High molecular weight organic matter in seawater

The presence and nature of high molecular weight organic matter in seawater was critically reviewed and its biogeochemical cycle was discussed.Organic matter that passes through a filter of 0.5–1 μm pore size is called dissolved and that which does not pass through such a filter is defined as particulate. The size of colloidal particles ranges from 0.001 to 1 μm, and therefore, they are included in the dissolved fraction having high molecular weight.High molecular weight organic matter greater than 100,000 molecular weight was found in the seawater of Tokyo Bay. The values ranged from 0.1 to 1.5 mgC/l, and accounted for 8–45% of the total dissolved organic matter.Decomposition experiments on dissolved organic matter showed that macromolecular organic matter is refractory to bacterial attack. However, macromolecular organic materials tend to aggregate or adsorb on small particles to a sufficient size for precipitation. Organic aggregates thus formed sink to the bottom of the sea and bioelements included in them are removed from water column. High molecular weight organic materials are, therefore, considered to play an important role in transportation and distribution of matter in seawater.In order to elucidate the chemical and biological properties of macromolecular organic matter, concentration and isolation of this material are important, using methods such as adsorption on organic adsorbents or ultrafiltration.     

Isolation and characterization of low molecular weight carbohydrates dissolved in seawater

Chemical characterization and quantitative determination of dissolved low molecular weight carbohydrates in Mikawa Bay near Nagoya, Japan were conducted. The water samples were collected during the period of algal bloom of the dinoflagellate Prorocentrum minimum on 31 May 1980.Low molecular weight carbohydrates in seawater samples from depths of 1 and 6 m were first retained on a charcoal column and then eluted with aqueous ethanol. The carbohydrates obtained were permethylated and then isolated into each of the components by thin layer chromatography. The sugars isolated were characterized by gas chromatography (GC), combined gas chromatography and mass spectrometry (GC-MS), proton nuclear magnetic resonance spectroscopy (¹H-NMR) and some chemical analyses. Laminaribiose, laminaritriose, sucrose, raffinose,

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were fully characterized and quantified with ranges from 2.3 to 27.7 μg l⁻¹ and from 0.5 to 17.8 μg l⁻¹.These low molecular weight carbohydrates were also identified, with some difference in their relative abundance, in particulate matter consisting mainly of dinoflagellate cells collected on the same occasion from this bay. These results indicate that low molecular weight carbohydrates dissolved in seawater are directly derived from those of phytoplankton through extracellular release or cell lysis.     

Decomposition and alteration of organic compounds dissolved in seawater

The dissolved organic matter in seawater is grouped into two fractions which are defined as having greatly differing reactivities with respect to both chemical and biological decomposition. One fraction, which constitutes the bulk of the material, is extremely stable and inert and is only slowly degraded by either chemical or biological processes. The other fraction, which is composed of trace organic compounds derived from living organisms, contains components such as free amino acids which are rapidly degraded, probably by marine organisms which use these compounds as a food source. Conclusions about the cycle of organic compounds in seawater based on total organic carbon concentrations could thus be misleading, since pathways involving labile trace organic constituents would be completely obscured. Investigations of the distribution, concentration, and possible diagenetic reactions of specific organic compounds are necessary in order to elucidate the cycle of these compounds in the oceans.     

Binding of monomeric organic compounds to marcomolecular dissolved organic matter in seawater

Several monomeric organic compounds, including amino acids, sugars, and fatty acids, were found to bind abiologically to dissolved macromolecular materials in particle-free seawater at natural substrate concentrations. The binding primarily occurred in ocean surface waters, at rates slower than in situ biological utilization rates of most of the compounds. Seasonal patterns of binding in Gulf of Maine waters may have been related to seasonal variations in macroalgal exudation of polyphenolic materials. Enhanced reactivity of relatively hydrophobic monomers implicated hydrophobic effects as potentially important in marine organic condensations. The resultant condensates showed high particle reactivity, consistent with low concentrations of dissolved condensed materials in seawater.     

