High Molecular Weight Dissolved Organic Carbon Research Articles

Reactive transport modeling of organic carbon degradation in marine methane hydrate systems

Natural methane hydrate has often been observed in sand layers that contain no particulate organic carbon (POC), but are surrounded by organic-rich, fine-grained marine muds. In this paper, we develop a reactive transport model (RTM) of a microbially-mediated set of POC degradation reactions, including hydrolysis of POC driven by extracellular enzymes, fermentation of the resulting high-molecular weight dissolved organic carbon (HMW-DOC), and methanogenesis that consumes low-molecular weight dissolved organic carbon (LMW-DOC). These processes are mediated by two groups of microbes, fermenters and methanogens that are heterogeneously distributed in different lithologies, with the largest numbers of microbes in the large pores of coarse-grained layers. We find that the RTM can reproduce methane hydrate occurrences observed in two different geological environments, at Walker Ridge Site 313-H (Gulf of Mexico) and IODP Site U1325 (Cascadia Margin). We also find that microbes can degrade POC even if they are physically separated, as extracellular enzymes and DOC can diffuse away from where they are produced by microbes. Microbial activity is highest at relatively early times after burial at shallow depths and near lithological boundaries, where concentration gradients transport solutes to intervals that contain the most microbes.

Molecular structure of dissolved organic carbon in a sandy soil receiving contrasting quality organic residues

Decomposing plant residues release plant-derived dissolved organic carbon (DOC) which can be transformed by microorganisms resulting in microbial-derived DOC (DOC biological function). The adsorbability of DOC to mineral surfaces, determined by DOC chemistry (DOC chemical function), can affect its vertical distribution down soil profiles (D). This study investigated the temporal characteristics (T) and functions (bioavailability, adsorbability) of DOC across the profile of a sandy soil by using chemically contrasting quality (Q) residues: 1) high-cellulose rice straw (RS), 2) nitrogen-rich groundnut stover (GN), 3) lignin-rich dipterocarp leaf litter (DP), and 4) medium-nitrogen and -lignin tamarind leaf litter (TM). Soil samples were collected at two depths (0–15 cm and 60–80 cm) two and eight weeks after residue application, and water-extractable DOC was determined. For the first time, the chemical structure of the extracted DOC was characterized using pyrolysis coupled with mid-infrared spectroscopy (Py-MIRS). The dominant functional groups found in all DOC samples were aromatic, carboxyl, and polysaccharide compounds. The interactive effects of Q, T, and D (soil depth) influenced DOC-polysaccharides, with the most pronounced effect observed in the RS treatment during the early stage, indicating that DOC-polysaccharides were mainly plant-derived. Evidence of partial adsorption of DOC-aromatics and DOC-carboxyls was found in topsoils. These high molecular weight-DOC functional groups may have displaced adsorbed polysaccharides, resulting in higher subsoil DOC-polysaccharides than topsoil, especially under DP and TM. During the later stage, topsoil DOC-polysaccharides were negatively related to microbial metabolic quotient (qCO2) (R2 = 0.938). This relationship led to our hypothesis that the dominant source of DOC-polysaccharides, particularly in the TM treatment, was of microbial origin. A novel discovery from this study is that the chemical quality of plant residues, which serve as the source of DOC, determines the origin of the polysaccharides in DOC. This origin transitioned from plant-derived to microbial-derived as decomposition progressed. Importantly, the presented findings were the first of its kind based on a field experiment.

Low Molecular Weight Dissolved Organic Carbon: Aging, Compositional Changes, and Selective Utilization During Global Ocean Circulation

