Ocean Water Column Research Articles

Metagenomics of the Deep Mediterranean, a Warm Bathypelagic Habitat

BackgroundMetagenomics is emerging as a powerful method to study the function and physiology of the unexplored microbial biosphere, and is causing us to re-evaluate basic precepts of microbial ecology and evolution. Most marine metagenomic analyses have been nearly exclusively devoted to photic waters.Methodology/Principal FindingsWe constructed a metagenomic fosmid library from 3,000 m-deep Mediterranean plankton, which is much warmer (∼14°C) than waters of similar depth in open oceans (∼2°C). We analyzed the library both by phylogenetic screening based on 16S rRNA gene amplification from clone pools and by sequencing both insert extremities of ca. 5,000 fosmids. Genome recruitment strategies showed that the majority of high scoring pairs corresponded to genomes from Rhizobiales within the Alphaproteobacteria, Cenarchaeum symbiosum, Planctomycetes, Acidobacteria, Chloroflexi and Gammaproteobacteria. We have found a community structure similar to that found in the aphotic zone of the Pacific. However, the similarities were significantly higher to the mesopelagic (500–700 m deep) in the Pacific than to the single 4000 m deep sample studied at this location. Metabolic genes were mostly related to catabolism, transport and degradation of complex organic molecules, in agreement with a prevalent heterotrophic lifestyle for deep-sea microbes. However, we observed a high percentage of genes encoding dehydrogenases and, among them, cox genes, suggesting that aerobic carbon monoxide oxidation may be important in the deep ocean as an additional energy source.Conclusions/SignificanceThe comparison of metagenomic libraries from the deep Mediterranean and the Pacific ALOHA water column showed that bathypelagic Mediterranean communities resemble more mesopelagic communities in the Pacific, and suggests that, in the absence of light, temperature is a major stratifying factor in the oceanic water column, overriding pressure at least over 4000 m deep. Several chemolithotrophic metabolic pathways could supplement organic matter degradation in this most depleted habitat.

Amino acid nitrogen isotopic fractionation patterns as indicators of heterotrophy in plankton, particulate, and dissolved organic matter

Bulk nitrogen (N) isotope signatures have long been used to investigate organic N source and food web structure in aquatic ecosystems. This paper explores the use of compound-specific δ 15N patterns of amino acids (δ 15N-AA) as a new tool to examine source and processing history in non-living marine organic matter. We measured δ 15N-AA distributions in plankton tows, sinking particulate organic matter (POM), and ultrafiltered dissolved organic matter (UDOM) in the central Pacific Ocean. δ 15N-AA patterns in eukaryotic algae and mixed plankton tows closely resemble those previously reported in culture. δ 15N differences between individual amino acids (AA) strongly suggest that the sharply divergent δ 15N enrichment for different AA with trophic transfer, as first reported by [McClelland, J.W. and Montoya, J.P. (2002) Trophic relationships and the nitrogen isotopic composition of amino acids. Ecology 83, 2173–2180], is a general phenomenon. In addition, differences in δ 15N of individual AA indicative of trophic transfers are clearly preserved in sinking POM, along with additional changes that may indicate subsequent microbial reworking after incorporation into particles. We propose two internally normalized δ 15N proxies that track heterotrophic processes in detrital organic matter. Both are based on isotopic signatures in multiple AA, chosen to minimize potential problems associated with any single compound in degraded materials. A trophic level indicator (ΔTr) is derived from the δ 15N difference between selected groups of AA based on their relative enrichment with trophic transfer. We propose that a corresponding measure of the variance within a sub-group of AA (designated Σ V) may indicate total AA resynthesis, and be strongly tied to heterotrophic microbial reworking in detrital materials. Together, we hypothesize that ΔTr and Σ V define a two dimensional trophic “space”, which may simultaneously express relative extent of eukaryotic and bacterial heterotrophic processing. In the equatorial Pacific, ΔTr indicates an average of 1.5–2 trophic transfers between phytoplankton and sinking POM at all depths and locations. The Σ V parameter suggests that substantial variation may exist in bacterial heterotrophic processing between differing regions and time periods. In dissolved material δ 15N-AA patterns appear unrelated to those in POM. In contrast to POM, δ 15N-AA signatures in UDOM show no clear changes with depth, and suggest that dissolved AA preserved throughout the oceanic water column have undergone few, if any, trophic transfers. Together these data suggest a sharp divide between processing histories, and possibly sources, of particulate vs. dissolved AA.

