Oxic Surface Layer Research Articles

Upwelling-driven high organic production in the late Ediacaran

The Ediacaran ocean (635-541 million years ago) is typically viewed as a stagnant, redox-stratified ocean with low phosphorus availability. However, this contradicts the high total organic carbon (up to 15 wt%) recorded in the upper Ediacaran Doushantuo Formation in South China. Here, we present a quantitative reconstruction of primary productivity and sediment total organic carbon using basin-scale simulations based on various redox-nutrient scenarios. Our results indicate that oceanic upwelling was the primary driver of enhanced primary productivity, with estimates over 2.5 times the modern ocean’s average, providing quantitative evidence for oceanic circulation in the Ediacaran Nanhua Basin. Our results also indicate the development of spatially linked but opposing redox trends, i.e., a ‘redox anticouple’, whereby the expansion of the oxic surface layer occurred concurrently with intensification of deep-water anoxia due to enhanced PP in surface and its consumption of oxidants in underlying waters. This model may explain the conflicting redox records across the Nanhua Basin. Our findings explain the unusually high rates of organic matter production and preservation in the low-pO2 Ediacaran ocean and provide a mechanism for its progressive oxygenation.

Ironstone as a proxy of Paleozoic ocean oxygenation

Marine ironstone is a Phanerozoic biochemical sedimentary rock that contains abundant primary iron. Although rare, ironstone is conspicuous in the Paleozoic sedimentary record. Its iron source remains contentious, with traditional models invoking a continentally derived source. Increasing sedimentologic evidence suggests that many Paleozoic ironstones formed along favourably oriented continental margins where coastal upwelling delivered ferruginous waters, with the postulated source of iron being deep-ocean hydrothermal fluids. Thus, deep-water redox conditions are potentially a critical control on the concentration of iron in early Phanerozoic seawater and the accumulation of ironstone. We report novel in situ strontium isotopic and europium anomaly data from five geographically and temporally distinct Ordovician-Silurian ironstones from the Rheic and Iapetus oceans to test the emerging ferruginous upwelling model. These data provide robust evidence that ironstone formed from marine fluids with a component of mid-ocean ridge hydrothermal waters that remained largely unmixed during transport to the locations of ironstone accumulation. These results imply that the deep oceans must have been oxygen deficient to allow local or regional enrichment in hydrothermal iron. Such conditions are interpreted to have been associated with a spatiotemporally variable redox-stratified global ocean with anoxia beneath an oxic surface layer. Thus, sedimentologic and geochemical data presented herein agree with other paleoredox proxies that suggest rising oxygen levels in the Neoproterozoic gave way to early Paleozoic oceans with dynamic redox conditions. The temporal record of Paleozoic ironstone suggests redox stratification, poor deep-ocean ventilation, and upwelling-related ironstone genesis persisted until the Devonian but not into the Carboniferous.

Diurnal changes in bacterial communities in oxic surface and hypoxic middle seawater layers of the Changjiang River Estuary

Diurnal changes in bacterial communities in oxic surface and hypoxic middle seawater layers of the Changjiang River Estuary

Potential of high pH and reduced sulfur for arsenic mobilization – Insights from a Finnish peatland treating mining waste water

Peatlands, used for purification of mining waste waters, have shown efficient solid-phase sequestration of contaminants such as arsenic (As). However, contaminant re-mobilization may occur related to management changes or chemical alteration of original peatland conditions. For a treatment peatland in Finnish Lapland, we here confirm efficient As retention in near-surface peat layers close to the mining waste water inflow, likely due to binding to FeIII-phases. Seven years into operation of the treatment peatland, there appears to be further retention potential, as large areas downstream still had solid-phase As concentrations at background levels. However, via depth-resolved pore water analysis we observed a hotspot 170 m from the inflow at 10–50 cm depth, where As pore water concentrations exceeded input concentrations by a factor of 20, indicating substantial As re-mobilization. At the same spot, a peak of reduced sulfur (S) species was found. Arsenic species detected were arsenite and up to 26% methylated oxyarsenates, 15% methylated and 7.9% inorganic thioarsenates. We postulate that As mobilization is a result of short-term re-equilibration to a changed inflow chemistry after installation of a process water treatment plant and a long-term consequence of changing pore water pH from acidic to near-neutral, releasing reduced S and As. We infer that the co-occurrence of reduced S and As leads to formation of methylated and/or thiolated As species with known low sorption affinity, thereby further enhancing As mobility. Laboratory incubation studies with two peat cores confirmed a high S-induced As mobilization potential, especially when As-Fe-rich, oxic surface layers were incubated anoxically at near-neutral pH. Highest risk of As re-mobilization from this treatment peatland is expected in a scenario in which mining waste water inflow has stopped but the peatland remains flooded, and near-surface layers transition from oxic to anoxic conditions.

