Oxic-anoxic Transition Research Articles

Aerobic methanotrophy in the oxic–anoxic transition zone of the Black Sea water column

Large amounts of dissolved methane (> 98%) are effectively oxidised by anaerobic and aerobic microorganisms in the Black Sea water column. In the oxic–anoxic transition zone methane concentrations rapidly decrease and an enrichment in 13CH 4 – most likely a result of microbial methane consumption – is observed. Using high performance liquid chromatography–mass spectrometry (HPLC–MS), we show that intact bacteriohopanoids, characteristic for aerobic methanotrophic bacteria, clearly increase in the sub-oxic part of the chemocline compared to water above and below this depth interval. Moreover, the co-occurrence of aminopentol (17 β(H),21 β(H)-35-aminobacteriohopane-30,31,32,33,34-pentol) with aminotetrol (17 β(H),21 β(H)-35-aminobacteriohopane-31,32,33,34-tetrol) argues for members of the type I and/or type X cluster as the dominating bacterial methanotrophs. Our data indicate that aerobic rather than anaerobic methanotrophs are responsible for the strong 13C enrichment in CH 4 occurring in sub-oxic waters within the oxic–anoxic transition zone of the Black Sea water column.

Spatial and temporal variability of the Black Sea suboxic zone

Abstract We coupled an in situ electrochemical analyzer to a CTD to conduct high-resolution, real-time profiling of redox species across the oxic–anoxic transition zone of the Black Sea water column. Voltammetry was performed using gold–amalgam working electrodes to measure simultaneously soluble oxygen and sulfur species (H2S/HS−, S x 2 - , S8) at a resolution of greater than one measurement per meter. In situ data agreed with measurements made in an on-deck voltammetry flow cell coupled to a pump profiling system, and from water samples collected with conventional CTD rosette bottle casts. In situ voltammetric analyses provided rapid redox information, thus enabling more accurate targeting of specific geochemical features by the CTD rosette package. We observed much less lateral oxygen injection from the Bosphorus in 2003 (less than 95 km from Bosphorus) than in 2001 (up to 150 km). This difference can be attributed to variability in physical processes including seasonal temperature and wind variations between winter conditions (2003) and early summer conditions (2001). Furthermore, suboxic zone thickness varied basin-wide, exhibiting changes in the depth of oxygen extinction (minimum detection limit=3 μM) and sulfide onset (minimum detection limit=30 nM). The density surface for oxygen extinction was more variable than the density for the onset of sulfide. Vertical shifts in oxygen extinction and sulfide onset also were observed at the western central gyre station for seven profiles measured over 21 days in 2003.

Stratified prokaryote network in the oxic–anoxic transition of a deep-sea halocline

The chemical composition of the Bannock basin has been studied in some detail. We recently showed that unusual microbial populations, including a new division of Archaea (MSBL1), inhabit the NaCl-rich hypersaline brine. High salinities tend to reduce biodiversity, but when brines come into contact with fresher water the natural haloclines formed frequently contain gradients of other chemicals, including permutations of electron donors and acceptors, that may enhance microbial diversity, activity and biogeochemical cycling. Here we report a 2.5-m-thick chemocline with a steep NaCl gradient at 3.3 km within the water column betweeen Bannock anoxic hypersaline brine and overlying sea water. The chemocline supports some of the most biomass-rich and active microbial communities in the deep sea, dominated by Bacteria rather than Archaea, and including four major new divisions of Bacteria. Significantly higher metabolic activities were measured in the chemocline than in the overlying sea water and underlying brine; functional analyses indicate that a range of biological processes is likely to occur in the chemocline. Many prokaryotic taxa, including the phylogenetically new groups, were confined to defined salinities, and collectively formed a diverse, sharply stratified, deep-sea ecosystem with sufficient biomass to potentially contribute to organic geological deposits.

Evidence of intense archaeal and bacterial methanotrophic activity in the Black Sea water column.