AbstractThe composition and cycling dynamics of marine dissolved organic carbon (DOC) have received increased interest in recent years; however, little research has focused on the refractory, low molecular weight (LMW) component that makes up the majority of this massive C pool. We measured stable isotopic (δ13C), radioisotopic (Δ14C), and compositional (C/N, 13C solid‐state NMR) properties of separately isolated high molecular weight (HMW) and LMW DOC fractions collected using a coupled ultrafiltration and solid phase extraction approach from throughout the water column in the North Central Pacific and Central North Atlantic. The selective isolation of LMW DOC material allowed the first investigation of the composition and cycling of a previously elusive fraction of the DOC pool. The structural composition of the LMW DOC material was homogeneous throughout the water column and closely matched carboxylic‐rich alicyclic material that has been proposed as a major component of the marine refractory DOC pool. Examination of offsets in the measured parameters between the deep waters of the two basins provides the first direct assessment of changes in the properties of this material with aging and utilization during ocean circulation. While our direct measurements largely confirm hypotheses regarding the relative recalcitrance of HMW and LMW DOC, we also demonstrate a number of novel observations regarding the removal and addition of DOC during global ocean circulation, including additions of fresh carbohydrate‐like HMW DOC to the deep ocean and large‐scale removal of both semilabile HMW and recalcitrant LMW DOC.

A mechanistic microbial underpinning for the size-reactivity continuum of dissolved organic carbon degradation

The reservoir of dissolved organic carbon (DOC) in the ocean is modified by multiple input and removal processes. Incubation experiments as well as measurements of oceanic DOC have demonstrated that the high molecular weight (HMW) fraction of DOC typically has a younger radiocarbon age and is more reactive biologically than the low molecular weight (LMW) fraction of DOC. These observations have been summarized as a ‘size-reactivity continuum’ of DOC reactivity, but mechanistic explanations for these observations have been lacking. Here we describe how our recent discovery of ‘selfish’ HMW organic matter uptake among bacteria in surface ocean waters may help explain the rapid removal of HMW DOC. ‘Selfish’ substrate uptake by bacteria encompasses rapid binding and partial hydrolysis of intact polysaccharides on the outer membrane of bacteria, seamlessly followed by the transport of large oligosaccharide fragments into the periplasm with little to no loss of LMW hydrolysis products. ‘Selfish’ bacteria therefore process HMW substrates in a manner distinct from bacteria that carry out extracellular hydrolysis that yields LMW hydrolysis products in the environment. Recognition of the presence and prevalence of selfish bacteria in the ocean has profound implications for carbon flow – the source and quantity of LMW substrates made available to non-extracellular-enzyme producing bacteria – as well as for efforts to model and measure bacterial interactions during organic matter degradation. This discovery also highlights the importance of targeted substrate binding and uptake as key (often understudied) factors in geochemical investigations of microbially driven carbon cycling in the ocean. We conclude with some speculative thoughts about the factors that may determine the prevalence of selfish substrate uptake in the environment.

Quality of DOC produced during litter decomposition of peatland plant dominants

Litter decomposition is an important source of dissolved organic matter (DOC). In peatlands with hardly degradable soil DOC, the input of DOC from vascular plant litter can represent an important source of nutrients and decomposable substrates for soil microorganisms. We established a laboratory incubation with the litters of three peatland plant dominants (Sphagnum fallax, Vaccinium myrtillus and Eriophorum vaginatum) for 200 days, aiming to study DOC production and its quality. The quality of the DOC leached from the litters was characterized by a distribution of C, N and P among molecular weight (MW) fractions (<1, 1-10, 10-100 and > 100 kDa), their aromaticity, composition of low molecular weight compounds (organic acids, sugars and amino acids) and DOC biodegradability.The leaves of vascular plants decomposed the fastest, releasing larger amounts of nutrients and easily degradable organic acids, sugars and amino acids to the leachate, when compared to their roots and Sphagnum. DOC distribution to the MW fractions did not differ among the litters. Neither the DOC distribution to the respective fractions nor leachate C/N/P stoichiometry were factors driving DOC biodegradability. Total dissolved P distribution to the MW fractions significantly differed among the litters, with Sphagnum being very specific: P was initially associated only with high molecular weight DOC, while later it was redistributed to the lower MW fractions and complexed with Al and Fe. The complexation may retard soluble reactive P leaching especially from Sphagnum litter. DOC biodegradability was higher for the vascular plant leaf litter than for the Sphagnum litter in the early stages of decomposition (20 days) but later decreased and became more uniform for all litters. These temporal differences (by decomposition stage) were more pronounced than those caused by litter origin. Our results indicate that mainly leaf litter of vascular plants can release significant amounts of DOC during the early stage of decomposition. This DOC is more aromatic with higher biodegradability and more nutrients (especially P) as compared to Sphagnum and can thus temporarily stimulate microbial activity in habitats dominated by the vascular plants.

Constraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses

We measured the concentrations and isotopic compositions of solid phase extracted (SPE) dissolved organic carbon (DOC) and high molecular weight (HMW) DOC and their constituent organic components in order to better constrain the sources and cycling of DOC in a large oligotrophic lacustrine system (Lake Superior, North America). SPE DOC constituted a significant proportion (41–71%) of the lake DOC relative to HMW DOC (10–13%). Substantial contribution of 14C-depleted components to both SPE DOC (Δ14C=25–43‰) and HMW DOC (Δ14C=22–32‰) was evident during spring mixing, and depressed their radiocarbon values relative to the lake dissolved inorganic carbon (DIC; Δ14C∼59‰). There was preferential removal of 14C-depleted (older) and thermally recalcitrant components from HMW DOC and SPE DOC in the summer. Contemporary photoautotrophic addition to HMW DOC was observed during summer stratification in contrast to SPE DOC, which decreased in concentration during stratification. Serial thermal oxidation radiocarbon analysis revealed a diversity of sources (both contemporary and older) within the SPE DOC, and also showed distinct components within the HMW DOC. The thermally labile components of HMW DOC were 14C-enriched and are attributed to heteropolysaccharides (HPS), peptides/amide and amino sugars (AMS) relative to the thermally recalcitrant components reflecting the presence of older material, perhaps carboxylic-rich alicyclic molecules (CRAM). The solvent extractable lipid-like fraction of HMW DOC was very 14C-depleted (as old as 1270–2320 14C years) relative to the carbohydrate-like and protein-like substances isolated by acid hydrolysis of HMW DOC. Our data constrain relative influences of contemporary DOC and old DOC, and DOC cycling in a modern freshwater ecosystem.

Allochthonous sources and dynamic cycling of ocean dissolved organic carbon revealed by carbon isotopes

AbstractWe present concentration and isotopic profiles of total, size, and polarity fractionated dissolved organic carbon (DOC) from Station ALOHA (A Long‐term Oligotrophic Habitat Assessment), an oligotrophic site in the North Pacific Ocean. The data show that, between the surface and 3500 m, low molecular weight (LMW) hydrophilic DOC, LMW hydrophobic DOC, and high molecular weight (HMW) DOC constitute 22–33%, 45–52%, and 23–35% of DOC, respectively. LMW hydrophilic DOC is more isotopically depleted (δ13C of −23.9‰ to −31.5‰ and Δ14C of −304‰ to −795‰; mean age of 2850 to 15000 years) than the LMW hydrophobic DOC (δ13C of −22‰ to −23‰ and Δ14C of −270‰ to −568‰; 2470 to 6680 years) and HMW DOC (δ13C of ~−21‰ and Δ14C of −24‰ to −294‰; 135–2700 years). Our analyses suggest that a large fraction of DOC may be derived from allochthonous sources such as terrestrial and hydrothermal DOC and cycle on much longer time scales of &gt;10000 years or enter the ocean as preaged carbon.

Different sources and degradation state of dissolved, particulate, and sedimentary organic matter along the Eurasian Arctic coastal margin

