Seawater Dissolved Organic Matter Research Articles

Dissolved organic matter released from beach wrack is source-specific and molecularly highly diverse

Beach wrack is an important supplier of nutrients and organic matter to sandy beach ecosystems and underlying subterranean estuaries (STEs), producing metabolic hotspots in these otherwise organic carbon- and nutrient-poor environments. To assess the impact of beach wrack type (e.g., marine, terrestrial, plant, animal) and environmental settings (e.g., tidal inundation, precipitation, and solar irradiation) on nutrient and dissolved organic matter (DOM) release, a series of leaching experiments was conducted. Quantities of leached nutrients and dissolved organic carbon (DOC) were determined, and DOM molecular composition was investigated using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Millimolar—to molar amounts of DOC and dissolved nitrogen were released from the beach cast per kg dry weight, with type of wrack and leaching medium (fresh- vs. saltwater) exerting the biggest influences. Exemplary for animal cast, jellyfish leached up to two 100-fold more, mostly organic, nitrogen compared to all other beach wrack types. FT-ICR-MS data of solid-phase extracted DOM indicated that beach wrack releases compounds with putative mono- and oligosaccharide-, amino acid- and vitamin-type molecular formulae, which likely serve as valuable substrate for heterotrophic microorganisms. DOM from the brown seaweed Fucus sp. was more aromatic than seawater DOM and even beach wrack of terrestrial origin, probably from structural components and secondary metabolites such as phlorotannins. We conclude that DOM and nutrient release from beach wrack strongly depends on wrack type and leaching medium, may obscure molecular provenance proxies (e.g., terrestrial indices), and adds a nutritional boost to infiltrating sea- and rainwater which likely impact microbial respiration rates in the STE.

Comparison of Coagulation-Integrated Sand Filtration and Ultrafiltration for Seawater Reverse Osmosis Pretreatment.

The removal of dissolved organic matter (DOM) from seawater before the reverse osmosis (RO) processes is crucial for alleviating organic fouling of RO membranes. However, research is still insufficiently developed in the comparison of the effectiveness of integrating coagulation with ultrafiltration (UF) or sand filtration (SF) in the pretreatment stage of seawater reverse osmosis (SWRO) for the removal of DOM. In this study, we investigated the effect of pretreatment technologies on RO fouling caused by DOM in seawater, including the integration of coagulation and sand filtration (C-S pretreatment) and the integration of coagulation and ultrafiltration (C-U pretreatment). Both integrated pretreatments achieved comparable DOM removal rates (70.2% for C-U and 69.6% for C-S), and C-S exhibited enhanced removal of UV-absorbing compounds. Although C-U was more proficient in reducing the silt density index (below 2) compared to C-S (above 3) and improved the elimination of humic acid-like organics, it left a higher proportion of tyrosine-protein-like organics, soluble microbial by-product-like organics, and finer organics in the effluent, leading to the formation of a dense cake layer on RO membrane and a higher flux decline. Therefore, suitable technologies should be selected according to specific water conditions to efficiently mitigate RO membrane fouling.

Unraveling sources of cyanate in the marine environment: insights from cyanate distributions and production during the photochemical degradation of dissolved organic matter

