Initial Dissolved Organic Carbon Research Articles

The influence of sources and solid phase extraction criteria on dissolved organic matter optical properties in aquatic systems

The integrity of studies investigating the composition of dissolved organic matter (DOM) is heavily dependent on the sampling and instrumental techniques employed. In this study, we aimed to assess the impacts of both the source of the DOM and the solid phase extraction (SPE) criteria on changes to the optical properties of DOM. Samples were collected from various aquatic environments, ranging from headwaters to the open ocean. The extraction kinetics of DOM were monitored throughout the SPE process, with alterations to the DOM verified using optical properties derived from chromophoric dissolved organic matter (CDOM), including absorbance and fluorescence measurements. The fluorescence excitation-emission matrices (EEMs) were further analyzed using PARAllel FACtor analysis (PARAFAC) to resolve the underlying chemical components. Results indicate that the reaction of DOM to acidification required for polystyrene-divinylbenzene polymer sorbent SPE varies across samples. And this variability may stem from differences in functional groups and DOM sources. The maximum carbon loading observed was 9.7 mg C per 1 g PPL, which is a widely used styrene-divinylbenzene polymer sorbent, and no overloading was noted. The dissolved organic carbon (DOC) extraction efficiency (EE) ranged from 51.5-61.5% and 27.6-40.0% for post- and extract- calculations, respectively, with no significant differences in DOC EE observed among different initial DOC concentrations, salinities, and flowrates. The differences in DOC EE across diverse sources and changes in optical proxies resulting from PPL extraction were mainly due to the heterogeneity of DOM. PPL extraction altered the overall molecular characteristics of DOM from more labile to more refractory or degraded in the open ocean and headwater source samples. Notably, high and low flowrates yielded identical SPE-DOM extraction and alterations. The results indicate that DOM sources primarily control SPE EE rather than methodological or other environmental factors. PPL SPE results in a convergence of DOM variability. These findings provide crucial insights into inter-study comparisons and the interpretability of SPE-derived DOM data from samples across diverse aquatic environments.

More about Persulfate-Assisted Ferrilanthanide-Mediated Photocatalysis of Textile Industry Dye Reactive Black 5: Surface Properties and Structural Assessment

Color and organic matter removal from dyehouse effluent remains a challenging issue for the environmentalist and textile dyer. Until now, various treatment processes have been proposed with limited success. In this study, the textile dye and model industrial pollutant Reactive Black 5 (RB5; 20 mg/L) could be rapidly degraded by persulfate (PS)-enhanced photocatalytic treatment using a novel, home-made lanthanum iron oxide (LF; 0.5 g/L). LF-mediated heterogeneous photocatalysis was effective when the solution pH was kept below 4. The photocatalytic degradation of RB5 solution was enhanced in the presence of 0.6 and 1.2 mM PS. The dissolved organic carbon (DOC) content of the aqueous, hydrolyzed RB5 solution (initial DOC = 5.15 mg/L) was effectively reduced by LF/UV-A (LF = 0.5 g/L; 52–54% DOC removal after 150–180 min) and LF/PS/UV-A (LF = 0.5 g/L; 60–66% DOC removal after 120 min) treatments. LF photocatalyst could be reused in four consecutive cycles for complete color and partial DOC removals without significant deterioration of the treatment performance with the LF/PS/UV-A/pH 3 process. Instrumental analyses of LF’s surface morphology/chemical composition and structural features via EDAX/SEM/Raman/FTIR/UV-vis/fluorescence spectroscopy indicated that LF remained practically intact throughout photocatalytic treatment, though slight changes/decreases in particle size/partial surface deformation and agglomeration coverage were observed, particularly during LF/PS/UV-A treatment. The presence of RB5 and its degradation products on the LF surface revealed that surface adsorption played a major role in LF-mediated photocatalysis. The Fe-content did not deviate appreciably from its original value after photocatalytic treatment.