Photochemical production of low-molecular-weight carbonyl compounds in seawater and surface microlayer and their air-sea exchange

Coastal and oceanic surface microlayer samples were collected using a stainless steel screen, along with subsurface bulk seawater, and were analyzed for low-molecular-weight (LMW) carbonyl compounds, including formaldehyde, acetaldehyde, propanal, glyoxal, methylglyoxal, glyoxylic acid and pyruvic acid. The enrichment factor in surface microlayer compared to corresponding subsurface seawater ranged from 1.2 to 21. A time-series measurement at a coastal site showed strong diurnal variations in concentrations of the LMW carbonyl compounds in the surface microlayer and in the enrichment factor, with maxima in the early afternoon and minima in the early morning. Exposure of samples to sunlight resulted in the higher yields of these compounds in the surface microlayer than in the bulk seawater, by a factor of 1.1–25, suggesting that the higher photoproduction rate of LMW carbonyl compounds in the surface microlayer accounts for the majority of the observed enrichment in these samples. Potential sinks include biological uptake and mixing. Air-sea exchange may be a source for soluble compounds and a sink for less soluble compounds. The enrichment of the LMW carbonyl in surface microlayer may alter their net air-sea exchange direction e.g., from the ocean as a potential sink to a source for atmospheric acetaldehyde and acetone. The residence times of the LMW carbonyl compounds in the microlayer were estimated to be on the order of tens of seconds to minutes using a modified two-layer model. However, to maintain the observed microlayer enrichment factor, the residence time should be on the order of ˜ 1 hour. This prolonged residence time may be due to organic enrichment in the surface microlayer (‘organic film’) which inhibited molecular transfer of carbonyl compounds into and out of the microlayer. The deviated behavior from model prediction may also be due to changes in the apparent partition coefficients of these species as a result of thier physical and chemical interactions with organic matrix in the surface microlayer.     

Sources and sinks of methylgermanium in natural waters

New data have extended our understanding of the distribution and behavior of methylgermanium in the environment. Laboratory attempts to induce aerobic methylation with known biological and abiotic methylating agents were unsuccessful; this confirmed previous field observations of methylgermanium's unreactive behavior. However, biomethylation of inorganic germanium was observed in the anaerobic digestor of a sewage treatment plant, which suggested a terrestrial methanogenic source. Attempts to locate such a source in methanogenic swamps and their drainages reveal very low methylgermanium concentrations typical of other remote, pristine rivers. Polluted rivers have monomethylgermanium (MMGe) and dimethylgermanium (DMGe) concentrations 3–100 times higher than those of pristine rivers, which suggests an anthropogenic source of methylgermanium as a result of the synergistic effects of sewage treatment and coal-ash derived inorganic germanium contamination. A new high-precision profile of MMGe and DMGe in the Sargasso Sea shows conservative behavior with no vertical gradients. However, marine anoxic basins have both inorganic germanium enrichment and methylgermanium depletion, which suggests that of marine anaerobic processes are responsible for demethylating marine organogermanium. These results all suggest that methylgermanium is produced on the continents, is unreactive in the open ocean, and is destroyed in marine anoxic environments. The residence time of organogermanium in the sea, based on a continental source (pristine rivers), is at least 1 Ma, consistent with its unreactive nature, its observed distribution in the ocean, and rates of destruction in anoxic basins.     

Sources, pathways and sinks of particulate organic matter in Hudson Bay: Evidence from lignin distributions

Hudson Bay is a large, estuarine, shelf-like sea at the southern margin of the Arctic, where changes in seasonal ice cover and river discharge appear already to be underway. Here we present lignin data for dated sediments from eleven box cores and evaluate sources of terrigenous carbon, transport pathways, and whether terrigenous organic matter has been influenced by recent environmental change. Lignin yields (0.04 to 1.46 mg/100 mg organic carbon) decreased from the margin to the interior and from south to north, broadly reflecting the distribution of river inputs. Lignin compositional patterns indicated distinct regional sources with boreal forest (woody gymnosperm) vegetation an important source in the south, vs. tundra (non-woody angiosperm) in the north. Lignin patterns suggest redistribution of a fine-grained, mineral-associated fraction of the southern-derived terrigenous carbon to the northeast part of the Bay and ultimately into west Hudson Strait with the Bay's cyclonic coastal circulation. A small component of the carbon makes it to the central basins of Hudson Bay but most of the terrigenous organic material in that area appears to derive from resuspension of older, isostatically-rebounding coastal and inner shelf deposits. Most modern plant debris appears to be retained near river mouths due to hydrodynamic sorting, with the exception of the southwest inner shelf, where these materials extend > 30 km from shore. Temporal changes in the composition of terrigenous organic carbon recorded in most of the southern Hudson Bay cores perhaps reflect increases in erosion and cross-shelf transport from coastal deposits, possibly mediated by change in ice climate. In contrast, temporal changes in the northwest may relate to changes in the supply of modern plant debris under recent warmer conditions. On the western shelf, changes may relate to ice climate and the distribution of northern coastal water and/or changes in the delivery of materials by the Churchill River due to water diversion. Although the cores show evidence of change related to the ice climate, there is little evidence that ice itself transports terrigenous organic carbon within the system.     

