of these activities is critical for understanding microbial feedbacks to current climate change. Theory and observations suggest that aerobic and anaerobic metabolisms can coexist in anoxic zones when some oxygen is supplied, even though oxygen concentrations may remain undetectable. This complicates traditional parameterizations in models where the presence or absence of a metabolism is dictated by oxygen concentration alone. I will use theory and modeling to show how obligately aerobic nitrite-oxidizing bacteria (NOB) can subsist alongside aerobic heterotrophy in anoxic zones when there is a variable supply of oxygen. Genome-based evidence supports the model implication that NOB can be considered as ``opportunists” that thrive in anoxic zones with infrequent oxygen intrusions. Results suggest that resolution of time-varying circulation is necessary for predicting nitrogen cycling in anoxic zones.
05:15 PM
Fuelling of aerobic processes under apparent anoxic conditions: nitrite oxidation in the Chlorophyll Secondary Maxima of the Oxygen Minimum Zones. (4937)
Primary Presenter: Emilio Garcia Robledo, Universidad de Cadiz (emilio.garcia@uca.es)
In the oceanic Oxygen Minimum Zones, the widespread picocyanobacterium Prochlorococcus forms a secondary chlorophyll maximum (SCM) below the oxycline, in waters without detectable O2. As a result of oxygenic photosynthesis, traces of oxygen released in this layer can support activity of aerobic microbial processes in apparent anoxia. Beside organotrophic aerobic respiration, other aerobic processes such as nitrite oxidation might use the released O2, altering the otherwise anaerobic element cycling in this layer below the oxycline. During two cruises in the Pacific OMZs, water collected in the SCM was incubated simulating the dim bluish light and trace O2 levels found in situ. Net O2 metabolism was measured using high-resolution O2 optodes and rates of incorporation of C isotopes. Isotopically labelled 15N-nitrite was also added to the incubations to measure its oxidation and reduction pathways. Our results indicate that a substantial fraction of the O2 produced by photosynthesis in the SCM was used for the oxidation of nitrite to nitrate, and could even account for the total net O2 consumption during some incubations. Nitrite oxidation in the SCM had a high affinity for O2, revealing half-saturation constants of only a few tens of nanomolar for O2, consistent with the trace levels of O2 found in situ. Nitrite oxidation was detected even under apparently anoxic conditions, suggesting an anaerobic pathway of nitrite oxidation. Regardless of O2 concentration, oxidation was the dominant nitrite transformation pathway, highlighting the role of the SCM in N retention in the OMZs
05:30 PM
TRACE OXYGEN STIMULATES NITROGEN REDUCTION AND SHIFTS NITROGEN METABOLISM TOWARDS RECYCLING THROUGH DNRA IN LOW-OXYGEN MARINE WATERS (7500)
Primary Presenter: Julia Huggins, University of British Columbia (julia.a.huggins@gmail.com)
The oceans are currently losing O2 with many unconstrained impacts and feedbacks on marine biology, biogeochemical cycles, and climate. As O2 concentrations decline, microorganisms transition to several possible NOx--based metabolisms: denitrification, anammox, and dissimilatory nitrite reduction to ammonium (DNRA), leading to either N-loss or N-retention, depending on pathway partitioning. This has important implications for ocean nutrient status and feedbacks on climate, but it remains unclear how these pathways are regulated in the transition to anoxia. Here, we use incubations of seawater collected from a model anoxic marine environment (Saanich Inlet, BC) with stable nitrogen isotope labeling (15N) to partition rates and pathways of microbial NOx--reduction in response to O2 dynamics. By measuring rates as a monthly time series, we show that denitrification and anammox remain relatively consistent, whereas DNRA occurs in intermittent periods of extremely high activity following mixing events and is, overall, the dominant pathway of NOx--reduction. Additionally, by manipulating O2 concentrations between 0.1-10 µM, we show that both DNRA and denitrification are unexpectedly stimulated by the addition of trace O2. The three pathways have different thresholds and response patterns across this O2 gradient, revealing interactions between competing pathways that could be otherwise overlooked. These findings challenge assumptions about the relationships between aerobic and anaerobic metabolisms and improve our capacity to predict microbially-driven responses to deoxygenation.