Contribution of plant-induced pressurized flow to CH4 emission from a Phragmites fen

The widespread wetland species Phragmites australis (Cav.) Trin. ex Steud. has the ability to transport gases through its stems via a pressurized flow. This results in a high oxygen (O2) transport to the rhizosphere, suppressing methane (CH4) production and stimulating CH4 oxidation. Simultaneously CH4 is transported in the opposite direction to the atmosphere, bypassing the oxic surface layer. This raises the question how this plant-mediated gas transport in Phragmites affects the net CH4 emission. A field experiment was set-up in a Phragmites-dominated fen in Germany, to determine the contribution of all three gas transport pathways (plant-mediated, diffusive and ebullition) during the growth stage of Phragmites from intact vegetation (control), from clipped stems (CR) to exclude the pressurized flow, and from clipped and sealed stems (CSR) to exclude any plant-transport. Clipping resulted in a 60% reduced diffusive + plant-mediated flux (control: 517, CR: 217, CSR: 279 mg CH4 m−2 day−1). Simultaneously, ebullition strongly increased by a factor of 7–13 (control: 10, CR: 71, CSR: 126 mg CH4 m−2 day−1). This increase of ebullition did, however, not compensate for the exclusion of pressurized flow. Total CH4 emission from the control was 2.3 and 1.3 times higher than from CR and CSR respectively, demonstrating the significant role of pressurized gas transport in Phragmites-stands.

Methylicorpusculum oleiharenae gen. nov., sp. nov., an aerobic methanotroph isolated from an oil sands tailings pond.

An aerobic methane oxidizing bacterium, designated XLMV4T, was isolated from the oxic surface layer of an oil sands tailings pond in Alberta, Canada. Strain XLMV4T is capable of growth on methane and methanol as energy sources. NH4Cl and sodium nitrate are nitrogen sources. Cells are Gram-negative, beige to yellow-pigmented, motile (via a single polar flagellum), short rods 2.0-3.3 µm in length and 1.0-1.6 µm in width. A thick capsule is produced. Surface glycoprotein or cup shape proteins typical of the genera Methylococcus, Methylothermus and Methylomicrobium were not observed. Major isoprenoid quinones are Q-8 and Q-7 at an approximate molar ratio of 71 : 22. Major polar lipids are phosphoglycerol and ornithine lipids. Major fatty acids are C16 : 1 ω8+C16 : 1 ω7 (34 %), C16 : 1 ω5 (16 %), and C18 : 1 ω7 (11 %). Optimum growth is observed at pH 8.0 and 25 °C. The DNA G+C content based on a draft genome sequence is 46.7 mol%. Phylogenetic analysis of 16S rRNA genes and a larger set of conserved genes place strain XLMV4T within the class Gammaproteobacteria and family Methylococcaceae, most closely related to members of the genera Methylomicrobium and Methylobacter (95.0-97.1 % 16S rRNA gene sequence identity). In silico genomic predictions of DNA-DNA hybridization values of strain XLMV4T to the nearest phylogenetic neighbours were all below 26 %. On the basis of the data presented, strain XLMV4T is considered to represent a new genus and species for which the name Methylicorpusculum oleiharenae is proposed. Strain XLMV4T (=DSMZ DSM 27269=ATCC TSD-186) is the type strain.