AbstractThawing Arctic permafrost causes massive fluvial and erosional releases of dissolved and particulate organic carbon (DOC and POC) to coastal waters. Here we investigate how different sources and degradation of remobilized terrestrial carbon may affect large‐scale carbon cycling, by comparing molecular and dual‐isotope composition of waterborne high molecular weight DOC (&gt;1 kD, aka colloidal OC), POC, and sedimentary OC (SOC) across the East Siberian Arctic Shelves. Lignin phenol fingerprints demonstrate a longitudinal trend in relative contribution of terrestrial sources to coastal OC. Wax lipids and cutins were not detected in colloidal organic carbon (COC), in contrast to POC and SOC, suggesting that different terrestrial carbon pools partition into different aquatic carrier phases. The Δ14C signal suggests overwhelmingly contemporary sources for COC, while POC and SOC are dominated by old C from Ice Complex Deposit (ICD) permafrost. Monte Carlo source apportionment (δ13C, Δ14C) constrained that COC was dominated by terrestrial OC from topsoil permafrost (65%) and marine plankton (25%) with smaller contribution ICD and other older permafrost stocks (9%). This distribution is likely a result of inherent compositional matrix differences, possibly driven by organomineral associations. Modern OC found suspended in the surface water may be more exposed to degradation, in contrast to older OC that preferentially settles to the seafloor where it may be degraded on a longer timescale. The different sources which partition into DOC, POC, and SOC appear to have vastly different fates along the Eurasian Arctic coastal margin and may possibly respond on different timescales to climate change.

富营养化湖泊溶解性有机碳生物可利用性研究进展

PDF HTML阅读 XML下载 导出引用 引用提醒 富营养化湖泊溶解性有机碳生物可利用性研究进展 DOI: 10.5846/stxb201209241349 作者: 作者单位: 南通大学地理科学学院,中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室,中国科学院南京地理与湖泊研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金资助项目（41201076，31270507，31070420）；湖泊与环境国家重点实验室开放基金资助项目（2012SKL006） The bioavailability of dissolved organic carbon in the eutrophic lakes Author: Affiliation: Geographical Sciences College,Nantong University,State key Laboratory of Lake Environment Science,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences,State key Laboratory of Lake Environment Science,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:富营养化湖泊溶解性有机碳（DOC）包括内源和外源性碳源，不同来源碳源在物质化学结构组成和分子量级等方面具有显著差异，进而影响到对细菌的生物可利用性和碳素在食物网中的传递效率。根据国内外文献，综述了内外源DOC在碳稳定同位素值域上的显著差异，建议通过对DOC碳稳定同位素的分析来识别富营养化湖泊中DOC的主要来源；通过对比内外源DOC在碳水化合物、结合态中性糖和腐殖质含量上的差异，并结合细菌生长参数如细菌二级生产力、细菌呼吸作用及细菌生长效率来分析内外源DOC对细菌的生物可利用性。从富营养化湖泊DOC来源的角度探讨其生物可利用性和碳素传递效率，有助于了解富营养化湖泊食物网中碳素循环特征，加强对湖泊生态学的认识，为湖泊环境治理与保护提供科学依据。 Abstract:The dissolved organic carbon (DOC) pool is composed of both autochthonous and allochthonous DOC, and its concentration in lakes generally increases with the trophic status. Accumulation of the autochthonous DOC was observed in the eutrophic lake, and the allochthonous DOC was highest in the dystrophic lake. Carbohydrates constitute a large component of the DOC, the consumption of DOC by heterotrophic bacteria is one of the largest fluxes of carbon in most aquatic ecosystems, but the bioavailability and the efficiency of carbon transfer in lakes food web is affected by the distribution of molecular weight and chemical composition. The DOC can be separated into high and low molecular weight DOC fractions by cross-flow ultrafiltration, but which fraction is more bioreactive is still in dispute.Stable carbon isotope can be used to trace the origins of organic carbon, and the approach depends on the fact that DOC from different origins has different stable isotopic compositions. The riverine DOC has a δ13C value of -27‰, which is different from freshwater phytoplankton, with a range from -35‰ to -25‰. This paper reviewed the researches on the stable carbon isotope ratio of the autochthonous and allochthonous DOC, suggesting that the main sources of DOC in eutrophic lakes can be identified by using natural stable carbon isotope ratio of DOC; the difference on the total dissolved carbohydrates (TCHO) and dissolved combined neutral sugar (DCNS) concentrations, as well as the humic substances (HS) was compared between the autochthonous and allochthonous DOC. Net increases in TCHO and DCNS were observed in the autochthonous DOC during phytoplankton blooms, whereas the HS fraction was quantitatively important in the allochthonous DOC. Many studies have reported the bacterial availability of TCHO and DCNS, and the ratio of TCHO/DOC is used to characterize the bioavailability of DOC, whereas HS can also increase bacterial secondary production and support bacterial growth if labile substrates are abundant.Furthermore, to elucidate the bioavailability of the two sources of DOC, the bacterial secondary production, bacterial respiration and bacteria growth efficiency (BGE) was analyzed together. The DOC can be either transformed to bacterial secondary production or respired to inorganic carbon. BGE is the fraction of assimilated organic carbon that supports growth. The source of DOC and its chemical composition could be a key regulator of BGE. Traditionally, the autochthonous DOC has been considered to be the main source for bacterial as well as other secondary production, the allochthonous DOC was long considered relatively recalcitrant to bacterial degradation. However, in lakes which are both humic-rich and oligotrophic, the ecosystem respiration exceeds gross primary production, suggesting that the allochthonous DOC can be incorporated into the bacteria biomass and makes a significant carbon and energy subsidy for lakes food web, but little of the allochthonous carbon assimilated by bacteria is likely to reach higher consumers. Recent studies suggest that bacterial BGE increases with the concentration of the low molecular weight DOC in allochthonous DOC.Discussing the bioavailability and efficiency of carbon transfer in the food web from the sources of DOC, will be helpful to investigate the characterization of carbon cycling in the eutrophic lakes, enhance our understanding of the lake ecology and to provide scientific references for the lake management and protection. 参考文献 相似文献 引证文献

