Microbial N2O consumption in and above marine N2O production hotspots

The ocean is a net source of N2O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N2O via microbial N2O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N2O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O2 tolerance, and community composition of N2O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N2O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N2O cycling. Microbes from the oxic layer consume N2O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N2O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O2 and N2O gradients right above the ODZ is a previously ignored potential gatekeeper of N2O and should be accounted for in the marine N2O budget.

Microbial sulfate reduction and organic sulfur formation in sinking marine particles

Climate change is driving an expansion of marine oxygen-deficient zones, which may alter the global cycles of carbon, sulfur, nitrogen, and trace metals. Currently, however, we lack a full mechanistic understanding of how oxygen deficiency affects organic carbon cycling and burial. Here, we show that cryptic microbial sulfate reduction occurs in sinking particles from the eastern tropical North Pacific oxygen-deficient zone and that some microbially produced sulfide reacts rapidly to form organic sulfur that is resistant to acid hydrolysis. Particle-hosted sulfurization could enhance carbon preservation in sediments underlying oxygen-deficient water columns and serve as a stabilizing feedback between expanding anoxic zones and atmospheric carbon dioxide. A similar mechanism may help explain more-extreme instances of organic carbon preservation associated with marine anoxia in Earth history.

A Green Sulfur Bacterium From Epsomitic Hot Lake, Washington, USA

Hot Lake is a small heliothermal and hypersaline lake in far north-central Washington State (USA) and is limnologically unusual because MgSO4 rather than NaCl is the dominant salt. In late summer, the Hot Lake metalimnion becomes distinctly green from blooms of planktonic phototrophs. In a study undertaken over 60 years ago, these blooms were predicted to include green sulfur bacteria but no cultures were obtained. We sampled Hot Lake and established enrichment cultures for phototrophic sulfur bacteria in MgSO4-rich sulfidic media. Most enrichments turned green or red within two weeks, and from green-colored enrichments, pure cultures of a lobed green sulfur bacterium (Phylum Chlorobi) were isolated. Phylogenetic analyses showed the organism to be a species of the prosthecate green sulfur bacterium Prosthecochloris. Cultures of this Hot Lake phototroph were halophilic and tolerated high levels of sulfide and MgSO4. In addition, unlike all recognized species of Prosthecochloris, the Hot Lake isolates grew at temperatures up to 45°C, indicating an adaptation to the warm summer temperatures of the lake. Photoautotrophy by Hot Lake green sulfur bacteria may contribute dissolved organic matter to anoxic zones of the lake, and their diazotrophic capacity may provide a key source of bioavailable nitrogen, as well.

Nested control loop configuration for a three stage biological wastewater treatment process

In every urban infrastructure, Wastewater Treatment Plant (WWTP) requires special attention because of its adverse effects on the environment and also for resource recovery. Therefore, there arises a need to treat the wastewater in order to meet the effluent norms prior to discharge. Different control strategies and various scenarios of plant layout can be tested and evaluated through modelling and simulation studies on the benchmark layouts. In this paper, a feedforward nested loop control structure based on ammonia concentration is implemented on Benchmark Simulation Model (BSM1-P) developed based on Activated Sludge Model No. 3 bioP (ASM3bioP) for controlling the dissolved oxygen in aerobic zones and nitrate level in anoxic zones and nutrient removal by adding two anaerobic zones. By using this control strategy, pumping energy, percentage violations of ammonia and nitrogen concentrations in the effluent, and effluent quality are reduced effectively.

Impact of Cold Temperatures on Nitrogen Removal in Denitrifying Down-Flow Hanging Sponge (DDHS) Reactors

Innovative and low-energy solutions for the removal of nitrogen from domestic wastewater are needed to achieve regulatory ambitions. However, there is a lack of appropriate technologies for use in non-centralised applications, where receiving waterbodies also are potentially sensitive. Denitrifying down-flow hanging sponge (DDHS) reactors are a promising solution but their performance has not been assessed under colder operating conditions pertinent to northern climates. Two DDHS reactor configurations (short and tall anoxic zones) were tested under “typical” UK winter, summer, and spring/autumn temperatures. At 22 °C, both reactors achieved >58% total nitrogen (TN) removal from domestic wastewater with no significant differences in removal rates between configurations. However, denitrification was lost at 13 °C in the reactor with the short anoxic zone, and was lost totally in both systems at 6 °C. Efficient nitrification was retained at 6 °C in both reactors (>90% removal NH4–N), suggesting that while elevated TN removal was not retained under colder conditions, the DDHS systems still effectively removed ammonia under UK winter conditions. DDHS reactors show promise for use under colder temperature conditions, although optimisation is needed, including the derivation of temperature correction factors for nitrogen removal.

Nitrous oxide emissions from aerobic granular sludge

The emissions of climate-relevant nitrous oxides from wastewater treatment with aerobic granular sludge (AGS) are of special interest due to considerable structural as well as microbiological differences compared with flocculent sludge. Due to the compact and large structures, AGS is characterised by the formation of zones with different dissolved oxygen (DO) and substrate gradients, which allows simultaneous nitrification and denitrification (SND). N2O emissions from AGS were investigated using laboratory-scale SBR fed with municipal wastewater. Special attention was paid to the effects of different organic loading rates (OLR) and aeration strategies. Emission factors (EF) were in a range of 0.54% to 4.8% (gN2O/gNH4-Nox.) under constant aerobic conditions during the aerated phase and different OLR. Higher OLR and SND were found to increase the N2O emissions. A comparative measurement of two similarly operated SBR with AGS showed that the reactor operated under constant aerobic conditions (DO of 2 mg L−1) emitted more N2O than the SBR with an alternating aeration strategy. Total nitrogen (TN) removal was significantly higher with the alternating aeration since non-aerated periods lead to increased anoxic zones inside the granules. The constant aerobic operation was found to promote the accumulation of NO2-N, which could explain the differences in the N2O levels.

Microbial ecosystem dynamics drive fluctuating nitrogen loss in marine anoxic zones

The dynamics of nitrogen (N) loss in the ocean’s oxygen-deficient zones (ODZs) are thought to be driven by climate impacts on ocean circulation and biological productivity. Here we analyze a data-constrained model of the microbial ecosystem in an ODZ and find that species interactions drive fluctuations in local- and regional-scale rates of N loss, even in the absence of climate variability. By consuming O2to nanomolar levels, aerobic nitrifying microbes cede their competitive advantage for scarce forms of N to anaerobic denitrifying bacteria. Because anaerobes cannot sustain their own low-O2niche, the physical O2supply restores competitive advantage to aerobic populations, resetting the cycle. The resulting ecosystem oscillations induce a unique geochemical signature within the ODZ—short-lived spikes of ammonium that are found in measured profiles. The microbial ecosystem dynamics also give rise to variable ratios of anammox to heterotrophic denitrification, providing a mechanism for the unexplained variability of these pathways observed in the ocean.