Effect of temperature during the hydrothermal carbonization of sewage sludge on the aerobic treatment of the produced process waters

The treatment of the highly contaminated process water produced during the hydrothermal carbonization (HTC) of waste activated sludge is of major concern for the full-scale implementation of the HTC process. So far, no satisfying treatment strategies have been elaborated and the biodegradability under aerobic conditions has hardly been studied. To fill these gaps, aerobic tests were first carried out in batches with HTC process waters produced at 190 °C, 218 °C and 249 °C, and two parallel sequencing batch reactors (SBR) were operated to treat process waters produced at 190 °C and 217 °C. Both experiments show that the HTC temperature has only a little effect on the elimination of dissolved organic carbon (DOC). In the aerobic batch tests, DOC removal were 80.5–81.9 %. In the SBR, 28 % of the initial DOC was found to be recalcitrant, and 25–28 % of the initial nitrogen. In the SBR experiments, the nitrification was also monitored, and nitrification inhibition test were conducted on both process waters obtained at 190 °C and 217 °C. Nitrification the initial SBR reactors was only possible after dilution of the process waters, which indicates the presence of inhibiting substances. The inhibition tests validated those observations, and showed that process waters derived at 217 °C had a higher inhibition potential. This study demonstrates that aerobically treated HTC process waters are still too polluted to be discharged in a wastewater treatment plant: model calculations showed an increase in effluent DOC of 8.3 mg C/L.

Tree Fresh Leaf- and Twig-Leached Dissolved Organic Matter Quantity and Biodegradability in Subtropical Plantations in China

Extreme weather events often cause the input of fresh plant tissues into soils in forests. However, the interspecific patterns of tree fresh plant tissue-leached dissolved organic matter (DOM) characteristics are poorly understood. In this study, we collected fresh leaves and twigs of two broadleaf trees (Liquidambar formosana and Schima superba) and two coniferous trees (Pinus massoniana and Pinus elliottii) in subtropical plantations in China, and measured tree fresh tissue-leached DOM quantity and biodegradability. The interspecific patterns of fresh plant tissue-leached DOM production varied with organ types. Broadleaf tree leaves leached greater amounts of dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP) than coniferous tree leaves, but an opposite pattern of DOC and DTN productions was observed between broadleaf and coniferous tree twigs. Regardless of tree species, leaves often leached greater quantities of DOC, DTN, and DTP than twigs. For both leaves and twigs, broadleaf tree tissue-leached DOM had greater aromaticity and lower biodegradability than coniferous tree tissue-leached DOM. Moreover, leaf-leached DOM had greater aromaticity and lower biodegradability than twig-leached DOM. In addition, DOM biodegradability negatively correlated with the initial aromaticity and DOC:DTN ratio, despite no relationship between DOM biodegradability and DOC:DTP ratio. These findings highlight the pivotal roles of leaf habit and organ type in regulating fresh tree tissue-leached DOM production and biodegradability and reveal that the substantial variations of fresh tissue-leached DOM biodegradability are co-driven by DOM aromaticity and N availability in subtropical plantations in China.

Effect of pH on the sorption of dissolved organic carbon derived from six tree species in forest soils

Dissolved organic carbon (DOC) is an important component of the total carbon (C) pool in temperate forest soils. The ability of forest soils to sorb DOC is an essential feature in the context of ecosystem changes (such as climate change) and the resulting changes in the C cycle. The aim of this study was to investigate the ability of forest soils with different pH levels to sorb DOC from litter of different forest tree species. To accomplish this, we conducted two laboratory experiments in which the soil differed in the depth from which they were sampled (topsoil vs. subsoil) and the initial DOC content. We modified the soil pH to obtain a range covering most soils in temperate forests (approximately 3.5–8). Soil samples were leached with extracts of similar DOC content obtained from the litter of six main forest-forming tree species in temperate zone. Increased DOC sorption was observed at approximate soil pH levels of 3.5 and 7.5, with the highest sorption capacity occurring at low pH. The DOC sorption capacity was decreasing with consecutive leaching with litter extract. The capacity of DOC sorption also depended on the litter species. In addition, the initial concentration of C in soil had no significant relationship with sorption capacity. Litter species were related to the sorption of released DOC, which was probably linked to the Al content in the litter. The increased DOC sorption at low pH was likely because of dissolved organic matter protonation (saturation of functional groups with H+ ions resulting in proton bridging and van der Waals forces), while Ca2+ ions causing cation bridging, flocculation or non-columbic interactions were likely responsible for the increased sorption at high pH. Our research shed light on the significant effects of soil pH on the stabilization of DOC, as well as the important role and positive influence of Al on DOC sorption by forest soils. Our findings provide new, significant insight by presenting that litter-derived DOC sorption is dependent both on soil pH and litter chemical composition.