Impact of euxinic holomictic conditions on prokaryotic assemblages in a marine meromictic lake

The water column of marine Rogoznica Lake (Croatia) is stratified throughout the year. However, on rare occasions, the lake exhibits complete mixing, primarily driven by meteorological conditions. On such occasions, anoxic, sulfide-rich bottom water mixes with the oxic surface layer, triggering both the oxidation of sulfide predominantly to colloidal sulfur and the establishment of euxinic conditions throughout the water column. We investigated prokaryotic communities during euxinic holomictic conditions. High-throughput sequencing of the 16S rRNA gene showed that these sulfur-driven communities were dominated by Planctomycetales, chemolithotrophic Epsilonbactereota (mainly Arcobacteraceae) and Gammaproteobacteria (Thioglobaceae), followed by Alpha- (Rhodobacteraceae) and Deltaproteobacteria (Desulfobulbaceae and Desulfobacteraceae). The majority of archaeal sequences belonged to Nanoarchaeota, all of which were designated as Woesearchaeia. This class has not been recorded in other meromictic lakes. Both bacterial and archaeal 16S rRNA gene abundance did not vary significantly across the vertical profile of the lake, as shown by quantitative PCR. However, hierarchical clustering showed a significant difference between the microbial assemblage structure in stratified periods and during a holomictic event. Besides the structure, this stressful event had a manifold effect on bacterial and archaeal assemblages by decreasing their diversity and abundance when compared to the usual stratified conditions.

Evidence for Highly Complex Redox Conditions and Strong Water‐Column Stratification in an Early Cambrian Continental‐Margin Sea

AbstractAlthough oxic surface, ferruginous deep, and euxinic intermediate waters have been documented, the redox heterogeneity of early Cambrian oceans remains largely unclear, precluding our understanding of the relationship between marine redox evolution and early animal diversification. In this study, we analyzed iron species, redox‐sensitive trace elements, and S‐N‐C isotopes of deepwater black shales in the Silikou section in the continental‐margin Nanhua Basin (South China), which represent extensive clastic input mainly from the Cathaysia Block during the early Cambrian (~541–509 Ma). Integrated data reveal a continuous shift in bottom‐water redox conditions at Silikou from euxinic in the lowermost black‐shale interval (BS1, 0–37.4 m) to oxic in the uppermost black‐shale interval (BS4, 296–372.9 m) accompanying a progressive movement of the O2/H2S redoxcline into the sediment. In between, most importantly, the BS2 interval (91–154 m) accumulated under manganous‐ferruginous conditions (i.e., Fe‐Mn reduction zone) characterized by an active Fe‐Mn particulate shuttle, and the BS3 interval (204–260 m) under nitrogenous conditions (i.e., nitrate reduction zone) characterized by strong denitrification processes, suggesting highly complex redox conditions in early Cambrian oceans. The observed sequence of redox conditions supports a strong lateral and depth‐related redox stratification in the early Cambrian ocean, including (from surface to deep and from nearshore to offshore) oxic, nitrate reduction, Fe‐Mn reduction, and euxinic zones, which were sequentially recorded at Silikou likely due to secular expansion of the oxic surface layer and/or long‐term relative sea level fall. Our study highlights the need for continued paleo‐redox studies to explore the redox heterogeneity of early Cambrian oceans.

The unique preservation of Sepia soft tissues in the Miocene deposits (Serravalian, Vienna Basin): Implications for the origin of microbodies in the fossil record

For the first time, we report an ancient cuttlefish ink sac possessing microbodies from the lower Serravallian (Middle Miocene) deposits of the Vienna Basin (Central Paratethys). We compare the geochemistry of these microbodies with the ink sac of the extant genus Sepia. The unique preservation of the cuttlefish soft tissue resulted from a high sedimentation rate and a low degree of sediment reworking and bioirrigation, which led to the dominance of anaerobic degradation pathways such as sulfate reduction below a thin oxic surface layer. The Recent Sepia melanin extracted from the ink sac is commonly used as a standard in biochemical analyses. However, it was unclear whether melanosomes can be preserved in fossil cuttlefishes. Although some portion of melanin-bearing microbodies is affected by pyritization, we show that some microbodies are still composed of eumelanin using Raman spectroscopy. They consist of particles corresponding to the size and shape of melanosomes observed in extant Sepia according to images produced with a scanning electron microscope. The Raman microspectroscopy of these microbodies in the Miocene Sepia shows identical chemical composition as in the Sepia officinalis ink melanin standard. Therefore, we confirm that these microbodies represent true fossil melanin pigment and melanosome remnants rather than being produced by microbes.