In the northwestern Black Sea, methane oxidation rates reveal that above shallow and deep gas seeps methane is removed from the water column as efficiently as it is at sites located off seeps. Hence, seeps should not have a significant impact on the estimated annual flux of approximately 4.1 x 10(9) mol methane to the atmosphere [W. S. Reeburgh, B. B. Ward, S. C. Wahlen, K. A. Sandbeck, K. A. Kilatrick, and L. J. Kerkhof, Deep-Sea Res. 38(Suppl. 2):S1189-S1210, 1991]. Both the stable carbon isotopic composition of dissolved methane and the microbial community structure analyzed by fluorescent in situ hybridization provide strong evidence that microbially mediated methane oxidation occurs. At the shelf, strong isotope fractionation was observed above high-intensity seeps. This effect was attributed to bacterial type I and II methanotrophs, which on average accounted for 2.5% of the DAPI (4',6'-diamidino-2-phenylindole)-stained cells in the whole oxic water column. At deep sites, in the oxic-anoxic transition zone, strong isotopic fractionation of methane overlapped with an increased abundance of Archaea and Bacteria, indicating that these organisms are involved in the oxidation of methane. In underlying anoxic water, we successfully identified the archaeal methanotrophs ANME-1 and ANME-2, eachof which accounted for 3 to 4% of the total cell counts. ANME-1 and ANME-2 appear as single cells in anoxicwater, compared to the sediment, where they may form cell aggregates with sulfate-reducing bacteria (A. Boetius, K. Ravenschlag, C. J. Schubert, D. Rickert, F. Widdel, A. Giesecke, R. Amann, B. B. Jørgensen, U. Witte, and O. Pfannkuche, Nature 407:623-626, 2000; V. J. Orphan, C. H. House, K.-U. Hinrichs, K. D. McKeegan, and E. F. DeLong, Proc. Natl. Acad. Sci. USA 99:7663-7668, 2002).

Bacterial magnetite produced in water column dominates lake sediment mineral magnetism: Lake Ely, USA

SUMMARY Environmental magnetic studies of annually laminated sediments from Lake Ely, northeastern Pennsylvania, USA indicate that bacterial magnetite is the dominant magnetic mineral in the lake sediment. In previous studies of Lake Ely sediment, the dark, organic-rich layers in the annual laminae were interpreted to have high-intensity saturation isothermal remanent magnetizations (SIRMs) while the light-coloured, silt-rich layers have low-intensity SIRMs. To test the hypothesis that the magnetic grains in the sediments were an authigenic product of magnetotactic bacteria rather than detrital magnetic grains eroded from the watershed, we analysed samples from the water column, the lake sediment, and a sediment trap installed near the lake bottom. Direct microscopic observation of the water column samples showed the presence of magnetotactic bacteria in and below the oxic-anoxic transition zone (OATZ). To characterize the magnetic minerals, rock magnetic parameters were measured for material from the water column, the sediment trap and the dark- and light-coloured lake sediments. Low-temperature magnetic measurements tested for the presence of magnetosomes in separated dark- and light-coloured layer samples. Numeric unmixing of the low-temperature results showed that biogenic magnetites were present in the lake sediment and contributed more significantly to the SIRM in the dark, organic-rich layers than in the light-coloured, inorganic silt-rich layers. Observations under the transmission electron microscope (TEM) of magnetic extracts also show the abundance of magnetosomes in the lake sediment. The presence of live magnetotactic bacteria in the water column and the predominance of bacterial magnetites in filtered particulate matter, sediment traps and recent lake sediment all suggest that bacterial magnetites are the main magnetic minerals in Lake Ely sediment. This finding suggests that changes in environmental factors that control the productivity of magnetic bacteria in the lake likely contribute to the variability of magnetic mineral concentrations observed in the lake sediments.

Uranium-series disequilibria as a means to study recent migration of uranium in a sandstone-hosted uranium deposit, NW China

Uranium concentration and alpha specific activities of uranium decay series nuclides 234U, 238U, 230Th, 232Th and 226Ra were measured for 16 oxidized host sandstone samples, 36 oxic-anoxic (mineralized) sandstone samples and three unaltered primary sandstone samples collected from the Shihongtan deposit. The results show that most of the ores and host sandstones have close to secular equilibrium alpha activity ratios for 234U/ 238U, 230Th/ 238U, 230Th/ 234U and 226Ra/ 230Th, indicating that intensive groundwater-rock/ore interaction and uranium migration have not taken place in the deposit during the last 1.0 Ma. However, some of the old uranium ore bodies have locally undergone leaching in the oxidizing environment during the past 300 ka to 1.0 Ma or to the present, and a number of new U ore bodies have grown in the oxic–anoxic transition (mineralized) subzone during the past 1.0 Ma. Locally, uranium leaching has taken place during the past 300 ka to 1.0 Ma, and perhaps is still going on now in some sandstones of the oxidizing subzone. However, uranium accumulation has locally occurred in some sandstones of the oxidizing environment during the past 1 ka to 1.0 Ma, which may be attributed to adsorption of U(VI) by clays contained in oxidized sandstones. A recent accumulation of uranium has locally taken place within the unaltered sandstones of the primary subzone close to the oxic–anoxic transition environment during the past 300 ka to 1.0 Ma. Results from the present study also indicate that uranium-series disequilibrium is an important tool to trace recent migration of uranium occurring in sandstone-hosted U deposits during the past 1.0 Ma and to distinguish the oxidation-reduction boundary.