Changes in the Dissolved Organic Matter Characteristics Released from Sediment According to Precipitation in the Namhan River with Weirs: A Laboratory Experiment.

In this study, changes in the properties of dissolved organic matter (DOM) released from sediments into water layers were investigated. To analyze the spatial and temporal variation in dissolved organic carbon (DOC), sediment and bottom water samples were collected upstream of the Gangcheon, Yeoju, and Ipo weirs of the Namhan River during the rainy and non-rainy seasons. The initial DOC was correlated with precipitation (R2 = 0.295, p = 0.034) and residence time (R2 = 0.275, p = 0.040). The change in the bottom water DOC concentration resulted from the DOC released from the sediments, which may cause water quality issues in the bottom water. The fluorescence analysis revealed that the DOM contained higher levels of hydrophilic and low-molecular-weight (LMW) organic matter in the non-rainy season and higher levels of hydrophobic and high-molecular-weight (HMW) organic matter in the rainy season. Since the Namhan River is the main resource of drinking water for the Seoul metropolitan area, our results can help to optimize the drinking water treatment process by reflecting the DOM characteristics that vary with the seasons. Furthermore, the statistical analysis confirmed that the nutrient content of pore-water and sediment can be used to estimate the DOM release rate from the sediment to the water layer. The results of this study provide a better understanding of DOM movement in aquatic ecosystems and the influences of rainfall on the water quality of the surface waterbody.

Limited utilization of extracted dissolved organic matter by prokaryotic communities from the subtropical North Atlantic

The ocean contains a large reservoir of dissolved organic matter (DOM) that persists for millennia. Both the very dilute concentrations of individual DOM molecules and intrinsic recalcitrance to microbial decay imparted by molecular structure are suggested mechanisms for this long residence time. Here, we report an experiment comparing the responses of surface and deep prokaryotes to DOM isolated and enriched by solid‐phase extraction from surface and deep waters of the subtropical North Atlantic Ocean. Extracts from both depths were qualitatively characterized as biologically recalcitrant given their similarly high C : N ratios of 26. Surface prokaryotes measurably drew down extracted dissolved organic carbon (DOC) concentrations, but the drawdown was only 4% of the initial enriched DOC concentration regardless of enrichment level or depth. Deep microbes, in contrast, did not cause observable changes in DOC concentrations. Surface and deep prokaryotes had similar temperature‐normalized growth responses to extracts from each depth. Biological indicators (e.g., kinetics) suggest that prokaryotes were less efficient at catalyzing surface than deep DOM (catalytic efficiencies of 0.003–0.005 vs. 0.02–0.03 h−1, respectively). These values indicate qualitative differences in extracted DOM from the two depths, perhaps suggesting a variable nature of the refractory DOC depending on depth. Moreover, only a small portion of the extracted DOM was biologically utilizable, regardless of concentration factor or depth, and essentially only a small fraction of it was incorporated into biomass. Microbial selection against substrates that meet modest energy but no growth demands may be a factor contributing to the long‐term stability of marine DOM.