Hydrogen Dynamics in Cyanobacteria Dominated Microbial Mats Measured by Novel Combined H2/H2S and H2/O2 Microsensors.

Hydrogen may accumulate to micromolar concentrations in cyanobacterial mat communities from various environments, but the governing factors for this accumulation are poorly described. We used newly developed sensors allowing for simultaneous measurement of H2S and H2 or O2 and H2 within the same point to elucidate the interactions between oxygen, sulfate reducing bacteria, and H2 producing microbes. After onset of darkness and subsequent change from oxic to anoxic conditions within the uppermost ∼1 mm of the mat, H2 accumulated to concentrations of up to 40 μmol L-1 in the formerly oxic layer, but with high variability among sites and sampling dates. The immediate onset of H2 production after darkening points to fermentation as the main H2 producing process in this mat. The measured profiles indicate that a gradual disappearance of the H2 peak was mainly due to the activity of sulfate reducing bacteria that invaded the formerly oxic surface layer from below, or persisted in an inactive state in the oxic mat during illumination. The absence of significant H2 consumption in the formerly oxic mat during the first ∼30 min after onset of anoxic conditions indicated absence of active sulfate reducers in this layer during the oxic period. Addition of the methanogenesis inhibitor BES led to increase in H2, indicating that methanogens contributed to the consumption of H2. Both H2 formation and consumption seemed unaffected by the presence/absence of H2S.

Cryptic oxygen cycling in anoxic marine zones

Oxygen availability drives changes in microbial diversity and biogeochemical cycling between the aerobic surface layer and the anaerobic core in nitrite-rich anoxic marine zones (AMZs), which constitute huge oxygen-depleted regions in the tropical oceans. The current paradigm is that primary production and nitrification within the oxic surface layer fuel anaerobic processes in the anoxic core of AMZs, where 30-50% of global marine nitrogen loss takes place. Here we demonstrate that oxygenic photosynthesis in the secondary chlorophyll maximum (SCM) releases significant amounts of O2 to the otherwise anoxic environment. The SCM, commonly found within AMZs, was dominated by the picocyanobacteria Prochlorococcus spp. Free O2 levels in this layer were, however, undetectable by conventional techniques, reflecting a tight coupling between O2 production and consumption by aerobic processes under apparent anoxic conditions. Transcriptomic analysis of the microbial community in the seemingly anoxic SCM revealed the enhanced expression of genes for aerobic processes, such as nitrite oxidation. The rates of gross O2 production and carbon fixation in the SCM were found to be similar to those reported for nitrite oxidation, as well as for anaerobic dissimilatory nitrate reduction and sulfate reduction, suggesting a significant effect of local oxygenic photosynthesis on Pacific AMZ biogeochemical cycling.

Porewater methane transport within the gas vesicles of diurnally migrating Chaoborus spp.: An energetic advantage

Diurnally-migrating Chaoborus spp. reach populations of up to 130,000 individuals m−2 in lakes up to 70 meters deep on all continents except Antarctica. Linked to eutrophication, migrating Chaoborus spp. dwell in the anoxic sediment during daytime and feed in the oxic surface layer at night. Our experiments show that by burrowing into the sediment, Chaoborus spp. utilize the high dissolved gas partial pressure of sediment methane to inflate their tracheal sacs. This mechanism provides a significant energetic advantage that allows the larvae to migrate via passive buoyancy rather than more energy-costly swimming. The Chaoborus spp. larvae, in addition to potentially releasing sediment methane bubbles twice a day by entering and leaving the sediment, also transport porewater methane within their gas vesicles into the water column, resulting in a flux of 0.01–2 mol m−2 yr−1 depending on population density and water depth. Chaoborus spp. emerging annually as flies also result in 0.1–6 mol m−2 yr−1 of carbon export from the system. Finding the tipping point in lake eutrophication enabling this methane-powered migration mechanism is crucial for ultimately reconstructing the geographical expansion of Chaoborus spp., and the corresponding shifts in the lake’s biogeochemistry, carbon cycling and food web structure.