Cyanate is a nitrogen and energy source for diverse marine microorganisms, playing important roles in the nitrogen cycle. Despite the extensive research on cyanate utilization, the sources of this nitrogen compound remain largely enigmatic. To unravel the sources of cyanate, distributions and production of cyanate during photochemical degradation of natural dissolved organic matter (DOM) were investigated across various environments, including freshwater, estuarine, coastal areas in Florida, and the continental and slope regions of the North American mid-Atlantic Ocean (NATL). Cyanate production was also examined during the photochemical degradation of exudates from a typical strain of Synechococcus, an important phytoplankton component. To deepen our understanding of the sources and production mechanisms of cyanate, its production was assessed during the photochemical degradation of a natural seawater DOM supplemented with five nitrogen–containing compounds with distinguishing structures and functional groups. Generally, cyanate exhibited higher concentrations in the Florida coastal, estuarine, and freshwater environments than the NATL. However, cyanate distribution did not consistently align with its production rates. Despite significantly low concentrations in the NATL, DOM from this region exhibited cyanate production rates comparable to estuarine and Florida coastal environments. Although relatively high cyanate concentrations were observed in the freshwaters, DOM in this environment exhibited very low cyanate production rates. A highly significant correlation was observed between cyanate and chlorophyll a (Chl a) concentrations in these areas. Moreover, in most estuarine and NATL stations, cyanate concentration and production rate in the Chl a maximum layer were significantly higher than in other layers. Cyanate was produced during the photochemical degradation of the Synechococcus exudates. The cyanate production was significantly enhanced when the natural seawater DOM was supplemented with GlycylGlycine, 4-(methylamino) benzoic acid, 4-[ethyl(methyl)amino] benzaldehyde or methyl 2-aminobenzoate. Our study implies that photochemical degradation of marine DOM, especially phytoplankton-derived DOM, is a substantial source of cyanate in the ocean. Additionally, cyanate may form during the degradation of peptides and small aromatic compounds in DOM, providing novel insights into the nitrogen cycle.

Molecular composition of dissolved organic matter across diverse ecosystems: Preliminary implications for biogeochemical cycling

Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely applied to characterize the molecular composition of dissolved organic matter (DOM) in different ecosystems. Most previous studies have explored the molecular composition of DOM focused on one or a few ecosystems, which prevents us from tracing the molecular composition of DOM from different sources and further exploring its biogeochemical cycling across ecosystems. In this study, a total of 67 DOM samples, including soil, lake, river, ocean, and groundwater, were analyzed by negative-ion electrospray ionization FT-ICR MS. Results show that molecular composition of DOM varies dramatically among diverse ecosystems. Specifically, the forest soil DOM exhibited the strongest terrestrial signature of molecules, while the seawater DOM showed the most abundant of biologically recalcitrant components, for example, the carboxyl-rich alicyclic molecules were abundant in the deep-sea waters. Terrigenous organic matter is gradually degraded during its transport along the river-estuary-ocean continuum. The saline lake DOM showed similar DOM characteristics with marine DOM, and sequestrated abundant recalcitrant DOM. By comparing these DOM extracts, we found that human activities likely lead to an increase in the content of S and N-containing heteroatoms in DOM, this phenomenon was commonly found in the paddy soil, polluted river, eutrophic lake, and acid mine drainage DOM samples. Overall, this study compared molecular composition of DOM extracted from various ecosystems, providing a preliminary comparison on the DOM fingerprint and an angle of view into biogeochemical cycling across different ecosystems. We thus advocate for the development of a comprehensive molecular fingerprint database of DOM using FT-ICR MS across a wider range of ecosystems. This will enable us to better understand the generalizability of the distinct features among ecosystems.

A snapshot on vertical variability of dissolved organic matter in the epilagic zone of the eastern Indian Ocean

Marine dissolved organic matter (DOM) plays an important role in aquatic ecosystems and is an essential reservoir of organic carbon in the marine carbon cycle. In this study, seawater DOM samples from 33 stations were collected in spring 2022 (April to May, 20 stations) and autumn 2020 (October to November, 13 stations) to better characterize and compare DOM variability within 200 m water layer in the eastern Indian Ocean (EIO). Hydrological parameters, nutrients and spectroscopic indices were calculated and evaluated for two cruises. In addition, excitation emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) was used to directly analyse seawater DOM samples. The sources and processes of DOM in the EIO were assessed by fluorescence index (FI), freshness index (β/α), Biological index (BIX), and humification index (HIX). Three fluorescent components were identified in DOM samples from two cruises, including: C1 (tryptophan- and tyrosine-like), C2 (marine and/or terrestrial humic-like), and C3 (terrestrial humic-like). The components of C1 accounted for 39.45 % ± 8.79 %, C2 for 33.05 % ± 6.42 %, and C3 for 27.20 % ± 4.47 % within 200 m water layer. The intensity of the DOM fluorescence seems to varied due to seasonal differences. In particular, the source of the DOM fraction varied at <100 m layer, which may also be related to the structure of the microbial community. Further, there is a strong correlation between the depth of seawater and hydrographic parameters, fluorescence indices and fluorescence components. Nutrients (nitrate, dissolved inorganic phosphate, and dissolved silicate) and humic-like fractions are more likely to accumulate in the deeper layers of the ocean. Thus, these results provide some data support for the variability of DOM fractions on a vertical scale in the EIO.