Tracking Br-DBPs and bromine substitution factors by two-stage differential characterization of water matrix and NOM during chlorination

Tracking disinfection byproducts (DBPs) upon chlorination becomes challenging whenever bromides occur because they alter DBP formation from chlorinated to brominated DBPs (Br-DBPs). Although epidemiological studies have emphasized the need to control Br-DBPs because of their notable carcinogen effects, work on Br-DBP monitoring is timely, since limited attention has to date been paid to this. This study proposes a two-stage differential characterization approach to a water matrix and natural organic matter (NOM) surrogates as a more robust way to trace Br-DBP precursors and bromine substitution factors (BSF). This two-stage approach monitors the differential variations (Δ) of both inorganic and organic precursors, e.g., dissolved organic carbon (DOC), bromide levels, Cl2 content, specific ultraviolet absorbance (SUVA), fluorescent spectra, before and after chlorination. To mimic practical conditions, chlorination experiments were conducted with two principle NOMs, including Suwannee River fulvic (SW) and M. aeruginosa organic matter (MA), with Br− levels from 0 to 2000 μg/L and chlorination times from 15 min-24 h. When Br− levels increased, all Br-DBP yields and BSF values correspondingly magnified, accounted for 70–90% of the total DBP yields from chlorinated SW or MA water. At each Br− level, prolonging chlorination time also increased the overall Br-DBPs and varied BSF values, but these differences were of no significance (p > 0.05). Correlation tests highlighted the significant insufficiency of the conventionally-used DBP surrogates (initial DOC, Cl2 dose, and SUVA) for Br-DBPs and BSF monitoring. By contrast, our two-stage characterization parameters are superior, with correlation coefficients ρ > 0.8 (p < 0.01), especially for those parameters including ΔBr as a numerator. Accordingly, regression models of Br-DBPs and BSFs built by our two-stage tertiary parameters exhibited high adjusted R2 values mostly >0.7 and >0.8 (n = 60), indicating a promising application for effective Br-DBPs and BSF monitoring.

Preferential loss of Yukon River delta colored dissolved organic matter under nutrient replete conditions

AbstractThe Yukon River exports a large amount of dissolved organic carbon (DOC), much of which is colored, absorbing visible and ultraviolet light. Yukon River water typically has low total dissolved nitrogen (TDN), with an average DOC : TDN ratio of 21.1 (mol C mol N−1) ~ 200 km upstream from the coast at Pilot Station, Alaska. TDN has been correlated to the bioavailability of DOC, but a direct estimate of the dependence of DOC reactivity on nutrient availability has not been conducted in the Yukon River delta. The biodegradability of DOC and colored dissolved organic matter (CDOM) was assessed along a gradient within the Yukon River delta and into the coastal ocean. Samples were dark incubated at 20°C and half of the incubations were amended with inorganic nutrients. At five time points through 27 d, CDOM absorption and DOC concentration were measured. Initial DOC concentration was 113–830 μmol C L−1 and CDOM absorption at 443 nm (a443) ranged from 0.16 to 7.8 m−1 from the ocean to a high CDOM lake. DOC was relatively unreactive, with 3–6% of DOC degrading in riverine samples and little difference between nutrient amended and control incubations. Control river delta a443 decreased 3–10%, while nutrient amended treatments saw a 6–22% decline over 27 d. In the nutrient amended riverine stations, CDOM absorption loss was greater at blue wavelengths and CDOM degradation was greater than total DOC loss. DOC was resistant to biodegradation in both nutrient amended and ambient conditions, while CDOM was preferentially degraded when nutrients were replete.

Potential bioavailability of organic matter from atmospheric particles to marine heterotrophic bacteria

Abstract. The surface ocean receives important amounts of organic carbon from atmospheric deposition. The degree of bioavailability of this source of organic carbon will determine its impact on the marine carbon cycle. In this study, the potential availability of dissolved organic carbon (DOC) leached from both desert dust and anthropogenic aerosols to marine heterotrophic bacteria was investigated. The experimental design was based on 16 d incubations, in the dark, of a marine bacterial inoculum into artificial seawater amended with water-soluble Saharan dust (D treatment) and anthropogenic (A treatment) aerosols, so that the initial DOC concentration was similar between treatments. Glucose-amended (G) and non-amended (control) treatments were run in parallel. Over the incubation period, an increase in bacterial abundance (BA) and bacterial production (BP) was observed first in the G treatment, followed then by the D and finally A treatments, with bacterial growth rates significantly higher in the G and D treatments than the A treatment. Following this growth, maxima of BP reached were similar in the D (879 ± 64 ng C L−1 h−1; n=3) and G (648 ± 156 ng C L−1 h−1; n=3) treatments and were significantly higher than in the A treatment (124 ng C L−1 h−1; n=2). The DOC consumed over the incubation period was similar in the A (9 µM; n=2) and D (9 ± 2 µM; n=3) treatments and was significantly lower than in the G treatment (22 ± 3 µM; n=3). Nevertheless, the bacterial growth efficiency (BGE) in the D treatment (14.2 ± 5.5 %; n=3) compared well with the G treatment (7.6 ± 2 %; n=3), suggesting that the metabolic use of the labile DOC fraction in both conditions was energetically equivalent. In contrast, the BGE in the A treatment was lower (1.7 %; n=2), suggesting that most of the used labile DOC was catabolized. The results obtained in this study highlight the potential of aerosol organic matter to sustain the metabolism of marine heterotrophs and stress the need to include this external source of organic carbon in biogeochemical models for a better constraining of the carbon budget.