Effects of magnetic ion exchange pretreatment on low pressure membrane filtration of natural surface water

Magnetic ion exchange (MIEX) pretreatment has been increasingly employed by water treatment plants for removal of dissolved organic carbon (DOC). In this study, the effects of MIEX pretreatment on low pressure membrane filtration of natural surface water were investigated under different feedwater qualities, membrane properties, and MIEX dosing conditions. Regardless of feedwater DOC, moderate decrease in the total and hydraulically irreversible fouling was observed for a polyvinylidene fluoride (PVDF) microfiltration membrane and a polyethersulfone ultrafiltration (UF) membrane after MIEX pretreatment, which was coincident with moderate removals of high molecular weight DOC in the feedwaters. Comparatively, the fouling of a PVDF UF membrane did not decrease after MIEX pretreatment, revealing the impact of membrane properties on membrane fouling in the presence of MIEX pretreatment. Reuse of virgin or regenerated MIEX resulted in similar membrane fouling as observed with single use of the virgin MIEX. The level of DOC removal by MIEX was similar to the removal of MS2 bacteriophage spiked in the feedwater, suggesting a potential similarity in the removal of organic and microbial particles. In conclusion, MIEX pretreatment was effective for DOC removal, but less effective in controlling short-term membrane fouling or removing viruses.

Large-volume ultrafiltration for the study of radiocarbon signatures and size vs. age relationships in marine dissolved organic matter

In recent decades, tangential-flow ultrafiltration (UF) technology has become a primary tool for isolating large amounts of “ultrafiltered” marine dissolved organic carbon (UDOC; 0.1 μm to ∼1 nm) for the detailed characterization of DOC chemical composition and radiocarbon (Δ 14C) signatures. However, while total DOC Δ 14C values are generally thought to be quite similar in the world ocean, previous studies have reported widely different Δ 14C values for UDOC, even from very similar ocean regions, raising questions about the relative “reactivity” of high molecular weight (HMW) DOC. Specifically, to what degree do variations in DOM molecular weight (MW) vs. composition alter its relative persistence, and therefore HMW DOC Δ 14C values? In this study we evaluate the effects of varying proportions of HMW vs. low molecular weight (LMW) DOC on UDOC Δ 14C values. Using concentration factor (CF) as a proxy for MW distributions, we modeled the retention of both OC and Δ 14C in several very large CF experiments (CF >3000), from three depths (20, 670, and 915 m) in the North Pacific Subtropical Gyre (NPSG). The resulting DOC and Δ 14C UF permeation coefficients generally increase with depth, consistent with mass balance trends, indicating very significant permeation of LMW, 14C-depleted DOC at depth, and higher recoveries of Δ 14C-enriched, HMW DOC in the surface. In addition, changes in CF during sample concentration and ionic strength during sample diafiltration had very large and predictable impacts on UDOC Δ 14C values. Together these results suggest that previously reported disparities in UDOC Δ 14C values are reconciled by linked trends of Δ 14C content vs. MW. At low CFs, UDOC samples have similar Δ 14C values to total DOC. In contrast, UDOC samples collected at extremely high CFs (and after diafiltration) have more positive Δ 14C values. We demonstrate that the observed relationships between UDOC Δ 14C and CF derived from our data can directly explain offsets in all previously published UDOC Δ 14C values for the NPSG. While CF is not traditionally considered in UF studies, our results indicate it can substantially influence the interpretation of UDOC 14C “age”, and thus reactivity, in the marine environment. In addition, our results indicate that CF can in fact be used as a proxy for average MW. We suggest that a variable-CF-UF approach, coupled with molecular-level Δ 14C analyses, presents a new tool for studying relationships between molecular size, age, and “labile” DOC distributions in the ocean.

Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH 3COO −) or strong (i.e., CO 3 2 - ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).

Aged black carbon identified in marine dissolved organic carbon

Produced on land by incomplete combustion of organic matter, black carbon (BC) enters the ocean by aerosol and river deposition. It has been postulated that BC resides in the marine dissolved organic carbon (DOC) pool before sedimentary deposition and may attribute to its great 14C age (1500–6500 14C years). Here we report the first radiocarbon measurements of BC in high molecular weight DOC (UDOM). BC exported from rivers is highly aromatic and &lt;500 14C years old, while open ocean samples contain less aromatic BC and have an age of 18,000 ± 3,000 14C years. The low abundance of BC in UDOM (0.5–3.5%) suggests that it is more labile than presently believed and/or the low molecular weight DOC contains a larger proportion of aged BC.

The model high molecular weight DOC compound, dextran, is ingested by the benthic ciliate Uronema marinum but does not supplement ciliate growth

Phagotrophic ciliates are capable of growth solely on dissolved compounds in laboratory cultures. Whether ciliates use dissolved compounds in the environment for growth is unclear. We investigated the ability of the marine benthic ciliate Uronema marinum to ingest a model high mole- cular weight dissolved organic carbon (HMW-DOC) compound, dextran, at concentrations typical for coastal salt marsh sediments (3 µM to 3 mM C). Ingestion was measured by incubating ciliates with fluorescein-labeled dextran (2000 kDa) and measuring the fluorescence signal of the labeled com- pound in cells via flow cytometry. Ciliates accumulated dextran (relative to formalin-killed controls) at concentrations as low as 0.1 mg l -1 dextran (3 µM C). Labeled dextran accumulated in food vac- uoles and near the buccal cavity; thus, the ingestion of dextran appears to be a consequence of feed- ing activities rather than transport across the cell membrane via parasomal sac formation. Dextran accumulation did not increase with higher bacterial ingestion rates. Instead, dextran accumulation was greatest at intermediate bacterial concentrations and grazing rates. Ciliate growth rates were measured in treatments amended with model carbon compounds—soluble starch, acetate, and glu- cose (3 mM C, final concentrations). There was no significant increase in ciliate growth rates with these compounds in either bacteria-free or bacteria-enriched treatments. Rather, growth rates were significantly lower in treatments with DOC addition, indicating that processing of these DOC com- pounds may incur some energetic cost to these ciliates.

Radiocarbon analysis of neutral sugars in high-molecular-weight dissolved organic carbon: Implications for organic carbon cycling