Quantification of groundwater versus fish-farm sources of nutrients in the coastal water off Jeju Island, Korea, using fluorescent dissolved organic matter as a tracer

The concentrations of nutrients and dissolved organic matter (DOM) in the coastal waters off Jeju Island, Korea, are known to be influenced mainly by submarine groundwater discharge (SGD) and fish-farm effluents, in addition to the background open-ocean seawater from the Tsushima Current. However, the differentiation of the effect of fish-farm effluents from SGD is very challenging since most of the farms in this island utilize groundwater as a rearing water. Thus, in this study, we measured the concentrations of nutrients and fluorescent DOM (FDOM) in seawater, fresh groundwater (FGW), and farm water in two coastal areas off Jeju Island, in May and August 2021, in order to determine the effect of SGD versus fish-farm effluents. The results showed that humic-like FDOM (FDOMH) and NH4+ concentrations were greatly enriched owing to organic matter decomposition in the farm waters. We used FDOMH, as a tracer for the farm effluent. The end-member mixing model for FDOMH and salinity showed that FGW contributed about 73% of NO3− (avg. 17 ± 20 μM), whereas the farm effluent contributed about 95% of NH4+ (avg. 5.6 ± 4.1 μM) and 71% of FDOMH (avg. 0.86 ± 0.49 R.U.) in the coastal waters. Thus, our result shows that fish-farm effluents can be an important source of anthropogenic DOM and nutrients in coastal waters, where farms are being operated intensively.

The detection of bacterial exometabolites in marine dissolved organic matter through ultrahigh‐resolution mass spectrometry

AbstractBacteria play a key role in sustaining the chemodiversity of marine dissolved organic matter (DOM), yet there is limited direct evidence of a major contribution of bacterial exometabolites to the DOM pool. This study tests whether molecular formulae of intact exometabolites can be detected in natural DOM via untargeted Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). We analyzed a series of quantitative mixtures of solid‐phase extracted DOM from the deep ocean, of a natural microbial community and selected model strains of marine bacteria. Under standard instrument settings (200 broadband scans, mass range 92–1000 Da), 77% of molecular formulae were shared between the mesocosm and marine DOM. However, there was &lt; 10% overlap between pure bacterial exometabolome with marine DOM, and in mixing ratios closest to mimicking natural environments (1% bacterial DOM, 99% marine DOM), only 4% of the unique bacterial exometabolites remained detectable. Further experiments with the bacterial exometabolome DOM mixtures using enhanced instrument settings resulted in increased detection of the exometabolites at low concentrations. At 1000 and 10,000 accumulated scans, 23% and 29% of the unique molecular formulae were detectable at low concentrations, respectively. Moreover, windowing a specific mass range encompassing a representative fraction of exometabolites tripled the number of unique detected formulae at low concentrations. Routine FT‐ICR‐MS settings are thus not always sufficient to distinguish bacterial exometabolome patterns from a seawater DOM background. To observe these patterns at higher sensitivity, we recommend a high scan number coupled with windowing a characteristic region of the molecular fingerprint.

Sponges With Microbial Symbionts Transform Dissolved Organic Matter and Take Up Organohalides

Seawater dissolved organic matter (DOM) is a large reservoir of carbon composed of a complex and poorly characterized mixture of molecules. Sponges have long been known to consume dissolved organic carbon (DOC) from this mixture, but the role of microbial sponge symbionts in this process is complex, and the molecules involved remain largely unknown. In order to better understand how sponge processing changes seawater DOM, we used untargeted metabolomics to characterize DOM in samples of incurrent and excurrent seawater taken from sponges on the fore-reef off Carrie Bow Cay, Belize, over 2 years. We collected samples from three sponge species each with either high or low microbial abundance (HMA, LMA) to explore the relationship between symbiont abundance and DOM alterations. Analyses revealed that sponges took up metabolites and changed the composition of seawater DOM, but only for the three HMA species, and none of the LMA species, implicating microbial symbionts in this uptake. Using a new mass spectra classification tool, we found that putative compositions of features depleted in the excurrent samples of HMA sponges were similar in both years and were dominated by organic acids and derivatives (74%) and organic nitrogen compounds (19%). Interestingly, HMA sponges also took up halogenated compounds (containing chlorine or bromine), providing evidence of a previously unknown mechanism of halide cycling. The metabolites taken up by HMA sponges may be used as a food source or as building blocks of chemical defenses, selective advantages that may have guided the evolution of microbial symbioses in sponges.