Enhanced nitrogen loss from rivers through coupled nitrification-denitrification caused by suspended sediment

Present-day estimations of global nitrogen loss (N-loss) are underestimated. Commonly, N-loss from rivers is thought to be caused by denitrification only in bed-sediments. However, coupled nitrification-denitrification occurring in overlying water with suspended sediments (SPS) where oxic and anoxic/low oxygen zones may coexist is ignored for N-loss in rivers. Here the Yellow and Yangtze Rivers were taken as examples to investigate the effect of SPS, which exists in many rivers of the world, on N loss through coupled nitrification-denitrification with nitrogen stable (15N) isotopic tracer simulation experiments and in-situ investigation. The results showed even when SPS was surrounded by oxic waters, there were redox conditions that transitioned from an oxic surface layer to anoxic layer near the particle center, enabling coupled nitrification-denitrification to occur around SPS. The production rate of 15N2 from 15NH4+-N (R15N2-production) increased with increasing SPS concentration ([SPS]) as a power function (R15N2-production=a·[SPS]b) for both the SPS-water and bed sediment-SPS-water systems. The power-functional increase of nitrifying and denitrifying bacteria population with [SPS] accounted for the enhanced coupled nitrification-denitrification rate in overlying water. SPS also accelerated denitrification in bed-sediment due to increased NO3− concentration caused by SPS-mediated nitrification. For these two rivers, 1gL−1 SPS will lead to N-loss enhancement by approximately 25–120%, and the enhancement increased with organic carbon content of SPS. Thus, we conclude that SPS in overlying water is a hot spot for nitrogen loss in river systems and current estimates of in-stream N-loss are underestimated without consideration of SPS; this may partially compensate for the current imbalance of global nitrogen inputs and sinks.

Short-term variation in benthic phosphorus transfer due to discontinuous aeration/oxygenation operation

The short-term dynamics of soluble reactive phosphorus (SRP) transport across the sediment surface in a brackish lake due to discontinuous aeration and oxygenation operations were investigated using laboratory and field experimental and analytical procedures. According to a laboratory incubation experiment using intact sediment cores, SRP release from the sediment was clearly suppressed by aeration, and substantial negative SRP transfer was observed during oxygenation treatment, while a positive value was observed for N2 bubbled cores. A remarkable but impermanent increase in SRP release rate was observed within 1 or 2 days of discontinuing the aeration and oxygenation, respectively, and the release rate rapidly deceased to a quasi-steady value under N2 bubbling conditions. An analytical model could quantitatively reproduce these laboratory experimental results for anoxic and aerated conditions, showing that this impermanent increase was attributable to the rapid release of accumulated SRP in the oxic surface layer of the sediment. Field experiments using an in situ oxygenator showed the same tendency as the laboratory experiments, but with much larger values of the benthic SRP transfer rate. Overall, the short-term dynamics of benthic SRP transport caused by discontinuous aeration and oxygenation are considered to be an important process for the phosphorus cycle in the field.

Genesis of Cryogenian Datangpo manganese deposit: Hydrothermal influence and episodic post-glacial ventilation of Nanhua Basin, South China

The Neoproterozoic Datangpo Formation of the Nanhua Basin in South China records interglacial sedimentation between the ~720–660-Ma Sturtian and ~654–635-Ma Marinoan glaciations. The lower part (1st Member) of this formation contains manganese-rich deposits that represent a mixture of two main components, Mn-carbonates and Mn-bearing aluminosilicates (clay minerals). The Mn-carbonate component is characterized by high La/Sc ratios, high initial 87Sr/86Sr ratios, and low initial εNd(t) values, and the siliciclastic component by low La/Sc ratios, low initial 87Sr/86Sr ratios, and high initial εNd(t) values. The likely source of the non-radiogenic siliciclastic material is weathering of Neoproterozoic continental flood basalts on the Yangtze Block. Discriminant plots show that the Mn-ore samples have compositions consistent with at least partial derivation from hydrothermal sources. Mn-carbonate deposition was the result of reactions between aqueous Mn and sedimentary organic matter during early diagenesis that led to elevated Mn2+ concentrations and alkalinity in sediment porewaters. Based on these observations, we propose a new metallogenic model for the Datangpo Formation manganese deposits. During the Sturtian glaciation, the anoxic Nanhua Basin accumulated abundant dissolved Mn, a substantial fraction of which was derived from hydrothermal sources. When glaciation ended and a redox-stratified water column developed in the basin with an oxic surface layer and an anoxic deep layer, the accumulated dissolved Mn precipitated as Mn-oxides on the basin floor during episodic ventilation events. After co-burial with organic-rich sediments, these Mn-oxides were reduced during organic matter oxidation, which led to the formation of secondary Mn-carbonates (rhodochrosite) through increases in sediment porewater Mn2+ and in alkalinity via microbial sulfate reduction and microbially mediated Mn reduction.