Distribution and Diversity of Archaeal Ammonia Monooxygenase Genes Associated with Corals

Corals are known to harbor diverse microbial communities of Bacteria and Archaea, yet the ecological role of these microorganisms remains largely unknown. Here we report putative ammonia monooxygenase subunit A (amoA) genes of archaeal origin associated with corals. Multiple DNA samples drawn from nine coral species and four different reef locations were PCR screened for archaeal and bacterial amoA genes, and archaeal amoA gene sequences were obtained from five different species of coral collected in Bocas del Toro, Panama. The 210 coral-associated archaeal amoA sequences recovered in this study were broadly distributed phylogenetically, with most only distantly related to previously reported sequences from coastal/estuarine sediments and oceanic water columns. In contrast, the bacterial amoA gene could not be amplified from any of these samples. These results offer further evidence for the widespread presence of the archaeal amoA gene in marine ecosystems, including coral reefs.

Oceanic Methane Biogeochemistry

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Quantitative distribution of presumptive archaeal and bacterial nitrifiers in Monterey Bay and the North Pacific Subtropical Gyre

The recent isolation of the ammonia-oxidizing crenarchaeon Nitrosopumilus maritimus has expanded the known phylogenetic distribution of nitrifying phenotypes beyond the domain Bacteria. To further characterize nitrification in the marine environment and explore the potential crenarchaeal contribution to this process, we quantified putative nitrifying genes and phylotypes in picoplankton genomic libraries and environmental DNA samples from coastal and open ocean habitats. Betaproteobacteria ammonia monooxygenase subunit A (amoA) gene copy numbers were low or undetectable, in stark contrast to crenarchaeal amoA-like genes that were broadly distributed and reached up to 6 x 10(4) copies ml(-1). Unexpectedly, in the North Pacific Subtropical Gyre, a deeply branching crenarchaeal group related to a hot spring clade (pSL12) was at times abundant below the euphotic zone. Quantitative data suggested that the pSL12 relatives also contain archaeal amoA-like genes. In both coastal and open ocean habitats, close relatives of known nitrite-oxidizing Nitrospina species were well represented in genomic DNA libraries and quantitative PCR profiles. Planktonic Nitrospina depth distributions correlated with those of Crenarchaea. Overall, the data suggest that amoA-containing Crenarchaea are more phylogenetically diverse than previously reported. Additionally, distributional patterns of planktonic Crenarchaea and Nitrospina species suggest potential metabolic interactions between these groups in the ocean's water column.

Oceanic Methane Biogeochemistry

This review shows that thermodynamic and kinetic constraints largely prevent large-scale methanogenesis in the open ocean water column. One example of open-ocean methanogenesis involves anoxic digestive tracts and fecal pellet microenvironments; methane released during fecal pellet disaggregation results in the mixed-layer methane maximum. However, the bulk of the methane in the ocean is added by coastal runoff, seeps, hydrothermal vents, decomposing hydrates, and mud volcanoes. Since methane is present in the open ocean at nanomolar concentrations, and since the flux to the atmosphere is small, the ultimate fate of ocean methane additions must be oxidation within the ocean. As indicated in the Introduction and highlighted in Table 3, sources of methane to the ocean water column are poorly quantified. There are only a small number of direct water column methane oxidation rates, so sinks are also poorly quantified. We know that methane oxidation rates are sensitive to ambient methane concentrations, but we have no information on reaction kinetics and only one report of the effect of pressure on methane oxidation. Our perspective on methane sources and the extent of methane oxidation has been changed dramatically by new techniques involving gene probes, determination of isotopically depleted biomarkers, and recent 14C-CH4 measurements showing that methane geochemistry in anoxic basins is dominated by seeps providing fossil methane. The role of anaerobic oxidation of methane has changed from a controversial curiosity to a major sink in anoxic basins and sediments. © 2007 American Chemical Society.