Photo-induced degradation and toxicity change of decabromobiphenyl ethers (BDE-209) in water: Effects of dissolved organic matter and halide ions

BDE-209 is a widely used brominated flame retardant that is ubiquitous in the aquatic environment, especially in marine water. However, photodegradation of BDE-209 in seawater is still not fully understood. In this work, the photodegradation kinetics of BDE-209 in water was studied and the effects of seawater dissolved organic matter (S-DOM) and halide ions (Cl−, Br−) were evaluated. S-DOM inhibited the degradation of BDE-209 through dynamic quenching and light shielding effect. However, with the coexistence of S-DOM, Cl− and Br−, the photodegradation of BDE-209 was significantly promoted. The promotional effect is attributed to the generation of excited triplet state S-DOM, singlet oxygen, and reactive halogen radicals. The results of density functional theory calculation showed that •Cl addition reaction on C-Br sites of BDE-209 is the main reaction pathway of BDE-209 with chlorine radical, which leads to the generation of mixed Cl/Br substituted intermediates. The acute toxicity and estrogenic effects of BDE-209 solution were enhanced during simulated sunlight irradiation. These results indicate that the environmental factors in seawater play important roles in the photodegradation of BDE-209, and contribute to the potential ecological risks of PBDEs in the marine environment.

Photochemistry of dissolved organic matter extracted from coastal seawater: Excited triplet-states and contents of phenolic moieties

Coastal seawater constitutes an important ecosystem receiving inputs of organic micropollutants (OMPs) such as sulfa antibiotics from land-based sources or mariculture activities. It is necessary to investigate photodegradation of OMPs in coastal seawaters for assessing their environmental fate and risks. However, effects of coastal seawater dissolved organic matter (S-DOM) on OMPs photodegradation are largely unknown, given that chemical compositions of S-DOM are different from those of freshwater DOM. Herein, photochemical characteristics of S-DOM extracted from Dalian coastal seawaters were investigated by simulating photochemical experiment adopting sulfachloropyridazine as a case. Results show that S-DOM accelerates the photodegradation mainly through excited triplet-state DOM (3DOM*) with an apparent rate constant (4.43 × 108 M−1 s−1) ten folds of that of freshwater DOM, which is mainly due to much lower phenol contents detected in the S-DOM (0.022 mg-Gallic acid mg-C−1). The S-DOM impacted by mariculture can photogenerate more high-energy 3DOM* than those less impacted by mariculture, further contributing to the high 3DOM* reactivity. The study shows that to accurately predict photolytic persistence of OMPs in field water bodies, it is of significance to determine the second-order reaction rate constants between 3DOM* and target OMPs using DOM extracted from relevant water bodies.

Effects of dissolved organic matter on photochemicaltransformation of organic micropollutants in natural waters