Separation of naturally occurring high molecular weight metal complexes from seawater

A filtration technique using Sephadex G-10 gel was adapted for differentiating metal-containing substances with a molecular weight >700 from all other compounds in seawater. The recovery was found to be good, as the metal concentration could be determined with flameless Atomic Absorption Spectrometry. This method was employed during a cruise in the Peruvian upwelling region. It showed that Fe is predominantly complexed in biologically active zones, whereas Cu is bound to higher molecular weight substances only in rare cases.     

Molecular weight distribution of dissolved organic materials in seawater determined by ultrafiltration: a re-examination

Most of the dissolved organic materials (DOM) in coastal Maine, Gulf of Maine, and North Atlantic seawaters had molecular weights (MW), determined by ultrafiltration, less than 1000. Only 34% of the DOM had MW greater than 1000, only 6% had MW greater than 30 000, and only 1% was of MW greater than 100 000. The majority of ultrafilter-derived MW data are consistent with patterns shown here; earlier reports of high-MW DOM in seawater were apparently exaggerated due to procedural and ultrafilter artifacts. It is also shown that most ultrafiltration data do not clearly indicate loss of high-MW DOM during estuarine mixing, and that apparent removal could result from decreased retention as ionic strength increases.     

A review of the effect of salts on the solubility of organic compounds in seawater

The presence of electrolytes (salts) in aqueous solution modifies the solubility and related properties of organic compounds in water. Reported data for salting-out constants (Setschenow constants) which relate solubility to the salt concentration of aromatic and alkane hydrocarbons, and their chlorinated derivatives, and some organic acids have been compiled for 25 aqueous salt solutions at 20–25 °C. The salting-out sequences for various electrolytes are discussed and it is shown that the salting-out effect is greater for organic solutes with large molar volumes. A compilation of salting-out constants for NaCl solutions and seawater (natural or synthetic) with a variety of solutes, shows that the Setschenow constants are similar for natural or artificial seawater (at salinity of 30–35%.) and NaCl solutions (at 3.0–3.5% or 0.5 M). A simple correlation is suggested for estimating the Setschenow constants for a variety of organic solutes in seawater which typically yields a reduction in solubility by a factor of 1.36. The hydrophobicity of organic solutes is therefore increased by this factor, as is the air-water partition coefficient, implying an increased partitioning from aqueous solution into air, organic carbon and lipid phases. The effect must be quantified when comparing the behavior of organic contaminants in freshwater and marine conditions.     

Optical properties of low molecular weight and colloidal organic matter: Application of the ultrafiltration permeation model to DOM absorption and fluorescence

Cross-flow ultrafiltration (CFF) is often used to obtain separation and concentration of colloids from bulk natural water samples. Application of the ultrafiltration permeation model allows the quantitative determination of the low molecular weight material (LMW, < 1 kDa) and colloids in bulk dissolved organic matter (DOM) from measurements of time series permeate samples obtained from CFF. Detailed analysis of a Yukon River water sample shows that DOM absorption coefficient and fluorescence follow the permeation model and that the complex spectral optical properties of LMW DOM can be reconstructed from CFF data. A combination of measured and modeled data indicates that the LMW contribution to bulk DOM optical properties obtained from CFF can be grossly underestimated by the use of a low concentration factor (CF, the ratio of initial sample volume to retentate volume). Even at a relatively high CF of 19, optical properties of LMW DOM calculated from measurements of the retentate or integrated permeate would underestimate true values by 5–36%. In the Yukon River sample, LMW dissolved organic carbon represented 26% of the bulk concentration, but only 3–14% of the colored DOM was in the LMW fraction while 31–33% of bulk DOM florescence was due to LMW DOM. The contrasting optical properties of LMW and colloidal DOM support the concept that analysis of bulk DOM absorption and fluorescence properties reveals information about DOM molecular weight.     