05:45 PM
DISTRIBUTION AND KINETICS OF NITROUS OXIDE CONSUMPTION IN A SEASONALLY ANOXIC FJORD (6941)
Primary Presenter: Laura Bristow, Univeristy of Gothenburg (laura.bristow@gu.se)
The ocean is a major source of atmospheric nitrous oxide (N2O), and oxygen (O2)-depleted marine waters are hotspots of N2O accumulation and emission. In such systems, N2O is both a product of and a substrate for microbial metabolism, but we lack understanding of the pathways involved and their environmental controls. In the seasonally anoxic Saanich Inlet, British Columbia, we explored N2O consumption in incubations of water with 15N-labelled N2O, and examined the kinetics of the process by manipulation of N2O, O2, and hydrogen sulfide (H2S) concentrations. N2O consumption was not detectable in oxycline waters but increased steeply below the oxic-anoxic interface along with the accumulation of H2S. Consistent with this distribution, the process was highly sensitive to O2, with 50% inhibition at ~150 nM O2 added. N2O consumption exhibited Michaelis-Menten kinetics with respect to N2O with apparent Km values of 46 – 92 nM, and was stimulated by low amounts of H2S (≤ 5 µM) while higher H2S concentrations were inhibitory, showing higher sensitivity closer to the oxic-anoxic interface than deeper in the anoxic zone. Our results imply that N2O consumption is to a wide extent coupled to the oxidation of H2S and that the N2O-consuming community is adapted to life near the oxic-anoxic interface, where opposing gradients of N2O and H2S intersect. Still, organisms deeper in the sulfidic zone retain a high capacity for N2O consumption, that may potentially be exploited during mixing events. The anoxic waters thus act as a sink for N2O despite the inhibitory effect of H2S.
06:00 PM
COMPARING THE MICROORGANISMS CAPABLE OF USING ALTERNATIVE REDUCED NITROGEN SOURCES BETWEEN OXIC AND ANOXIC OCEAN REGIONS (6217)
Primary Presenter: Paulina Alejandra Huanca Valenzuela, UMCES (phuanca@umces.edu)
Ammonium is the preferred reduced nitrogen form many microbes use for assimilation and growth, but is often absent from offshore waters. Microorganisms can metabolize alternative organic reduced nitrogen forms if they possess genes encoding for cyanase (cynS), or urease (ureC), which catalyze the decomposition of cyanate and urea respectively. Little is known about which microbes contain these genes in the environment. We quantified the microbes that can use cyanate and/or urea in oxic and anoxic environments by using a phylogenetic read placement technique with depth profiles of metagenomes from two Pacific Ocean regions: an oxic region represented by the nutrient limited Hawaii Ocean Time series, and two Oxygen Deficient Zone (ODZ) environments represented by the Eastern Tropical South and the North Pacific. Ammonia-oxidizing Thaumarchaeota all had the ability to use urea in deep oxic waters. Contrastingly, ~40% of heterotrophic SAR11 bacteria had the ureC gene in surface water where nitrate was undetectable, but none did in deep waters. In ODZs, N2 producing anammox bacteria all contained genes to use cyanate, but 50-70% had genes to use urea. Contrastingly, all Thioglobaceae had the ability of using urea in the ODZ, but few did in oxic waters. Nitrite-oxidizing Nitrospina all had genes to use urea with ~35% having genes to use cyanate in the lower euphotic zone, but not below. This structuring of who can utilize which reduced nitrogen form could both reflect competition between microbes and substrate availability.
06:15 PM
Novel bacterial and fungal lineages link carbon, nitrogen, and sulfur cycling in an oxygen deficient zone (5550)
Primary Presenter: Xuefeng Peng, University of South Carolina (xpeng@seoe.sc.edu)
Both prokaryotes and microbial eukaryotes are key drivers of biogeochemical processes in oxygen deficient zones (ODZs). We combined genome-resolved metagenomics and metatranscriptomics to study the diversity, functions, activities, and potential interactions of prokaryotes and fungi in the eastern tropical North Pacific. Among the 426 high-quality metagenome-assembled genomes (MAGs) reconstructed, we identified novel bacterial lineages interfacing carbon (C), nitrogen (N), and sulfur (S) cycling. The highest level of nitric oxide (NO) reductase expression was found in a Rhodospirillales MAG, which expressed a complete dissimilatory pathway of sulfate reduction to sulfide at the secondary nitrite maximum. The highest level of nitrous oxide (N2O) reductase expression was found in a Poribacteria MAG, of which the abundance and activity peaked at the oxic-anoxic interface and showed a positive correlation with N2O reduction rates. This Poribacteria MAG lacks genetic capacity to reduce NO but it expressed the complete pathway for dissimilatory nitrate reduction to ammonia (DNRA), potentially fueling anaerobic ammonia oxidation (anammox) at the top of the anoxic layer. The eukaryotic metatranscriptomes revealed that 1/3 of the fungal activities were from early diverging fungi. Expressed even at anoxic depths, Dikarya GH7 was the dominant fungal hydrolytic enzyme and potentially fueled the growth of bacteria involved in nitrogen and sulfur cycling. These findings advance our understanding of the role of bacteria and fungi in linking C, N, and S cycling in ODZs.
SS107B Oxic-Anoxic Interfaces: Pathways, Dynamics and Exchanges
Description
Time: 5:00 PM
Date: 5/6/2023
Room: Sala Portixol 2