<p indent="0mm">Organic micropollutants (OMPs) are ubiquitous in natural water bodies and can have negative impacts to health of aquatic ecosystems and human beings. It is of importance to investigate fate of the OMPs so as to assess their environmental risks. Photodegradation is an important pathway to transform the OMPs in natural water bodies, including direct photolysis and indirect photodegradation induced by photosensitizers. Dissolved organic matter (DOM) is an important photosensitizer that is ubiquitous in water bodies. DOM plays an important role in photodegradation of the OMPs especially for those recalcitrant to direct photolysis. On one hand, DOM can produce photochemically produced reactive intermediates (PPRIs), e.g., hydroxyl radical (HO^•), singlet state oxygen (¹O₂), and excited triplet-state DOM (³DOM*), after absorbing solar irradiation. The PPRIs are capable of inducing indirect photodegradation of the various OMPs. On the other hand, DOM can exhibit inhibition effects (light screening effects and quenching effects) on photodegradation of the OMPs. Chromophores in DOM can compete photons with the OMPs and thus inhibit direct photolysis of some OMPs, exhibiting the light screening effects. Antioxidants like phenolic moieties in DOM can quench the formed PPRIs and reduce the reactive intermediates of OMPs to the parent OMPs, manifesting the quenching effects. Generally, the effects of DOM on photodegradation of the OMPs depend on sources of DOM. DOM can derive from autochthonous (microbial) and allochthonous (terrestrial) sources based on its forming processes. Many previous studies on DOM mainly focused on formation pathways of PPRIs, reactivity of ³DOM* from different sources, and effects of different DOM on photodegradation of the OMPs. These efforts in early phase laid a foundation for emerge of the discipline of environmental (aquatic) photochemistry. In recent decades, many studies focused on photochemical properties of autochthonous and allochthonous DOM extracted from freshwater bodies. Very limited studies focused on seawater DOM (S-DOM). S-DOM may undergo longer period of photobleaching than freshwater DOM. Due to sources of riverine inputs, atmospheric depositions, wastewater discharges and mariculture activities, S-DOM in coastal seawaters can be quite different from S-DOM in distant seas or DOM in inland freshwater bodies. Therefore, S-DOM may potentially exhibit photochemical properties different from that of freshwater DOM. As seawater is a final sink of many OMPs, it is necessary to investigate photochemical property of S-DOM and its effects on photodegradation of the OMPs. Some recent studies did prove that S-DOM and freshwater DOM has different photochemical reactivities towards some OMPs. Data on environmental photodegradation kinetics are necessary for assessing fates, persistence and ecological risks of the OMPs. However, translation and extrapolation of reaction rates from environment to environment and laboratory to environment is a challenging task. Underwater downward sunlight attenuation in field water bodies should be taken into consideration when estimating photolytic persistence of the OMPs. More research efforts are needed to develop prediction models on photodegradation kinetics of OMPs in different field water bodies, and to develop prediction models (such as quantitative structure-activity relationship models that can predict second-order reaction rate constants between OMPs and PPRIs) on photodegradation kinetics parameters of the vast and ever-increasing number of OMPs (or environmental organic chemicals). In this review, the recent progresses in aquatic photochemistry of DOM were summarized into three aspects, mechanisms for photogeneration of PPRIs from DOM of different sources, effects of DOM from different sources on photodegradation of OMPs, and photodegradation kinetics prediction models of OMPs in natural water bodies. In addition, potential differences in the effects of freshwater DOM and S-DOM on photodegradation of OMPs were discussed. Based on the research progresses, future efforts in the field of aquatic environmental photochemistry were suggested.

Production of tyrosine-like fluorescence and labile chromophoric dissolved organic matter (DOM) and low surface accumulation of low molecular weight-dominated DOM in a productive Antarctic sea

The Antarctic seas play critical roles in global carbon cycling. Yet, little is known about the dissolved organic matter (DOM) characteristics and the dynamics there. Here, we conducted an extensive study on the seawater DOM in the Amundsen Sea in the Pacific sector of the Southern Ocean. We found that low molecular weight fractions quantitatively dominated the DOM composition at the surface of the highly productive Amundsen Sea Polynya with the relative abundance reaching up to ~89%. Moreover, CDOM and tyrosine-like fluorescence generation were observed, with the average values of~2.4–2.6 m−1 (a254) and ~0.3 RU, respectively. While there is a net positive accumulation of dissolved organic carbon (DOC) at the ocean's surface, the net accumulation was negative for the chromophoric DOM (CDOM), which suggests a labile nature for the freshly produced CDOM. The estimated net DOC production ratio was only ~9 ± 6%, which was less than the global level (~17%). This finding signified a low surface accumulation of DOM in the austral summer, which is potentially explained by its nonlimiting nutrients, photo- and/or bio-labile nature of produced DOM, and long water residence time.