Ultra-Microscale (5–25 μg C) Analysis of Individual Lipids by 14C AMS: Assessment and Correction for Sample Processing Blanks

Measurements of the natural abundance of radiocarbon in biomarker molecules can be used to elucidate the biogeochemical roles of marine bacteria and archaea in the oceanic water column. However, the relatively low concentration of biomass, especially below the euphotic zone, inevitably results in small sample sizes for compound-specific analyses. In ultra-microscale Δ14C measurements, which we define as measurements on samples smaller than 25 μg C, the process of isolating pure compounds and preparing them for measurement adds significant background carbon. This additional blank carbon can contribute up to 40% of the total sample mass; therefore, it is necessary to quantify all components of the processing blank in order to make appropriate corrections. Complete propagation of error is critical in order to report the correct analytical uncertainty. The carbon blank is composed of at least 3 different sources: i) those that scale in proportion to the mass of the sample; ii) sources that contribute a constant mass of blank, e.g. closed-tube combustion; and iii) contaminants from vacuum lines and/or other aspects of sample handling that are difficult to quantify. We approached the problem of correcting for the total sample processing blank by deriving a 4-part isotopic mass balance based on separating the 3 exogenous components from the sample. Subsequently, we derived the appropriate equations for the full propagation of error associated with these corrections. Equations for these terms are presented. Full treatment of a set of raw data is demonstrated using compound-specific Δ14C data from the North Central Pacific water column.

Pressure and life: some biological strategies

All biological processes of life on Earth experience varying degrees of pressure. Aquatic organisms living in the deep-sea, as well as chondrocytic cells of articular cartilage are exposed to hydrostatic pressures that rise up to several hundred times that of atmospheric pressure. In the case of marine larvae that disperse through the oceanic water column, pressure changes might be responsible for stress conditions during development, limiting colonisation capabilities. In a number of biological systems, life strategies may be significantly influenced by pressure. In this review, we will focus on the consequences of pressure changes on various biological processes, and more specifically on animals living in the deep-sea. Revisiting general principles of pressure effects on biological systems, we present recent data illustrating the diversity of effects pressure may have at different levels in biological systems, with particular attention to effects on gene expression. After a review of the main pressure equipments available today for studying species living naturally at high pressure, we summarise what is known concerning pressure impact during animal development.

Hydroacoustic contributions to understanding the December 26th 2004 great Sumatra–Andaman Earthquake

Within the era of modern digital-recording, the December 26th 2004 Sumatra–Andaman Earthquake represents an event of unprecedented scale. Hydroacoustic observations have made significant contributions toward our understanding of this great rupture and serve to reiterate the potential use of tertiary (T) waves as a tool in tsunami warning. Small-aperture arrays of hydrophones operated by the International Monitoring System (IMS) recorded the seismically generated, water-borne T-wave within the Indian Ocean. Due to the velocity structure of the oceanic water column, T-wave propagation is both slower and more efficient than radiation within the solid earth. This results in a relatively large amplitude signal that arrives within a time window distinct from the more complex and overlapping pattern of solid earth seismic phases. Hydroacoustic analysis has constrained the rupture length of the fault to be ~1,200 km and the duration on the order of 8 min, with 2–3 phases exhibiting progressively decreasing rupture velocity. These data also indicate that aftershock rates in the first hours following the mainshock correlate with spatial variability in the sourced T-wave amplitude, with far fewer events along the northern section of the main rupture. Although IMS stations telemeter data in near real time, data access for scientists was restricted due to the provisions of the Comprehensive Test Ban Treaty. The swift dissemination of data will be critical in using hydroacoustic methods to assess the magnitude and tsunamigenic potential of future events.

A Preliminary Assessment of Ocean Thermal Energy Conversion Resources

Worldwide power resources that could be extracted from Ocean Thermal Energy Conversion (OTEC) plants are estimated with a simple one-dimensional time-domain model of the thermal structure of the ocean. Recently published steady-state results are extended by partitioning the potential OTEC production region in one-degree-by-one-degree “squares” and by allowing the operational adjustment of OTEC operations. This raises the estimated maximum steady-state OTEC electrical power from about 3TW(109kW) to 5TW. The time-domain code allows a more realistic assessment of scenarios that could reflect the gradual implementation of large-scale OTEC operations. Results confirm that OTEC could supply power of the order of a few terawatts. They also reveal the scale of the perturbation that could be caused by massive OTEC seawater flow rates: a small transient cooling of the tropical mixed layer would temporarily allow heat flow into the oceanic water column. This would generate a long-term steady-state warming of deep tropical waters, and the corresponding degradation of OTEC resources at deep cold seawater flow rates per unit area of the order of the average abyssal upwelling. More importantly, such profound effects point to the need for a fully three-dimensional modeling evaluation to better understand potential modifications of the oceanic thermohaline circulation.