We used compound-specific natural-abundance radiocarbon analyses of neutral sugars to study carbon cycling of high-molecular-weight (HMW) dissolved organic carbon (DOC) at two sites in the North Pacific Ocean. Sugars released from HMW DOC by acid hydrolysis were purified by high-pressure liquid chromatography and analyzed for radiocarbon content via accelerator mass spectrometry. The seven most abundant sugars recovered from HMW DOC have similar radiocarbon values, supporting the hypothesis that these sugars are incorporated into a common family of polysaccharides. Neutral sugar Δ14C values from surface waters collected in 1999 and 2001 are 89 ± 13‰ and 57 ± 6‰, respectively; these values are much more enriched in radiocarbon than those found in previous studies that used operationally defined carbohydrate fractions. Radiocarbon values for HMW DOC neutral sugars are the same as, or only slightly depleted relative to, dissolved inorganic carbon (DIC), which is consistent with rapid cycling and a short (<3-yr) residence time. In addition, the Δ14C value of neutral sugars at 600 m is 20‰ enriched relative to DIC Δ14C, suggesting that a fraction of dissolved neutral sugars at this depth are introduced by dissolution from large, rapidly sinking particles.

Flood pulse influences on terrestrial organic matter export from an agricultural watershed

Riverine export of terrestrial organic carbon (TOC) plays an important role in the global C cycle through influences on coastal productivity, sedimentary preservation, and CO2 efflux. In order to examine the influence of hydrologic variability and upland processes on TOC export from a midwestern agricultural watershed, we applied lignin phenol and stable carbon isotope analyses to assess quantity, source, and relative degradation state of TOC in soils, sediments, and size‐fractioned aquatic OC from stream water. Flood conditions exported 90 times more dissolved organic carbon (DOC) than baseflow owing to increases in both discharge and concentration, indicating mobilization of additional DOC pools. These observations are supported by increased TOC contributions to aquatic OC during flooding. Furthermore, stable carbon isotope values show that OC exported during flooding is enriched in C4‐derived C by up to 38% (for high molecular weight DOC) with respect to baseflow, indicating different TOC sources are mobilized dependant on hydrologic conditions. Lignin phenols isolated from the colloid size fraction show progressive increases in the ratio of carboxylic acid to aldehyde functional groups (a measure of relative degradation state) with distance downstream suggesting selective partitioning to mineral surfaces and/or degradation to this size fraction. This study shows that TOC quantity, source, and relative degradation state can change dependant on hydrologic conditions and in‐stream transport in a small agricultural watershed. Moreover, these results are important in showing variability in TOC source and chemistry exported from headwater systems that might otherwise not be detected in studies of larger rivers.

Origin and migration of dissolved organic carbon fractions in a clay‐rich aquitard: 14C and δ13C evidence

The origin and long‐term transport of dissolved organic carbon (DOC) and its high molecular weight (HMW) and low molecular weight (LMW) fractions were investigated in a clay‐rich till aquitard system. DOC concentrations decreased systematically with depth from 168 mg L−1 in the surficial fractured zone (1.2 m) to minimum values of 12–16 mg L−1 from 15 to 43 m depth in the nonfractured aquitard. The HMW and LMW DOC fractions and their corresponding δ13C values also decreased systematically with depth. Numerical transport simulations of the downward migration of DOC from the soil zone into the aquitard closely approximated field‐measured values and show transport of DOC was controlled by diffusive mixing. The depth trends for HMW and LMW DOC fractions and δ13C support hypothesized transport via diffusion, as well as earlier findings that microbial respiration in this nonfractured aquitard was negligible over the time period studied. The 14C content of the HMW DOC fraction decreased systematically with depth from 87 percent modern carbon (pmC) at 1.2 m to 13–15 pmC from 15 to 43 m depth. A simple model accounting for diffusive mixing and radioactive decay described the 14C profile in the aquitard. Radiocarbon measurements, transport modeling, and δ13C data suggest DOC and its fractions were derived from two distinct end‐members: soil organic carbon formed since the Holocene (∼10 ka B.P.) and DOC in connate pore water (∼15 ka B.P.).