RICE UPTAKE AND LEACHING OF 99TC IN DIFFERENT PADDY SOILS CONTAMINATED ACCORDING TO TWO CONTRASTING SCENARIOS

Four different paddy soils collected around the Gyeongju nuclear site were treated with 99TcO4− solution under the assumption of two contrasting contamination scenarios. Scenario I (SN-I) is for a pre-transplanting deposition of 99Tc followed by plowing, whereas SN-II is for its deposition onto the water surface shortly after transplanting. Rice plants were grown in lysimeters in a greenhouse. Plant uptake of 99Tc was quantified with the TFarea (m2·kg−1-dry). The SN-II TFarea values for straws and brown rice, having been generally higher than the SN-I values, were within the ranges of 6.9x10−3~4.1x10−2 and 5.2x10−6~7.3x10−5, respectively. Sorption onto clay seems to have decreased 99Tc uptake in SN-I, whereas it may have had an insignificant effect in SN-II. A phenomenon characteristic of submerged paddy soil, i.e., the development of a thin oxic surface layer may have greatly affected the rice uptake of SN-II 99Tc. The surface-water concentrations of 99Tc were much higher in SN-II than in SN-I. For the percolating water, however, the opposite was generally true. At most 1.3% of the applied 99Tc were leached through such percolation. The use of empirical deposition time-dependent TFarea values was considered desirable in assessing the radiological impact of a growing-season deposition of 99Tc onto paddy fields.

Pyritic event beds and sulfidized Fe (oxyhydr)oxide aggregates in metalliferous black mudstones of the Paleoproterozoic Talvivaara formation, Finland

The Paleoproterozoic, 2.0–1.9 Ga Talvivaara formation of Finland was deposited during the Shunga Event, a worldwide episode of enhanced accumulation of organic-rich sediments in the aftermath of the Lomagundi–Jatuli carbon isotope excursion. Sulfidic carbonaceous mudstones in the Talvivaara formation contain one of the largest known shale-hosted nickel deposits. In order to gain new insight into this Shungian sedimentary environment, sedimentological, petrographical and in situ S and Fe isotopic microanalyses were carried out on samples representing depositional and early-diagenetic conditions. The event-bedded lithology with tidal signatures in the organic-rich mudstones strongly indicates deposition from predominantly river-delivered mud on a highly-productive coastal area, below storm-wave base. The riverine supply of phosphorus, sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water. The stratigraphic upper part of the Talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds. The pyrite beds were deposited by seaward excursions of the concentrated, acidic Fe-rich river plume subsequent to droughts or dry seasons, which led to intense pyrite precipitation upon mixing with euxinic waters. δS34 and δFe56 values of the bedded pyrite (median δS34=−10.3‰ and δFe56=−0.79‰) are consistent with the reaction of dissolved Fe(II) with H2S from bacterial sulfate reduction. Organic-rich clayey Fe-monosulfide-bearing granules were transported from the muddy estuary, and enclosed in Fe (oxyhydr)oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river-delivered iron. The isotopic composition of these presently pyrrhotitic inclusions (median δS34=−3.3‰ and δFe56=−1.6‰) indicates microbial iron reduction. The Fe (oxyhydr)oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor. Their Fe (oxyhydr)oxide matrix was replaced by pyrite (median δS34=+5.8‰ and δFe56=+0.81‰) at shallow sediment depths with 34S and 56Fe-enriched porewater. Wavy-crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation. These results indicate episodic deposition at seasonal to multiannual time scales. δS34 and δFe56 values in the studied Fe-sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early-diagenetic origin, finding no support for the previously proposed local hydrothermal activity in the Talvivaara mudstones.