Statistical Analysis of Sonar Performance Prediction in Littoral Environments

AbstractThis paper presents the Statistical Analysis of Passive Sonar Performance Prediction in Littoral Environments. Passive sonar performance and acoustic prediction mainly refer to the detection range. The inputs for estimating the sonar detection range include the Figure of Merit (FOM), Transmission Loss (TL), and Ambient Noise (NL) of the operation region. These inputs are directly related to the ocean environment; hence, the detection range is, too. A littoral environment is highly variable both in time and space. This paper proposes a methodology for analyzing the statistical properties of the detection range from measurements of ocean water column properties. It is found that the detection range of the southwestern region of Taiwan in the summer is 13.8km with 5.9km as the standard deviation and in the winter is 37.8km with 33.6km as the standard deviation.

Low dissolved DMSP concentrations in seawater revealed by small‐volume gravity filtration and dialysis sampling

A variety of filtration and passive dialysis protocols were tested with seawater for determination of the dissolved concentrations of dimethylsulfoniopropionate (DMSPd), a compound originating in the cytosol of many marine phytoplankton. Commonly used sampling procedures, such as in‐line filtration, syringe pressure filtration, and gravity filtration of relatively large‐volume samples (5‐50 mL) through glass fiber filters, caused release of DMSPd from particulate material as evidenced by increasing DMSPd concentrations with volume filtered. Exposure of filters to air at the end of filtration caused particularly severe DMSPd release. Dialysis bags or Slide‐A‐Lyzer dialysis cassettes equilibrated with seawater DMSPd within 4 h, even in polar waters (−1.8 °C), and appeared to give accurate DMSPd concentrations in some circumstances. However, incubation of seawater in laboratory containers (e.g., glass jars) during dialysis sometimes caused artifactual release of DMSPd. We therefore adopted a small‐volume gravity drip filtration (SVDF) procedure, the essential elements of which were: (i) collection of 20 to 50 mL seawater directly from the primary sample container (e.g., Niskin bottle) into a dry, all‐plastic filtration tower; (ii) use of a 47‐mm‐diameter Whatman GF/F filter; (iii) rapid (< 3 min) filtration by gravity (hydrostatic) pressure; (iv) collection of only the first 3.5 mL filtrate for DMSPd analysis; and (v) never exposing the filtered plankton to air. The SVDF procedure appeared to yield the same low DMSPd concentrations as dialysis samplers incubated in the ocean water column. Using the SVDF procedure, DMSPd was found to be < 2.8 nM over a broad range of ocean water types and particulate DMSP concentrations. The maximum DMSPd concentration observed in our study (2.8 nM) was far lower than the reported worldwide average DMSPd concentration of 16.9 nM, raising the possibility that past data collections may have been influenced by filtration artifacts.

Reconstruction of changes in global sulfur cycling from marine sulfate isotopes

Stable isotope ratios of sulfur ( δ 34S) and oxygen ( δ 18O) in marine sulfate respond to changes in fluxes and processes in the global sulfur cycle. The two isotope systems respond on different time scales to different factors affecting the global sulfur cycle. Sulfur isotopes respond primarily to the relative fluxes of weathering-derived sulfate to the oceans versus the net flux of sulfur exported from the oceans as pyrite in marine sediments. At the present day, this response is relatively slow (∼ 20 Ma) because of the large size of the marine sulfate reservoir; ancient oceans had lower sulfate concentrations and thus sulfate residence times may have been shorter. The δ 34S of marine sulfate is also sensitive to the development of a significant reservoir of H 2S in ancient stratified oceans. Sulfate–oxygen isotopes respond primarily to changes in the cycling of sulfur in the oceans and marine sediments. Sulfur disproportionation, oxic and anaerobic sulfide oxidation all impart different δ 18O signatures to marine sulfate; the relative importance of these different processes is in turn controlled by the distribution of redox conditions and microbial ecology of sea-floor sediments. The mutual interpretation of changes in marine sulfate δ 34S and δ 18O can thus give rather precise information on the nature of changes in global sulfur cycling. Recently, records of isotope compositions of ancient marine sulfate have become available through the analysis of carbonate-associated sulfate (CAS) in limestones and marine biogenic barite, both of which provide far more continuous records than previous data based primarily on marine evaporite sulfate minerals. We review a number of case studies which use sulfur or combined sulfur and oxygen isotopic records derived from CAS and biogenic barite. Proterozoic CAS-sulfur isotope datasets often show marked variations over very small stratigraphic intervals (e.g. 20‰ over 300 m). When examined in conjunction with estimates of deposition rates, they are used to infer a new evolutionary curve of marine sulfate concentrations and therefore a protracted oxidation of the Earth's biosphere. Through the Cretaceous–Tertiary interval, the marine sulfate–sulfur record is characterised by significant periods of relative stability punctuated by phases of more rapid change (limited to 7‰ variation over the last 130 Ma). The Carboniferous was characterized essentially by a gradual change of 9‰ over 60 Ma. By contrast, at the Permian–Triassic boundary, extreme changes occur over very short geological time scales (14‰ over < 1 Ma); simple mass balance considerations suggest the development of a large, relatively short-lived, reservoir of H 2S in the oceanic water column at this time. In this case, it is the short time scale of the response of the sulfate–oxygen record that enables the interpretation of rapid changes in oceanic redox state. Over the last 10 Ma, sulfate–oxygen δ 18O in biogenic barite exhibits large isotope changes (∼ 7‰) that occur independently of those in sulfate–sulfur. These changes must reflect changes in depositional/redox environment at the ocean margins and/or changes in microbial cycling of sulfur, but the exact cause remains controversial. Our limited understanding of the influence of redox environment and sediment microbial ecology on sulfur cycling currently restricts the level to which we are able to interpret such changes in marine sulfate δ 18O.

Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp. strain SG-1

Natural Mn-oxide nanoparticles and grain coatings are ubiquitous in the environment and profoundly impact the water quality and quality of sediments through their ability to degrade and sequester contaminants. These oxides, which are believed to form dominantly via oxidation of Mn2+ by marine and freshwater bacteria, have extremely high sorptive capacities for heavy metals. We have used XANES, EXAFS, and synchrotron (SR)-XRD techniques to study biogenic Mn oxides produced by spores of the marine Bacillus sp. strain SG-1 in seawater as a function of reaction time under in-situ conditions. An EXAFS model was developed to fully account for the structure and features in the data, providing realistic structural information. The first observed biogenic solid-phase Mn-oxide product is a layered phyllomanganate with hexagonal sheet symmetry and an Mn-oxidation state similar to that in δ-MnO2, between 3.7 and 4.0. XRD and SEM-EDS data show the biooxides to have a phyllomanganate 10 Å basal plane spacing and an interlayer containing Ca. With time, a phyllomanganate oxide with pseudo-orthogonal sheet symmetry appears. Fits to these EXAFS spectra suggest the octahedral layers have relatively few Mn octahedral site vacancies in the lattice and the layers bend to accommodate Jahn-Teller distortions creating the change in symmetry. A reaction mechanism is proposed to account for the observed products. The phyllomanganate oxides observed in this study may be the same as the most abundant Mn-oxide phases suspended in the oxic and sub-oxic zones of the oceanic water column that are of global importance in trace metal and nutrient cycling

Chemoherms on Hydrate Ridge — Unique microbially-mediated carbonate build-ups growing into the water column

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as −48.1 ‰ PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The δ18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 ‰ PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 ‰ PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

Incidence of novel and potentially archaeal nitrogenase genes in the deep Northeast Pacific Ocean

Archaea have been detected throughout the oceanic water column and are quantitatively important members of picoplankton in the deep ocean. Two common groups, group I Crenarchaeota and group II Euryarchaeota, are consistently detected in warm hydrothermal fluid and are assumed to have been drawn into the subseafloor, mixed with hydrothermal fluid and then expelled. However, because they remain resistant to cultivation, very little is known about their physiology. Here we show that cold deep-seawater from the axial valley of Endeavour Segment on the Juan de Fuca Ridge contains not only groups I and II archaea as expected, but also unique potentially archaeal nitrogenase (nifH) genes, which are required for nitrogen fixation. These nifH genes are phylogenetically distinct and have dissimilar G+C content compared with those of hydrothermal vent archaea, suggesting that they belong to non-thermophilic deep-sea archaea. Furthermore, this sample did not contain mcrA genes, which are present in methanogens, the only known archaeal nitrogen fixers. These nifH genes were not detected in upper water column samples, or in a deep-seawater sample 100 km away from the spreading axis of the Juan de Fuca Ridge. We propose that these unique nifH genes may be localized to archaea that circulate through the nitrogen-poor subseafloor at the mid-ocean ridge as part of their life cycle.