Fluxes of dissolved combined neutral sugars (polysaccharides) in the Delaware estuary

This project examined concentrations, composition and turnover neutral sugars in the Delaware estuary to gain insights into dissolved inorganic nitrogen (DIN) use by heterotrophic bacteria and into the lability and diagenetic state of dissolved organic material (DOM) during passage through the estuary. Dissolved free monosaccharides were not measurable (<5 nM) in the estuary whereas concentrations of dissolved combined neutral sugars (DCNS) were high, much higher than observed in oceanic waters. DCNS made up a similar fraction of dissolved organic carbon (DOC) as in the oceanic waters examined to date, and the monosaccharide composition of the DCNS pool was similar to that of oceanic waters. The composition did not vary substantially within the estuary or seasonally, but it did vary among three size fractions of the organic matter pool. Glucose was enriched in the low molecular weight fraction of DOC and in particulate material, whereas the high molecular weight DOC fraction was slightly depleted in glucose. Depletion experiments indicated that DCNS is not used extensively on the day time scale in the Delaware estuary, although freshly-produced polysaccharides may still be important carbon sources for heterotrophic bacteria. The very low concentrations of free monosaccharides in the Delaware estuary help to explain why DIN use by heterotrophic bacteria is relatively low in this estuary. Although DOC-DIN interactions in the Delaware apparently differ from oceanic waters, the portion of DOM traced by DCNS, which is thought to be the labile fraction, appears to be similar to that of oceanic DOM, suggesting that organic material in the estuary is degraded extensively before being exported to the coastal ocean.

Photochemical transformations and bacterial utilization of high-molecular-weight dissolved organic carbon in a southern Louisiana tidal stream (Bayou Trepagnier)

UVirradiation of dissolved organic carbon (DOC) in the laboratory can producesmall, labile organic compounds utilizable by microbes, but few studies haveattempted to document this process in situ. 13Cnuclear magnetic resonance (NMR) was used to examine the bulk chemicalcomposition of natural and laboratory-irradiated high-molecular-weight DOC(HMW-DOC) from shaded (150 μmol m−2s−1 average light in surface water) and open (1500μmol m−2 s−1) field sitesoverone and a half years. 13C NMR revealed only small differences incarbon functional groups between laboratory irradiated and non-irradiatedHMW-DOC. However, bacterial protein productivity per cell (BPP) was enhanced innaturally irradiated samples of HMW-DOC in a field mesocosm experiment (p <0.05), suggesting that bacterial growth was enhanced by photochemicalproductionof labile DOC substrates. Absorbance characteristics such as spectral slope,absorbance at 350 nm, and the absorbance ratio 250nm/365 nm revealed that HMW-DOC was photoreactive,yetno differences in these values were found between samples irradiated with andwithout UV-B. In experiments conducted with simulated solar radiation in thelaboratory and with natural light in the field mesocosm experiment, UV-A(320–400 nm) and photosynthetically active radiation (PAR;400–700 nm) were more effective than UV-B (280–320nm) in HMW-DOC photolysis.

Chemical characterization of high molecular weight dissolved organic matter in fresh and marine waters

The high molecular weight fraction of dissolved organic matter in a suite of lakes, rivers, seawater, and marine sediment interstitial water samples was collected by ultrafiltration and characterized by molecular level and spectroscopic techniques. Proton nuclear magnetic resonance spectra of all samples show a high degree of similarity, with major contributions from carbohydrates, bound acetate, and lipids. Molecular level analyses of neutral sugars show seven monosaccharides, rhamnose, fucose, arabinose, xylose, mannose, glucose, and galactose, to be abundant, and to occur in comparable relative amounts in each sample. Previous studies have emphasized the distinctive composition of dissolved humic substances in fresh and marine waters, and have attributed these differences to sources and transformations of organic matter unique to each environment. In contrast we find a large fraction of freshwater high molecular weight dissolved organic matter (HMWDOM; > 1kD) to be indistinguishable from marine HMWDOM in bulk and molecular-level chemical properties. Aquatic HMWDOM is similar in chemical composition to biologically derived acylated heteropolysaccharides isolated from marine algal cultures, suggesting a biological source for some fraction of persistent HMWDOM. High molecular weight DOC contributes 51 ± 26% of the total DOC, and monosaccharides 18 ± 8% of the total HMWDOC in our freshwater samples. These contributions are on average higher and more variable, but not significantly different than for surface seawater (30% and 16% respectively). Biogeochemical processes that produce, accumulate, and recycle DOM may therefore share important similarities and be broadly comparable across a range of environmental settings.