Dissolved Organic Carbon Turnover Research Articles

Mapping turnover of dissolved organic carbon in global topsoil

Dissolved organic carbon (DOC), the labile fraction of organic carbon, is a predominant substrate for microbes. Therefore, the turnover of DOC dominates microbial respiration in soils. We compiled a global dataset (1096 data points) of the turnover rates of DOC in 0–30 cm soil profiles and integrated the data with a machine learning algorithm to develop a global map of DOC turnover rate in global topsoil. The global DOC turnover rate in 0–30 cm soil was averaged as 0.0087 day−1, with a considerable variation among biomes. The fastest DOC turnover rate was found in tropical forests (0.0175 day−1) and the lowest in tundra (0.0036 day−1), exhibiting a declining trend from low to high latitudes. The DOC turnover rate is primarily controlled by edaphic and climate factors, as confirmed by the analyses with the structural equation model and the Mental's test. With a machine learning algorithm, we produced global maps of DOC turnover rate at a monthly scale, which were further combined with a global dataset of DOC density to produce monthly maps of carbon mineralization from DOC turnover in topsoil. The annual carbon release from DOC was estimated as 27.98 Pg C year−1 from topsoil across the globe, with the largest contribution from forest biomes, followed by pasture and grassland. Tundra released the least carbon from DOC due to its low turnover rate suppressed by low temperatures. The biome- and global-scale information of DOC turnover rate and carbon release from DOC provide a benchmark for ecosystem models to better project soil carbon dynamics and their contributions to global carbon cycling in the changing environment.

Unified understanding of intrinsic and extrinsic controls of dissolved organic carbon reactivity in aquatic ecosystems.

Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land‐to‐ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services.

Turnover of dissolved organic carbon fuels nocturnal CO2 emissions from a headwater catchment reservoir, Southeastern China: Effects of ecosystem metabolism on source partitioning of CO2 emissions

Dam reservoirs in headwater catchments, as critical zones for their proximity to terrestrial sources, play important roles in dissolved organic carbon (DOC) cycling. However, the effects of ecosystem metabolism (EM) on DOC cycling are not well known. Here, in-situ diurnal and monthly observations were conducted to measure EM (including gross primary production (GPP), ecosystem respiration (ER) and heterotrophic respiration (HR)), DOC turnover and CO2 emissions in a headwater catchment reservoir in Southeastern China in 2020. Our study showed the nocturnal CO2 emission rate was about twice as high as in daytime, and was strongly driven by EM. The values for DOC turnover velocity ranged from 0.10 to 1.59 m/day, and the average DOC turnover rate was 0.13 day−1, with the average removal efficiency of 12%. The contribution of respired DOC to daily CO2 emissions ranged from 17% to 61%. The accumulated efficiencies were estimated to be 13% for the selected 15 reservoirs throughout the Changjiang River network, corresponding to about 0.34 Tg C/year of the respired DOC. The modified CO2 flux was 0.75 Tg C/year, and respired DOC accounted for about 45% of total emitted CO2 from the 15 larger reservoirs. Our research emphasizes the necessity of incorporating the effects of EM into studies of reservoir DOC removal and CO2 emissions.

Bio- and photo-lability of dissolved organic matter in the Pearl River (Zhujiang) estuary

We investigated the bio- and photo-lability of dissolved organic matter (DOM) from the head, mixing zone, and mouth of the Pearl River estuary. At all three sites, bio- and photo-refractory dissolved organic carbon (DOC) and biorefractory chromophoric DOM (CDOM) dominated over the corresponding bio- and photo-labile constituents, while photolabile CDOM dominated over photo-refractory CDOM. Relative to the mixing-zone and mouth waters, the headwater was enriched with bio- and photo-labile DOC and photolabile CDOM and depleted with biolabile CDOM. Biolabile DOC was richer than photolabile DOC in the headwater, while photolabile CDOM was richer than biolabile CDOM at all three sites. Pre-biotransformation inhibited, stimulated, or had little impact on DOM photodegradation, depending on site. Ultra-violet absorption coefficients are indicators of bio- and photo-refractory DOC. The relative proportions of transparent and chromophoric DOM control the turnover of biolabile DOC and the effect of pre-biotransformation on DOM photodegradation.

Soothsaying DOM: A Current Perspective on the Future of Oceanic Dissolved Organic Carbon

The vast majority of freshly produced oceanic dissolved organic carbon (DOC) is derived from marine phytoplankton, then rapidly recycled by heterotrophic microbes. A small fraction of this DOC survives long enough to be routed to the interior ocean, which houses the largest and oldest DOC reservoir. DOC reactivity depends upon its intrinsic chemical composition and extrinsic environmental conditions. Therefore, recalcitrance is an emergent property of DOC that is analytically difficult to constrain. New isotopic techniques that track the flow of carbon through individual organic molecules show promise in unveiling specific biosynthetic or degradation pathways that control the metabolic turnover of DOC and its accumulation in the deep ocean. However, a multivariate approach is required to constrain current carbon fluxes so that we may better predict how the cycling of oceanic DOC will be altered with continued climate change. Ocean warming, acidification, and oxygen depletion may upset the balance between the primary production and heterotrophic reworking of DOC, thus modifying the amount and/or composition of recalcitrant DOC. Climate change and anthropogenic activities may enhance mobilization of terrestrial DOC and/or stimulate DOC production in coastal waters, but it is unclear how this would affect the flux of DOC to the open ocean. Here, we assess current knowledge on the oceanic DOC cycle and identify research gaps that must be addressed to successfully implement its use in global scale carbon models.