Bacterial Iron Oxidation in Circumneutral Freshwater Habitats: Findings from the Field and the Laboratory

Lithotrophic Fe-oxidation at neutral pH is becoming recognized as an important microbial process. An overview of the microbial iron cycle is presented with an emphasis on the role of microbes that grow under microaerobic conditions at oxic-anoxic transition zones where Fe(II) is abundant. Examples of these environments from freshwater are considered. Contrary Creek is a spring-fed wetland in Virginia. Measurements over the course of a year showed that it had a consistent pH around 6, and Fe(II) concentrations ranged from 25 to 300 μ M, with the highest concentrations in the summer months. At all times abundant flocs of Fe-oxides composed principally of Lepthothrix ochracea sheaths were present. Based on observations at this site, and other sites, a model for microbial Fe mat formation is presented. A thermal site in Yellowstone National Park that had consistent circumneutral pH and high Fe(II) concentrations was also studied. This site did not have evidence for Fe-oxidizing bacteria, but was, instead, dominated by a cyanobacterial photosynthetic mat. Consideration is given to growth conditions for pure cultures of Fe-oxidizing bacteria (FeOB) in the laboratory. A novel method of growing FeOB on gradient plates was developed. This led to an increase of cell yields to 2 × 108 cells/ml, which is nearly an order of magnitude greater than previous methods have yielded. Finally, speculation is made as to the potential for conditions on Mars that might have been conducive for microbial Fe-oxidation.

Chemolithoautotrophy in the marine, magnetotactic bacterial strains MV-1 and MV-2.

Magnetite-producing magnetotactic bacteria collected from the oxic-anoxic transition zone of chemically stratified marine environments characterized by O2/H2S inverse double gradients, contained internal S-rich inclusions resembling elemental S globules, suggesting they oxidize reduced S compounds that could support autotrophy. Two strains of marine magnetotactic bacteria, MV-1 and MV-2, isolated from such sites grew in O2-gradient media with H2S or thiosulfate (S2O3(2-)) as electron sources and O2 as electron acceptor or anaerobically with S2O3(2-) and N2O as electron acceptor, with bicarbonate (HCO3-)/CO2 as sole C source. Cells grown with H2S contained S-rich inclusions. Cells oxidized S2O3(2-) to sulfate (SO4(2-)). Both strains grew microaerobically with formate. Neither grew microaerobically with tetrathionate (S4O6(2-)), methanol, or Fe2+ as FeS, or siderite (FeCO3). Growth with S2O3(2-) and radiolabeled 14C-HCO3- showed that cell C was derived from HCO3-/CO2. Cell-free extracts showed ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) activity. Southern blot analyses indicated the presence of a form II RubisCO (cbbM) but no form I (cbbL) in both strains. cbbM and cbbQ, a putative post-translational activator of RubisCO, were identified in MV-1. MV-1 and MV-2 are thus chemolithoautotrophs that use the Calvin-Benson-Bassham pathway. cbbM was also identified in Magnetospirillum magnetotacticum. Thus, magnetotactic bacteria at the oxic-anoxic transition zone of chemically stratified aquatic environments are important in C cycling and primary productivity.

Evidence for a dynamic cycle between Mn and Co in the water column of a stratified lake.

The geochemical behavior of Co in aquatic systems has often been related to the presence of Fe and Mn particles. A few studies have shown that Co is exclusively associated with particulate Mn, but the dynamics of Co and Mn cycling have never been determined in real time under natural conditions. In this study, we used a combination of analytical techniques to study the temporal and spatial evolution of Mn microparticles (MnOx) over 2 weeks in the water column of a shallow stratified lake (Paul Lake, MI). We report a temporal accumulation of dissolved Mn at the oxic-anoxic transition, and we show that this accumulation is due to the reductive dissolution of Mn particles. The reductant has not been identified, but abiotic reduction by sigmaH2S and ferrous iron is excluded because they are produced below the zone of MnOx reduction. Hybridization of RNA isolated from Paul Lake with oligonucleotide probes targeting the delta proteobacteria, which include metal-reducing species, suggests that their activity is greatest at and just below the oxic-anoxic transition, so that Mn reduction may be influenced by bacterial activity. Mn-oxidizing bacteria were isolated from this zone as well. We also demonstrate that the dynamic evolution of MnOx has a direct influence on the distribution of Co in the water column of this lake: dissolved Co is released during the reductive dissolution of MnOx and accumulates at the redox interface.