LMWOA (low molecular weight organic acid) exudation by salt marsh plants: Natural variation and response to Cu contamination

This work aimed to evaluate, in vitro, the capability of roots of two salt marsh plants to release low molecular weight organic acids (LMWOAs) and to ascertain whether Cu contamination would stimulate or not organic acids exudation. The sea rush Juncus maritimus and the sea-club rush Scirpus maritimus, both from the lower Douro river estuary (NW Portugal), were used. Plants were collected seasonally, four times a year in 2004, during low tide. After sampling, plant roots were washed for removal of adherent particles and immersed for 2 h in a solution that matched salinity (3) and pH (7.5) of the pore water from the same location to obtain plant exudates. In one of the seasons, similar experiments were carried out but spiking the solution with different amounts of Cu in order to embrace the range between 0 and 1600 nM. In the final solutions as well as in sediment pore water LMWOAs were determined by high performance liquid chromatography. Plants were able to release, in a short period of time, relatively high amounts of LMWOAs (oxalate, citrate, malate, malonate, and succinate). In the sediment pore water oxalate, succinate and acetate were also detected. Therefore, plant roots probably contributed to the presence of some of these organic compounds in pore water. Exudation differed between the plant species and also showed some seasonally variation, particularly for S. maritimus. The release of oxalate by J. maritimus increased with Cu increase in the media. However, exudation of the other LMWOAs did not seem to be stimulated by Cu contamination in the media. This fact is compatible with the existence of alternative internal mechanisms for Cu detoxification, as denoted by the fact that in media contaminated with Cu both plant species accumulated relatively high amounts (29–83%) of the initially dissolved Cu. This study expands our knowledge on the contribution of globally dominant salt marsh plants to the release of LMWOAs into the environment.     

Sources and sinks of sediment to the Amazon margin: The Amapa coast

Modern and Holocene muddy strata were studied along the shoreline adjacent to the Amazon river mouth using sedimentological, radiochemical, physical, and seismic methods. The present paper is a synthesis of the results, collected during the AmasSeds project, that is used to outline a regional shoreline sediment budget. Erosion of relict Amazon muds in southern Amapa supplies 10⁶ tons yr^–1 to the Amazon advective mud stream. Local rivers are sediment-poor (total suspended discharge ~ 1 × 10⁶ tons yr^–1), but form depositional sandflats on the shoreface downdrift of the river mouths. Mudflat accumulation in northern Amapa sequesters 10⁶–10⁷ tons yr^–1 by tidal-flat aggradation, alongshore mudcape accretion, and sediment trapping by mangroves. The processes temporarily store 1.5 × 10⁸ tons of Amazon mud in January–June.     

Distributions of low molecular weight dicarboxylic acids in the North Pacific aerosol samples

Marine aerosol samples collected from the North Pacific atmosphere were studied for molecular distributions of dicarboxylic acids by using a capillary gas chromatography and mass spectrometery. A homologous series of dicarboxylic acids (C₂–C₁₀) was detected in the marine aerosol samples as dibutyl esters. All the samples showed that the smallest diacid (oxalic acid: C₂) was the most abundant and comprised 41–67% of the total diacids. The second most abundant species was malonic acid (C₃) or succinic acid (C₄). The diacids with more carbon numbers were generally less abundant. Total diacid concentration range was 17–1040 ng m^–3, which accounted for up to 1.6% of total aerosol mass. This indicates that low molecular weight dicarboxylic acids are important class of organic compounds in the marine atmosphere. The concentrations were generally higher in the western North Pacific and lower in the central North Pacific. The major portion of diacids is probably derived from the Asian Continent and East Asian countries by long-range atmospheric transport and partly fromin situ photochemical production in the marine atmosphere.     

The stability of molecular iodine in seawater

Both theoretical considerations and laboratory studies suggest that molecular iodine undergoes rapid hydrolysis to form hypoiodite when it is added to seawater. Previous experimental designs were not suitable for studying the fate of molecular iodine in seawater.     