Thermally altered marine dissolved organic matter in hydrothermal fluids

Hydrothermal vent fluids contain thermally modified dissolved organic matter (DOM) originally entrained from sediments and seawater. We hypothesized that in hydrothermal systems DOM molecular composition is modulated by (i) fluid contribution, (ii) thermal decomposition and pH, and (iii) aspects particular to the vent system. Hence, solid phase extracted (SPE) DOM samples collected along the Mid-Atlantic Ridge (MAR) were molecularly characterized via 15 Tesla Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The molecular character of an oceanic DOM sample was also determined before and after thermal (300°C) decomposition at acidic and neutral pH. Multivariate statistical analysis indicated that DOM composition was strongly influenced by fluid contribution (Mg2+ concentration between 12 and 54mM), which correlated positively with measured temperature (between 8 and 375°C). In comparison, pH of the fluids (between 2.5 and 6.9) had a minor influence. Seafloor pressure, used as a theoretical maximum fluid temperature at the seafloor, separated the fluids collected at Menez Gwen from deeper locations, due to the higher abundance of peptide, carboxyl-rich alicyclic (CRAM) and aromatic molecular formulae at Menez Gwen. Compared with seawater DOM, thermally decomposed DOM had on average lower molecular mass, lower O/C ratios, fewer double bond equivalents, and fewer CRAM formulae but higher aromaticity - the same molecular features displayed by MAR hydrothermal fluids. The study provides evidence that thermal reworking plays a major role in shaping DOM mixtures from hydrothermal fluids along the MAR, which partly represent thermally reworked marine DOM that survived hydrothermal circulation.

Probing and Comparing the Photobromination and Photoiodination of Dissolved Organic Matter by Using Ultra-High-Resolution Mass Spectrometry.

Photochemical halogenation of dissolved organic matter (DOM) may represent an important abiotic process for the formation of natural organobromine compounds (OBCs) and natural organoiodine compounds (OICs) within surface waters. Here we report the enhanced formation of OBCs and OICs by photohalogenating DOM in freshwater and seawater, as well as the noticeable difference in the distribution and composition pattern of newly formed OBCs and OICs. By using negative ion electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry, various OBCs and OICs were identified during the photohalogenation processes in sunlit waters. The respective number of OBCs and OICs formed in artificial seawater (ASW) under light radiation was higher than that in artificial freshwater (AFW), suggesting a possible role of the mixed reactive halogen species. OBCs were formed mainly via substitution reactions and addition reactions accompanied by other reactions and distributed into three classes: unsaturated hydrocarbons with relatively low oxygen content, unsaturated aliphatic compounds, and saturated fatty acids and carbohydrates with relatively high hydrogen content. Unlike the OBCs, OICs were located primarily in the region of carboxylic-rich alicyclic molecules composed of esterified phenolic, carboxylated, and fused alicyclic structures and were generated mainly through electrophilic substitution of the aromatic proton. Our findings call for further investigation on the exact structure and toxicity of the OBCs and OICs generated in the natural environment.

Molecular Fractionation of Dissolved Organic Matter in a Shallow Subterranean Estuary: The Role of the Iron Curtain.

Iron that precipitates under aerobic conditions in natural aquatic systems scavenges dissolved organic matter (DOM) from solution. Subterranean estuaries (STEs) are of major importance for land–ocean biogeochemical fluxes. Their specific redox boundaries, coined the “iron curtain” due to the abundance of precipitated iron(III) (oxy)hydroxides, are hot spots for the removal and redissolution of iron, associated nutrients, and DOM. We used ultra-high-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry to molecularly characterize the iron-coagulating fractions of 32 groundwater and seawater DOM samples along a salinity gradient from a shallow STE on Spiekeroog Island, North Sea, Germany, and linked our findings to trace metal and nutrient concentrations. We found systematic iron coagulation of large (>450 Da), oxygen-rich, and highly aromatic DOM molecules of terrestrial origin. The extent of coagulation increased with growing terrestrial influence along the salinity gradient. Our study is the first to show that the iron curtain may be capable of retaining terrigenous DOM fractions in marine sediments. We hypothesize that the iron curtain serves as an inorganic modulator for the supply of DOM from groundwaters to the sea, and that the STE has the potential to act as a temporal storage or even sink for terrigenous aromatic DOM compounds.