Effects of dissolved oxygen and pH on nitrous oxide production rates in autotrophic partial nitrification granules

The effects of dissolved oxygen (DO) and pH on nitrous oxide (N2O) production rates and pathways in autotrophic partial nitrification (PN) granules were investigated at the granular level. N2O was primarily produced by betaproteobacterial ammonia-oxidizing bacteria, mainly Nitrosomonas europaea, in the oxic surface layer (<200μm) of the autotrophic PN granules. N2O production increased with increasing bulk DO concentration owing to activation of the ammonia (i.e., hydroxylamine) oxidation in this layer. The highest N2O emissions were observed at pH 7.5, although the ammonia oxidation rate was unchanged between pH 6.5 and 8.5. Overall, the results of this study suggest that in situ analyses of PN granules are essential to gaining insight into N2O emission mechanisms in a granule.

The squat lobster Pleuroncodes monodon tolerates anoxic “dead zone” conditions off Peru

The squat lobster Pleuroncodes monodon is a key species of the highly productive, but oxygen-poor upwelling system of the Eastern Tropical South Pacific. Observations of P. monodon in the water column off Peru have led to the hypothesis that anoxic conditions force this otherwise primarily benthic species to adopt a pelagic lifestyle. Here we show that off Peru, P. monodon can be found in the oxygenated surface water, but also on the anoxic seafloor. Our physiological experiments demonstrate that juvenile and adult specimens have a very low critical respiratory pO2 of 0.5 kPa and that adults survive anoxia for 30.5–70.5 h. Anoxic conditions at the seafloor should therefore force P. monodon to regularly migrate to the oxic surface layer in order to restore energy reserves and recycle metabolic end products of anaerobic metabolism. It was recently estimated that the ammonium supply mediated by diel vertical migrations (DVMs) of zooplankton and nekton considerably fuels bacterial anaerobic ammonium oxidation—a major loss process for fixed nitrogen in the ocean. These estimates were based on the implicit assumption that anoxia does not result in a down-regulation of ammonium excretion. We here show that exposure to anoxia elicits a fourfold reduction in ammonium excretion from 2.1 ± 0.6 µmol h−1 g dry weight−1 under normoxic to 0.5 ± 0.6 µmol h−1 g DW−1 under anoxic conditions in P. monodon. Estimates of ammonium supply to the anoxic core of oxygen minimum zones via DVM therefore are likely too high.

Previously unknown evolutionary groups dominate the ssDNA gokushoviruses in oxic and anoxic waters of a coastal marine environment.

Metagenomic studies have revealed that ssDNA phages from the family Microviridae subfamily Gokushovirinae are widespread in aquatic ecosystems. It is hypothesized that gokushoviruses occupy specialized niches, resulting in differences among genotypes traversing water column gradients. Here, we use degenerate primers that amplify a fragment of the gene encoding the major capsid protein to examine the diversity of gokushoviruses in Saanich Inlet (SI), a seasonally anoxic fjord on the coast of Vancouver Island, BC, Canada. Amplicon sequencing of samples from the mixed oxic surface (10 m) and deeper anoxic (200 m) layers indicated a diverse assemblage of gokushoviruses, with greater richness at 10 m than 200 m. A comparison of amplicon sequences with sequences selected on the basis of RFLP patterns from eight surface samples collected over a 1-year period revealed that gokushovirus diversity was higher in spring and summer during stratification and lower in fall and winter after deep-water renewal, consistent with seasonal variability within gokushovirus populations. Our results provide persuasive evidence that, while specific gokushovirus genotypes may have a narrow host range, hosts for gokushoviruses in SI consist of a wide range of bacterial taxa. Indeed, phylogenetic analysis of clustered amplicons revealed at least five new phylogenetic groups of previously unknown sequences, with the most abundant group associated with viruses infecting SUP05, a ubiquitous and abundant member of marine oxygen minimum zones. Relatives of SUP05 dominate the anoxic SI waters where they drive coupled carbon, nitrogen, and sulfur transformations along the redoxline; thus, gokushoviruses are likely important mortality agents of these bacteria with concomittant influences on biogeochemical cycling in marine oxygen minimum zones.