Reactive transport modeling of trace elements in the water column of a stratified lake: iron cycling and metal scavenging

A simple one-dimensional reactive transport model was developed to determine the importance of the dynamics of major redox species such as iron on the distribution of trace elements in aquatic environments. The model simulates the distribution of iron and lead in the water column of a stratified lake (Paul Lake, MI) assuming a quasi steady-state is reached. The reactions described in the model include oxidation of reduced iron to colloidal ferric iron at the oxic–anoxic transition in the water column, aggregation of colloidal iron to particulate iron, and reduction of colloidal and particulate iron in the deep anoxic waters. The model includes sedimentation of both colloidal and particulate species. The cycling of lead is described by adsorption on colloidal iron, transfer to the particulate phase by aggregation, and redissolution in the deep waters during reduction of colloidal and particulate iron. Generally, this model reproduces experimental data collected during three field trips from 1994 to 1996 well. However, these calculations clearly show that the processes regulating the cycling of iron and lead in the water column are more complex than what is generally admitted. Additional reactions not considered in the model affect the transformation of Fe, and thus affect the removal of Pb. These reactions have to be fully characterized in order to predict the chemical distribution of trace elements at oxic–anoxic transitions in the water column of aquatic systems.

Speciation, reactivity, and cycling of Fe and Pb in a meromictic lake

A suite of analytical techniques were combined to study the chemical speciation of Fe and Pb in the water column of a lake characterized by a biogenic meromixis (Paul Lake, MI). Depth profiles of Fe 2+ and “dissolved” Pb display significant concentration gradients below the chemocline, i.e., they increase from below detection limit to ca. 100 μM for Fe 2+ and 2 nM for Pb d. Significant correlations between particulate organic matter, hydrous iron oxides, and particulate Pb suggest that Pb is scavenged by Fe-rich particles formed at the oxic-anoxic transition. Transmission electron microscopy shows that particles of hydrous iron oxides form complex aggregates with natural organic matter at and below the oxic-anoxic transition. Experiments with batch reactors show that these organo-mineral moieties remove Pb rapidly during their formation. Thermodynamic calculations predict that FeS and PbS are respectively saturated and oversaturated in the monimolimnion, although the presence of neither FeS nor PbS was observed. This suggests that the solubilities of Fe and Pb are influenced by complexation. Voltammetric experiments on filtered samples show that Pb is weakly complexed in the mixolimnion and strongly complexed in the monimolimnion. A conditional stability constant for Pb complexation is determined using metal titration curves assuming a simple 1:1 stoichiometry and gives logK cond = 9.4 ± 0.8 M −1 in the monimolimnion. These speciation results are confirmed by ion exchange chromatography, which demonstrates that more than 98% of Pb is complexed by natural organic matter.

Inorganic Geobiochemical Implications of the Oxic-Anoxic Transition in Ancient Earth Oceans

The development of free dioxygen in the Earth's atmosphere around 2.2Ga led to considerable biological changes. In particular, the early Earth biota was dominated by anaerobic prokaryotes including archaebacteria, possibly the most primitive life forms. Subsequent to the development of an oxic environment, aerobic bacteria and eventually prokaryotes developed. The inorganic biochemistry of the two groups are characterized in part by the relative dominance of nickel and cobalt proteins in the primitive anoxic group and the appearance of copper proteins in aerobic bacteria and multicellular organisms (Frausto di Silva and Williams 1996). Nickel and cobalt proteins are less important in aerobic organisms and copper is insignificant in the primitive anaerobes. Iron and zinc appear equally important throughout, although their dominant biochemical functions change. It is well established that base metal concentrations constitute limiting growth factors for present-day aerobic and anaerobic communities. For example, increasing oceanic iron concentrations enhances the growth of planktonic algae (Martin et al 1991). Iron has long been known as a limiting factor for the development of anaerobic sulphate reducing bacteria because of its importance to cytochrome c 3. (Postgate 1965). The present paper presents the results of a study of thermodynamic of oxic and anoxic ocean models based on the recent definition and discovery of a large number of base metal sulphide and polysulphide complexes (Luther et al. 1996; Chadwell et al. in prep). I assume that biological availability potentially embraces any soluble metal species and not only free metal aquo ions. The chemica l models were made using Geochemists Workbench | version 3.4 (GWB). The GWB seawater model has been discussed by Bethke (1996). The input thermodynamic database is a modified selection of the Lawrence Livermore National Labora tory thermodynamic dataset THERMO.COM.V8.R6.FULL. based on a compilation by Thomas Wolery and co-workers which is in turn largely based on Helgeson et al.(1978). The composition of early Earth oceans is not rigorously constrained and both acid and alkaline oceans have been proposed. I take a conservative view in this study, assuming that ocean composition is currently buffered by reaction with oceanic lithosphere and that this effect is likely to have been enhanced in the higher heat flow regimes of the ancient Earth. As a starting point, I therefore take the composition of the modem oxic oceans, which is fairly well defined. I then reduce it through the GWB reaction progress algorithm, basically by titrating electrons. During this process, the total major element concentration (excluding dioxygen) of the oceans remains constant. The pH remains around 8.0. In oxic oceans, inorganic base metal chemistry is dominated by free metal aquo ions, hydroxide, oxyhydroxide and chloride complexes. The database includes a number of simple base metal organic complexes, but these do not appear to affect the model and their stability constants may not be wellconstrained. In a n o x i c oceans , base metal speciation is simplified and their chemistry is largely controlled by sulphide and bisulphide complexes. The introduction of base metals (bi)sulphide complexes into the anoxic ocean model increases the relative dissolved concentrations of all base metals except Mn. The enhancement is approximately 2 magnitudes or more. This relative enhancement is independent of the sulphide mineral substrate. The total dissolved metal concentrations decrease markedly with peak concentrations around -200mV. The concentrations increase from around -300mV through -400mV. A feature of the anoxic ocean is the diminution of the free metal aquo ion concentrations. I took a computed dissolved metal concentration of 10 -15 M as being a limiting value for the presence of the metal in solution. The absolute concentrations of base metals in anoxic systems is not well constrained because of the lack of information about the nature of the metastable mineral sulphides. The increase in solubility of metastable compared with stable iron sulphides is better known. The first precipitated FeS