Quantification and characterization of coastal dissolved organic matter by high‐performance size exclusion chromatography with ultraviolet absorption, fluorescence, and total organic carbon analyses

AbstractMolecular size and dissolved organic carbon (DOC) distributions of dissolved organic matter (DOM) in seawater were determined qualitatively and quantitatively by high‐performance size exclusion chromatography (HPSEC). To estimate the relationship between molecular size distribution and the DOC concentration of seawater DOM, seawater was desalinated with an electric dialyzer. The recent technological advancements of desalination make us possible to recover a high percentage of seawater DOM. Surface coastal seawater samples were collected from three locations in Japan: Tokyo Bay, Kashima Port, and Cape Inubo. Samples were evaluated by a chromatography system coupled with ultraviolet (UV) absorbance, fluorescence, and non‐dispersive infrared total organic carbon detectors in series. Two peaks, 1 (1.24 × 105−1.54 × 105 Da) and 2 (9.48 × 102−1.81 × 103 Da), were detected in all sample chromatograms. Peak 1 showed little fluorescence and UV absorption in all samples. It has been shown that carbohydrates, one of the main components of seawater DOM, exhibit almost no fluorescence or UV absorption; thus, Peak 1 could mainly consist of carbohydrates which are usually labile DOM. Total DOC concentrations of the Tokyo Bay, Kashima Port, and Cape Inubo were 1.38, 0.92, and 0.80 mg C L−1, respectively. Peak 2 accounted for 75–80% of the total DOC and showed a substantial humic‐like fluorescence and UV absorption in all samples. These results indicate that the photochemical characteristics of seawater DOM significantly differ depending on its molecular size. Thus, analysis of molecular size with our HPSEC provides a new approach for quantitative and qualitative characterization of seawater DOM.

Simple, quantitative method for low molecular weight dissolved organic matter extracted from natural waters based upon high performance counter-current chromatography

A simple, high-performance counter-current chromatography method with sequential UV absorbance (254 nm) and evaporative light scattering detection (ELSD) was developed for the quantification of pre-extracted low molecular weight dissolved organic matter (DOM) extracted from natural waters. The method requires solid-phase extraction (SPE) extraction of only small volumes of water samples, here using poly(styrenedivinylbenzene)-based extraction cartridges (Varian PPL). The extracted and concentrated DOM was quantified using reversed-phase high-performance counter-current chromatography (HPCCC), with a water/methanol (5:5) mobile phase and hexane/ethyl acetate (3:7) stationary phase. The critical chromatographic parameters were optimised, applying a revolution speed of 1900 rpm and a flow-rate of 1 mL min−1. Under these conditions, 50 μL of extracted DOM solution could be injected and quantified using calibration against a reference natural dissolved material (Suwannee River), based upon UV absorbance at 254 nm and ELSD detection. Both detection methods provided excellent linearity (R2 > 0.995) for DOM across the concentration ranges of interest, with limits of detection of 4 μg ml−1 and 7 μg ml−1 for ELSD and UV absorbance, respectively. The method was validated for peak area precision (<5%), and accuracy and recovery based upon spiking seawater samples prior to extraction, together with DOM solutions post-extraction (>95% recovery). The developed method was applied to the determination of the concentration of DOM in seawater, based upon initial sample volumes as small as 20 mL.

Marine sequestration of carbon in bacterial metabolites.

Linking microbial metabolomics and carbon sequestration in the ocean via refractory organic molecules has been hampered by the chemical complexity of dissolved organic matter (DOM). Here, using bioassay experiments and ultra-high resolution metabolic profiling, we demonstrate that marine bacteria rapidly utilize simple organic molecules and produce exometabolites of remarkable molecular and structural diversity. Bacterial DOM is similar in chemical composition and structural complexity to naturally occurring DOM in sea water. An appreciable fraction of bacterial DOM has molecular and structural properties that are consistent with those of refractory molecules in the ocean, indicating a dominant role for bacteria in shaping the refractory nature of marine DOM. The rapid production of chemically complex and persistent molecules from simple biochemicals demonstrates a positive feedback between primary production and refractory DOM formation. It appears that carbon sequestration in diverse and structurally complex dissolved molecules that persist in the environment is largely driven by bacteria.