Aspen Soils Retain More Dissolved Organic Carbon Than Conifer Soils in a Sorption Experiment

The effect tree species have on soil organic carbon (SOC) has been hotly debated but, so far, few clear patterns have emerged. One example of a differing tree species effect on SOC are aspen forests in North America, which have been found to have more stable SOC than adjacent conifer forest stands. An important source for the formation of stable organo-mineral complexes in soil is dissolved organic carbon (DOC). DOC concentrations in mineral soil are often higher under the thick O-horizons of conifer forests than under aspen forests, but this does not correspond to more stable mineral SOC. This suggests that, instead of DOC concentration, DOC quality could be driving the observed differences in SOC. Therefore, we quantified the retention of contrasting forest detritus DOC in soils. Using a batch sorption experiment approach, we compared the retention of detritus leachates from four sources – aspen leaves (AL), aspen roots (AR), conifer (subalpine fir) needles (CN), and conifer (subalpine fir) roots (CR) – on soils sampled under aspen and conifer (subalpine fir and Douglas fir) overstories. The calculated sorption isotherms showed a higher retention of AL DOC than AR DOC, as indicated by all four sorption parameters – k and n (curve-fitting parameters), null point concentration (NPC; net sorption = net desorption), and endpoint (EP, retention at the highest initial DOC concentration). Leachates from CN and CR showed very similar retention behavior, and between the two species the retention of root leachates was more similar than the retention of foliage leachates. Soils sampled from aspen forests showed higher affinity for new DOC than conifer soils [higher sorption rate (n), lower NPC, and higher EP] regardless of the DOC source. The findings suggest that the higher DOC sorption on aspen soils might be a major driver for more stable SOC under aspen stands in North America.

Dissolved Organic Matter Quality and Biofilm Composition Affect Microbial Organic Matter Uptake in Stream Flumes

Agriculture delivers significant amounts of dissolved organic matter (DOM) to streams, thereby changing the composition and biodegradability of the aquatic DOM. This study focuses on the interactive effects of DOM quality and biofilm composition on the degradation of DOM in a laboratory flume experiment. Half of the flumes were exposed to light to stimulate algal growth, the other half was shaded. Leachates of deciduous leaves, maize leaves, and cow dung were added to the flumes in a single pulse and changes of DOC (dissolved organic carbon) and nutrient concentrations, DOM composition (absorbance and fluorescence data), chlorophyll-a concentrations, bacterial abundances, and enzymatic activities were recorded over a week. DOM was taken up with rates of 50, 109, and 136 µg DOC L−1 h−1 for dung, leaf, and maize leachates, respectively, in the light flumes and 37, 80, and 170 µg DOC L−1 h−1 in the dark flumes. DOC uptake correlated strongly with initial SRP (soluble reactive phosphorus) and DOC concentrations, but barely with DOM components and indices. Algae mostly stimulated the microbial DOC uptake, but the effects differed among differently aged biofilms. We developed a conceptual model of intrinsic (DOM quality) and external (environmental) controlling factors on DOM degradation, with the microbial community acting as biotic filter.