Controlled Biomineralization of Magnetite (Fe(inf3)O(inf4)) and Greigite (Fe(inf3)S(inf4)) in a Magnetotactic Bacterium

A slowly moving, rod-shaped magnetotactic bacterium was found in relatively large numbers at and below the oxic-anoxic transition zone of a semianaerobic estuarine basin. Unlike all magnetotactic bacteria described to date, cells of this organism produce single-magnetic-domain particles of an iron oxide, magnetite (Fe(inf3)O(inf4)), and an iron sulfide, greigite (Fe(inf3)S(inf4)), within their magnetosomes. The crystals had different morphologies, being arrowhead or tooth shaped for the magnetite particles and roughly rectangular for the greigite particles, and were coorganized within the same chain(s) in the same cell with their long axes along the chain direction. Because the two crystal types have different crystallochemical characteristics, the findings presented here suggest that the formation of the crystal types is controlled by separate biomineralization processes and that the assembly of the magnetosome chain is controlled by a third ultrastructural process. In addition, our results show that in some magnetotactic bacteria, external environmental conditions such as redox and/or oxygen or hydrogen sulfide concentrations may affect the composition of the nonmetal part of the magnetosome mineral phase.

Distribution of microaerophilic bacteria through the oxic-anoxic transition zone of lagoon sediments

In marine sediments, where soluble gases diffuse only very slightly, many organisms struggle for molecular oxygen. Microaerophilic bacteria, able to grow at reduced pO2 between 0.2 and 12%, have an advantage. Distribution of aerobes, microaerophiles and anaerobes was compared with the oxygen gradient in seawater and sediment samples collected in a northern Mediterranean lagoon. In the near bottom seawater and in the 0–10 mm upperlayer of sediment, the microaerophilic counts were less than 1% of aerobe densities. In the 10–15 mm zone, these two groups were equivalent in density (1 × 105 cells ml−1). As expected, the microaerophiles took advantage of their low oxygen tension requirements in the subsurface sediments, between the well aerated zone (0–5 mm depth) and the low redox potential zone. Then, beyond a depth of 20 mm, the anaerobes prevailed in this sandy clay.

Sauerstoffzehrungsraten von Profundalsediment des Bodensee-Obersees unter oxischen und anoxischen Bedingungen

Oxygen uptake rates by the sediment have been determined in a natural stratified sediment-water system from Lake Constance (Obersee, max. depth). After oxic preconditioning of the system the uptake rates ranged between 855 and 1,062 mg·m−2·d−1; after anoxic preconditioning of the system they ranged between 3,405 and 3,794 mg·m−2·d−1. These data, and the electron activity buffer capacity and oxygen consumption intensity as found in Lake Constance profundal water, show that the oxic-anoxic transition will happen here at the earliest after 142 days (about 4.5 months) of total oxygen isolation. Reoxygenation requires at least 3.7 times higher O2 input than supposed for a ‘normal’ winter circulation in Obersee.