In vitro, batch-dissolution of biogenic silica in seawater – the application of recent modelling to real data

Both for straight-forward oceanographic needs as well as application to climate-change amelioration, there is a need for quantitative description of the spatial and temporal variability in the recycled flux of silicic acid in the ocean water column in terms of governing processes. As part of that, the need for a fuller chemical kinetics treatment of biogenic silica dissolution, involving laboratory rate measurements, rate equations and mechanisms, is stressed. Advantage is taken of recent modelling of batch reactor dissolutions, which relies upon simple and sum of exponentials models. This is applied to archived data, and to some new data for Cyclotella cryptica, to stiffen an earlier four-point classification of dissolution behaviour which relies upon plots of ln{( C ∞ − C t )/ C ∞} versus time. The models are also used to demonstrate the potential for serious errors in the earlier rate constants derived from the batch approach. By extending recent modelling of non-linearity in the back reaction of the O’Connor and Greenberg equation, it is shown how simple exponential behaviour during dissolution might still be approximated, even though a net reaction is involved. The earlier approach is further extended to introduce non-linearity in the forward term of the O’Connor and Greenberg equation, caused by adsorption of silicic acid on the silica surface. The study of marine biogenic silica dissolution is reviewed in the wider context of general mineral dissolution, with an explanation of the relative merits of Transition State Theory used generally in mineral dissolution, and the more pragmatic approach to kinetics used here. The relative strength of chemo-stat (flow-through) and batch reactors is discussed.

Production of evanescent acoustic waves and their scattering by resonant targets

An environmentally-friendly liquid mixture, denser than water and immiscible with it, and with a sound speed slower than water’s, has been produced and incorporated into a scattering facility. This liquid mixture is submerged in a volume of water. In this way, the water above the denser fluid simulates a sandy ocean bottom, with the dense fluid simulating the ocean water column. When insonified by an acoustic beam from within the denser liquid, evanescent wave fields are produced in the water volume. Various targets and detectors are scanned in the water (the simulated sandy bottom) or in the mixture (the simulated ocean water column), and coupling of the evanescent waves to low-frequency resonances of cylindrical targets has been observed. Such coupling may reveal important structural characteristics of scatterers. Additionally, the complicated soundfield structure in the simulated bottom has been mapped. The structure is associated with the production of evanescent acoustic waves by a beam of sound. A model for this structure was developed using numerical wavenumber integration. [Work supported by the Office of Naval Research.]

Evidence for the presence of Mn(III) intermediates in the bacterial oxidation of Mn(II).

Bacterial oxidation of Mn(II) to Mn(IV) is believed to drive the oxidative segment of the global biogeochemical Mn cycle and regulates the concentration of dissolved Mn(II) in the oceanic water column, where it is a critical nutrient for planktonic primary productivity. Mn(II) oxidizing activity is expressed by numerous phylogenetically diverse bacteria and fungi, suggesting that it plays a fundamental and ubiquitous role in the environment. This important redox system is believed to be driven by an enzyme or enzyme complex involving a multicopper oxidase, although the biochemical mechanism has never been conclusively demonstrated. Here, we show that Mn(II) oxidation by spores of the marine Bacillus sp. strain SG-1 is a result of two sequential one-step electron transfer processes, both requiring the putative multicopper oxidase, MnxG, in which Mn(III) is a transient intermediate. A kinetic model of the oxidation pathway is presented, which shows that the Mn(II) to Mn(III) step is the rate-limiting step. Thus, oxidation of Mn(II) appears to involve a unique multicopper oxidase system capable of the overall two-electron oxidation of its substrate. This enzyme system may serve as a source for environmental Mn(III), a strong oxidant and competitor for siderophore-bound Fe(III) in nutrient-limited environments. That metabolically dormant spores catalyze an important biogeochemical process intimately linked to the C, N, Fe, and S cycles requires us to rethink the role of spores in the environment.

Double-diffusive fluxes of salt and heat in the upper layer of north pacific intermediate water

Almost half of the oceanic water columns exhibit double-diffusion. The importance of double-diffusion in global oceans’ salt and heat fluxes, water-mass formation and mixing, and circulation is increasingly recognized. However, such an important physical process in the ocean has not been well studied. One of the reasons is the difficulty of parameterizing and quantifying the processes. The paper presented here attempts to quantify the double-diffusive fluxes of salt and heat in the ocean. Previous qualitative analysis by applying the water-mass Turner angle, mTu, to the North Pacific Intermediate Water (NPIW) layer showed a favorable condition for salt-fingering in the upper NPIW due to the overlying warm/salty water above the cold/fresh NPIW core, and a doubly-stable condition in the lower NPIW where potential temperature decreases with depth while salinity increases, inducing double stratification with respect to both potential temperature and salinity. The present study gives a quantitative estimate of double-diffusive fluxes of salt and heat contributed by salt-fingering in the upper NPIW layer.