New insights into mechanisms of sunlight- and dark-mediated high-temperature accelerated diurnal production-degradation of fluorescent DOM in lake waters

The production of fluorescent dissolved organic matter (FDOM) by phytoplankton and its subsequent degradation, both of which occur constantly under diurnal-day time sunlight and by night time dark-microbial respiration processes in the upper layer of surface waters, influence markedly several biogeochemical processes and functions in aquatic environments and can be feasibly related to global warming (GW). In this work sunlight-mediated high-temperature was shown to accelerate the production of FDOM, but also its complete disappearance over a 24-h diurnal period in July at the highest air and water temperatures (respectively, 41.1 and 33.5 °C), differently from lower temperature months. Extracellular polymeric substances (EPS), an early-state DOM, were produced by phytoplankton in July in the early morning (6:00–9:00), then they were degraded into four FDOM components over midday (10:00–15:00), which was followed by simultaneous production and almost complete degradation of FDOM with reformation of EPS during the night (2:00–6:00). Such transformations occurred simultaneously with the fluctuating production of nutrients, dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and the two isotopes (δ15N and δ18O) of NO3−. It was estimated that complete degradation of FDOM in July was associated with mineralization of approximately 15% of the initial DOC, which showed a nighttime minimum (00:00) in comparison to a maximum at 13:00. FDOM identified by excitation-emission matrix spectroscopy combined with parallel factor analysis consisted of EPS, autochthonous humic-like substances (AHLS) of C- and M-types, a combined form of C- and M-types of AHLS, protein-like substances (PLS), newly-released PLS, tryptophan-like substances, tyrosine-like substances (TYLS), a combined form of TYLS and phenylalanine-like substances (PALS), and their degradation products. Finally, stepwise degradation and production processes are synthesized in a pathway for FDOM components production and their subsequent transformation under different diurnal temperature conditions, which provided a broader paradigm for future impacts on GW-mediated DOM dynamics in lake water.

Dissolved organic matter biodegradation along a hydrological continuum in permafrost peatlands

Arctic regions contain large amounts of organic carbon (OC) trapped in soil and wetland permafrost. With climate warming, part of this OC is released to aquatic systems and degraded by microorganisms, thus resulting in positive feedback due to carbon (C) emission. In wetland areas, water bodies are spatially heterogenic and separated by landscape position and water residence time. This represents a hydrological continuum, from depressions, smaller water bodies and lakes to the receiving streams and rivers. Yet, the effect of this heterogeneity on the OC release from the soil and its processing in waters is largely unknown and not accounted for in C cycle models of Arctic regions. Here we investigated the dissolved OC (DOC) biodegradation of aquatic systems along a hydrological continuum located in two discontinuous permafrost sites: in western Siberia and northern Sweden. The biodegradable dissolved OC (BDOC15; % DOC lost relative to the initial DOC concentration after 15 days incubation at 20 °C) ranged from 0 to 20% for small water bodies located at the beginning of the continuum (soil solutions, small ponds, fen and lakes) and from 10 to 20% for streams and rivers. While the BDOC15 increased, the removal rate of DOC decreased along the hydrological continuum. The potential maximum CO2 production from DOC biodegradation was estimated to account for only a small part of in-situ CO2 emissions measured in peatland aquatic systems of northern Sweden and western Siberia. This suggests that other sources, such as sediment respiration and soil input, largely contribute to CO2 emissions from small surface waters of permafrost peatlands. Our results highlight the need to account for large heterogeneity of dissolved OC concentration and biodegradability in order to quantify C cycling in arctic water bodies susceptible to permafrost thaw.

Dissolved organic carbon chemical composition controls the rate of CO2 release from rewetted soil