Luminescence quenching of dissolved organic matter in seawater

To characterize more fully the nature of the fluorophores present in the dissolved organic matter found in seawater, steady state and time-resolved measurements of the luminescence quenching of a number of samples of marine dissolved organic matter with known quenchers, such as iodide, acrylamide and methyl viologen (MV) (1,1′-dimethyl-4,4′-bipyridinium), were compared. Quenching characteristics of these systems were analyzed using Stern-Volmer plots for both intensity and lifetime measurements. The bimolecular quenching constants, κ_q, for these quenchers were found to decrease in the order MV²⁺ (κ_q 10¹⁰M⁻¹s⁻¹) > I⁻ (κ_q 2 × 10⁹ M⁻¹ s⁻¹) >CH₂CHCONH₂ (κ_q 2 × 10⁸ M⁻¹ s⁻¹) for the samples measured. The results also show that different samples are quenched to differing extents by the quenchers studied, that ionic strength alters the quenching constants, and that both static and diffusional quenching mechanisms may operate.Such studies are appropriate to the quantification of the reactivity of the singlet states of the chromophores found within marine dissolved organic matter. Although excess energy of the singlet state may be readily transferred to another chemical species, the combination of competing physical deactivation paths and the low concentrations of efficient quenches in the oceans serves to lessen the direct chemical impact of this process.     

The fluorescence of dissolved organic matter in seawater

A total of 28 vertical profiles of seawater fluorescence was measured in the Sargasso Sea, the Straits of Florida, the Southern California Borderlands, and the central Pacific Ocean. In all cases, surface seawater fluorescence was low as a result of photochemical bleaching which occurs on the timescale of hours. Fluorescence of deep water was 2–2.5 times higher than that of surface waters, and was constant, implying a long residence time for fluorescent organic matter, possibly of the order of thousands of years. Fluorescence correlates well with nutrients (NO₃⁻, PO₄³⁻) in mid-depth waters (100–1000 m) in the Sargasso Sea and the central North Pacific, consistent with results in the central Pacific and the coastal seas of Japan. This suggests that regeneration or formation of fluorescent materials accompanies the oxidation and remineralization of settling organic particles.The various sources and sinks of fluorescent organic matter in the global oceans are assessed. The major sources are particles and in situ formation; rivers, rain, diffusion from sediments, and release from organisms are minor sources. The major sink is photochemical bleaching.     

Evidence for organic complexation of iron in seawater

Iron occurs at very low concentrations in seawater of oceanic origin and its low abundance is thought to limit primary production in offshore waters (Martin and Fitzwater, 1988). A new electrochemical method, cathodic stripping voltammetry (CSV), is used here to determine the speciation of iron in seawater originating from the Western Mediterranean taking advantage of ligand competition of an added electroactive ligand with the natural organic complexing matter to evaluate whether iron is organically complexed. The measurements indicate that iron occurs 99% (or 99.9% depending on which value is selected for α_Fe′) complexed by organic complexing ligands throughout the water column of the Western Mediterranean and by analogy probably also in other oceanic waters. The composition of the organic complexing ligands is as yet unknown, but the data indicate a major source from microorganisms (bacteria or phytoplankton) in and immediately below the fluorescence maximum in the upper water column. The organic complexes are apparently reversible releasing iron when the competing ligand is added and binding more iron when its concentration is increased. The organic complexing ligands occur at concentrations well above those of iron ensuring full complexation of this biologically essential element, and buffer the free iron concentration at a very low level against fluctuations as a result of removal by primary producers or inputs from atmospheric sources. The new data indicate that a re-evaluation of the concept of the bioavailable fraction of iron is required.     

低分子量琼胶的抗氧化活性研究

采用化学发光法研究了低分子量琼胶清除羟基自由基(·OH)和超氧阴离子(O2·-)的活性。结果表明,4个组均有清除自由基的作用,其中A3的清除羟基自由基(·OH)的能力比阳性对照硫脲的效果好得多。S2-2,S2-3,S2-4,A3对超氧阴离子(O2·-)和羟基自由基的IC50值分别为:3.7,1.3,0.96,0.83g/L和1.94,1.28,0.85,0.26g/L。