Sunlight-driven photochemical halogenation of dissolved organic matter in seawater: a natural abiotic source of organobromine and organoiodine.

Reactions of dissolved organic matter (DOM) with photochemically generated reactive halogen species (RHS) may represent an important natural source of organohalogens within surface seawaters. However, investigation of such processes has been limited by difficulties in quantifying low dissolved organohalogen concentrations in the presence of background inorganic halides. In this work, sequential solid phase extraction (SPE) and silver-form cation exchange filtration were utilized to desalt and preconcentrate seawater DOM prior to nonspecific organohalogen analysis by ICP-MS. Using this approach, native organobromine and organoiodine contents were found to range from 3.2-6.4 × 10(-4) mol Br/mol C and 1.1-3.8 × 10(-4) mol I/mol C (or 19-160 nmol Br L(-1) and 6-36 nmol I L(-1)) within a wide variety of natural seawater samples, compared with 0.6-1.2 × 10(-4) mol Br/mol C and 0.6-1.1 × 10(-5) mol I/mol C in terrestrial natural organic matter (NOM) isolates. Together with a chemical probe method specific for RHS, the SPE+ICP-MS approach was also employed to demonstrate formation of nanomolar levels of organobromine and organoiodine during simulated and natural solar irradiation of DOM in artificial and natural seawaters. In a typical experiment, the organobromine content of 2.1 × 10(-4) mol C L(-1) (2.5 mg C L(-1)) of Suwannee River NOM in artificial seawater increased by 69% (from 5.9 × 10(-5) to 1.0 × 10(-4) mol Br/mol C) during exposure to 24 h of simulated sunlight. Increasing I(-) concentrations (up to 2.0 × 10(-7) mol L(-1)) promoted increases of up to 460% in organoiodine content (from 8.5 × 10(-6) to 4.8 × 10(-5) mol I/mol C) at the expense of organobromine formation under the same conditions. The results reported herein suggest that sunlight-driven reactions of RHS with DOM may play a significant role in marine bromine and iodine cycling.

Evaluation of tangential flow ultrafiltration procedures to assess trace metals bound to marine dissolved organic matter

A procedure based on two-dimensional size exclusion chromatography (SEC) and anion exchange chromatography (AEC) with UV (205nm) and ICP-MS detection was used to assess dissolved organic matter (DOM) and trace metals associated to DOM in surface seawater. Marine DOM was isolated by tangential ultrafiltration (UF) using two different polyethersulfone membranes exhibiting different molecular weight cut-off (MWCO), 3 and 10kDa. The procedures require a volume of seawater sample of 100L, and marine DOM of molecular weight higher than 3 and 10kDa (UF membranes of 3 and 10kDa MWCO) was pre-concentrated in 0.5L (retentate), which implies a pre-concentration factor of 200. Retentate fractions obtained after the different UF procedures were further desalted by using HI Trap desalting mini-columns before two-dimensional SEC/AEC. Apparent molecular weights of isolated compounds after SEC with UV detection ranged from 1.5kDa (close to the permeable volume of the SEC column fixed by injecting vitamin B12) to 16 and 22kDa (UF membranes of 3 and 10kDa MWCO, respectively). Further AEC/UV characterization of SEC fractions showed a large group of macromolecules eluted at 4.5min, and small signals at shorter retention times (2.5 and 3.5min). In addition, AEC experiments of the isolated SEC fractions when using 10kDa MWCO UF membranes showed a group of substances eluted at high retention times (13min). SEC hyphenation with ICP-MS proved the existence of several trace elements (Ni, Co, Cu, Zn, Mn, Mo and Sr) bound to the isolated marine DOM. Mass balance studies after analyzing the retentate and permeate fractions for trace elements indicate good recoveries (close to 100%) for elements such as Mo, Sr, Ba and Zn when performing the UF with both 3 and 10kDa MWCO membranes. However, recoveries from 36 to 81% were obtained for the remaining studied elements after either UF procedure. SEC-ICP-MS experiments showed percentages of metals bound to the isolated marine DOM ranging from 0.055 and 0.077% (Zn) to 4.1 10-4 and 1.4 10-4% (Sr). © 2013 Elsevier B.V.