AbstractThe CO2 flush from dried and rewetted soil has been proposed as an indicator of soil health. Drying and rewetting releases mineralizable carbon (C) due to effects on microbial populations and aggregate stability. It is a complex, integrative process, and its relationship to soil health is incompletely understood. In order to increase our understanding of this process, we investigated changes in the quantity and chemistry of dissolved organic carbon (DOC) in a laboratory incubation study during 48 h after rewetting dried soil. A LI‐COR infrared gas analyzer was used to quantify CO2 released. DOC was extracted by water and quantified by a Shimadzu TOC‐VCPH. Changes in chemical composition of the DOC were followed during decomposition using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT‐ICR‐MS). The initial DOC was 67% lignin‐like, and 6.5% N‐containing aliphatic material (number‐averaged formula distribution). The contribution of N‐containing aliphatics to total DOC declined to 0.2% by 48 h, although the amount of DOC declined only 34% by 48 h. The lignin‐like material increased to 76% of the total DOC by 48 h of incubation, suggesting it is less microbially available than other DOC constituents. The decline in CO2 release rate was related to the loss of readily mineralizable N‐containing compounds rather than to total DOC, supporting the use of the CO2 flush as an indicator of mineralizable N. Further studies designed to elucidate the biological and abiotic mechanisms controlling the CO2 flush will improve its usefulness as a soil health indicator.

Long-term stability of marine dissolved organic carbon emerges from a neutral network of compounds and microbes

Dissolved organic carbon (DOC) is the main energy source for marine heterotrophic microorganisms, but a small fraction of DOC resists microbial degradation and accumulates in the ocean. The reason behind this recalcitrance is unknown. We test whether the long-term stability of DOC requires the existence of structurally refractory molecules, using a mechanistic model comprising a diverse network of microbe-substrate interactions. Model experiments reproduce three salient observations, even when all DOC compounds are equally degradable: (i) >15% of an initial DOC pulse resists degradation, but is consumed by microbes if concentrated, (ii) the modelled deep-sea DOC reaches stable concentrations of 30–40 mmolC/m3, and (iii) the mean age of deep-sea DOC is several times the age of deep water with a wide range from <100 to >10,000 years. We conclude that while structurally-recalcitrant molecules exist, they are not required in the model to explain either the amount or longevity of DOC.

Non-conservative patterns of dissolved organic matter degradation when and where lake water mixes

In this study, we experimentally investigated the degradation of dissolved organic matter (DOM) during lateral and vertical mixing of different water masses in a peri-alpine lake. River intrusions and vertical winter turnover in Lake Geneva (Switzerland, France) were simulated through short-term laboratory incubations by mixing riverine and lacustrine waters or lacustrine waters collected at different depths in winter. Dissolved organic carbon (DOC) degradation was monitored by dissolved oxygen (DO) measurements and changes in DOM composition were tracked by fluorescence spectroscopy in pure and mixed treatments during 72 h. Initial DOC content and DOM composition were relatively similar between end-members. The amount of DOC respired at the end of the incubation was similar between treatments, but decay constants in mixed treatments derived from a first-order decay model were significantly higher than expected values calculated based on a simple mass balance model. Fluorescent dissolved organic matter (FDOM) in mixed treatments followed non-conservative patterns that could not be predicted based on observations made in pure treatments. Hence, one protein-like and one microbial-like fluorophore that were consumed in lake waters were continuously produced in mixed treatments although lake waters represented 90% of the mix. No relationships were observed between the rate of DOC consumption and the initial DOM composition, suggesting that other factors such as nutrients and/or interactions in microbial communities were involved. Moreover, no relationships were found between DOC and FDOM patterns during incubations, suggesting that these measurements targeted different facets of microbial metabolism of DOM, respectively microbial respiration (catabolism) and microbial production (anabolism). While additional investigations are required in order to identify the drivers of these changes, this study provides evidence of non-conservative behavior of DOM degradation in mixing zones in lakes.

Treatment of Organic Matter and Tetracycline in Water by Using Constructed Wetlands and Photocatalysis

In this study, the ability of a bench-scale simulated constructed wetland (CW) to remove organic matter (OM) and tetracycline (TC) from water was examined. The performance of CW was evaluated by varying the initial concentrations of the target compounds and the hydraulic retention times (HRTs). Findings showed that OM removal efficiencies were 55.2–80.8%, 28.1–71.9% and 72.1–79.7% for ultraviolet absorbance at 254 nm (UV-254), dissolved organic carbon (DOC) and soluble chemical oxygen demand (sCOD) respectively, under 1 day-HRT, whereas higher initial DOC concentration achieved better removal efficiencies. Changing from 1 day-HRT to 2 day-HRT, the removal efficiency of OMs remained practically unchanged, while that of NH3-N increased considerably, from 61.7% to 73.0%, implying that the removal of ammonia in CW needs a longer time for complete treatment. CW also showed an excellent performance in removing TC, especially in the first two hours of operation through the absorption process. In addition, the findings from this research revealed an improvement in effluent water quality when photocatalysis (TiO2/α-Al2O3, with ultraviolet A (UVA) irradiation) was used as the post-treatment following CW, presented by the increase in removal efficiency of OMs of the combined system compared to that of CW alone. This study points to the possible and promising application of the low-cost water treatment system for dealing with OMs and TC in water.

Reused Cultivation Water Accumulates Dissolved Organic Carbon and Uniquely Influences Different Marine Microalgae.

Reusing growth medium (water supplemented with nutrients) for microalgae cultivation is required for economical and environmentally sustainable production of algae bioproducts (fuels, feed, and food). However, reused medium often contains microbes and dissolved organic matter that may affect algae growth. While the accumulation of dissolved organic carbon (DOC) in reused medium has been demonstrated, it is unclear whether DOC concentrations affect algae growth or subsequent rates of algal DOC release. To address these questions, lab-scale experiments were conducted with three marine microalgae strains, Navicula sp. SFP, Staurosira sp. C323, and Chlorella sp. D046, grown in medium reused up to four times. Navicula sp. and Chlorella sp. grew similarly in reused medium as in fresh medium, while Staurosira sp. became completely inhibited in reused medium. Across the three algae, there was no broad trend between initial DOC concentration in reused medium and algae growth response. Navicula sp. released less DOC overall in reused medium than in fresh medium, but DOC release rates did not decrease proportionally with increased DOC concentrations. Net DOC accumulation was much lower than gross DOC released by Navicula sp. and Staurosira sp., indicating the majority of released DOC was degraded. Additionally, biodegradation experiments with reused media showed no further net decrease in DOC, suggesting the accumulated DOC was recalcitrant to the associated bacteria. Overall, these results suggest that taxa-specific factors may be responsible for algae growth response in reused medium, and that DOC release and accumulation are insensitive to prior cultivation rounds. Choosing an algae strain that is uninhibited by accumulated DOC is therefore critical to ensure successful water reuse in the algae industry.

Controls on fluvial carbon efflux from eroding peatland catchments

AbstractGlobal peatlands store an unparalleled proportion of total global organic carbon but it is vulnerable to erosion into fluvial systems. Fluvial networks are being recognized as areas of carbon transformation, with eroded particulate organic carbon processed to dissolved organic carbon and CO2. Existing studies indicate biodegradation and photodegradation as key processes controlling the transformation of organic carbon in fluvial systems, with initial concentrations of dissolved organic carbon (DOC) identified as a control on the rate of carbon mineralization. This study manipulates temperature and incident light intensity to investigate carbon mineralization rates in laboratory simulations of peatland sediment transport into fluvial systems. By directly measuring gaseous CO2 emissions from sampled stream water, the relationship of temperature and light intensity with carbon efflux is identified. In simulations where sediment (as particulate organic matter, POM) is absent, temperature is consistently the dominant factor influencing carbon efflux rates. This influence is independent of the initial DOC concentration of the water sample. In simulations where POM was added, representing a peatland river receiving eroded terrestrial sediment, initial DOC concentration predicts 79% of the variation in total gaseous carbon efflux whereas temperature and light intensity predict 12% and 3%, respectively. When sampled stream water's mineralization rates in the presence of added POM are analysed independently, removing DOC as a model variable, the dominant variable affecting CO2 efflux is opposite for each sample. This study presents novel data suggesting peatland erosion introduces further complexity to dynamic stream systems where rates of carbon transformation processes and the influence of specific environmental variables are interdependent. Anthropogenic climate change is identified as a leading risk factor perpetuating peatland erosion; therefore, understanding the fate of terrestrial sediment in rivers and further quantifying the benefits of protecting peatland soils will be of increasing importance to carbon budgeting and ecosystem function studies.