Lability of dissolved organic carbon from boreal peatlands: interactions between permafrost thaw, wildfire, and season

Boreal peatlands are major sources of dissolved organic carbon (DOC) to downstream aquatic ecosystems, where it influences carbon cycling and food web structure. Wildfire and permafrost thaw alter peatland vegetation and hydrology and may affect the quantity and chemical composition of exported DOC. We studied the influence of wildfire and thaw on microbial and photochemical lability of near-surface porewater DOC, assessed through 7 d incubations. We carried out these incubations in spring, summer, and fall but only found differences in spring when DOC biodegradability (% loss during dark incubations) increased with lower DOC aromaticity and C/N ratios. During spring, the most labile DOC was found in recently formed thermokarst bogs along collapsing peat plateau edges (25% loss), which was greater than in mature sections of thermokarst bogs (3%), and peat plateaus with intact permafrost (9%). Increased DOC lability following thaw was likely linked to high DOC production and turnover associated with productive hydrophilic Sphagnum mosses and sedges, rather than thawed permafrost peat. A wildfire (3 yr prior) reduced DOC biodegradability in both peat plateaus (4%) and rapidly collapsing peat plateau edges (14%). Biodegradability of DOC in summer and fall was low across all sites; 2% and 4%, respectively. Photodegradation was shown to potentially contribute significantly to downstream DOC degradation but did not vary across peatland sites. We show that disturbances such as permafrost thaw and wildfire have the potential to affect downstream carbon cycling, particularly as the largest influences were found in spring when peatlands are well connected to downstream aquatic ecosystems.

Oxygen dynamics in a boreal lake responds to long‐term changes in climate, ice phenology, and DOC inputs

AbstractBoreal lakes are impacted by climate change, reduced acid deposition, and changing loads of dissolved organic carbon (DOC) from catchments. We explored, using the process‐based lake model MyLake, how changes in these pressures modulate ice phenology and the dissolved oxygen concentrations (DO) of a small boreal humic lake. The model was parametrized against year‐round time series of water temperature and DO from a lake buoy. Observed trends in air temperature (+0.045°C yr−1) and DOC concentration (0.11 mg C L−1 yr−1, +1% annually) over the past 40 years were used as model forcings. A backcast of ice freezing and breakup dates revealed that ice breakup occurred on average 8 days earlier in 2014 than in 1974. The earlier ice breakup enhanced water column ventilation resulting in higher DO in the spring. Warmer water in late summer led to longer anoxic periods, as microbial DOC turnover increased. A long‐term increase in DOC concentrations caused a decline in lake DO, leading to 15% more hypoxic days (<3 mg L−1) and 10% more anoxic days (<15 µg L−1) in 2014 than in 1974. We conclude that climate warming and increasing DOC loads are antagonistic with respect to their effect on DO availability. The model suggests that DOC is a stronger driver of DO consumption than temperature. The browning of lakes may thus cause reductions in the oxythermal habitat of fish and aquatic biota in boreal lakes.

Microbial activities and organic matter degradation at three sites in the coastal North Atlantic: Variations in DOC turnover times and potential for export off the shelf

The Mid-Atlantic Bight is a highly productive region where considerable quantities of organic matter are processed, transformed, and respired. Much of this organic matter is in the form of dissolved organic carbon (DOC). We investigated the activities of heterotrophic microbial communities responsible for DOC processing at three sites in the coastal Mid-Atlantic Bight/northern end of the South Atlantic Bight: near the Chesapeake Bay mouth, off Cape Hatteras, and near Cape Lookout, NC. Activities of endo- and exo-acting extracellular enzymes (polysaccharide hydrolases, and glucosidase and peptidases, respectively), and bacterial protein production were measured in surface and subsurface waters at the three stations. Total suspended material, cell counts, and particle-associated enzyme activities were also quantified. Water masses at each station showed distinct physical characteristics (temperature, salinity, total suspended material), and differed notably in rates and spectrum of enzyme activities, as well as in the fraction of particle-associated activities. Despite similar cell counts, bacterial productivity differed by a factor of ca. 30 among depths and stations. Glucosidase and peptidase activities differed by a factor of ca. 40 among stations, but the differences in enzyme activities among stations did not scale with the differences in bacterial productivity. Theoretical turnover times of dissolved carbohydrates – calculated from total dissolved carbohydrate concentrations and polysaccharide hydrolase activities – range from weeks to months, suggesting that dissolved carbohydrates may be among the components of DOC that can be advected from the Mid-Atlantic Bight into the open ocean, connecting the carbon cycle in the ocean margin with the ocean's interior.

Modeling the rate of turnover of DOC and particulate organic carbon in a UK, peat‐hosted stream: Including diurnal cycling in short‐residence time systems

AbstractThis study proposes a multicomponent, multiprocess scheme to explain the turnover of organic matter (particulate and dissolved organic matter) in streams. The scheme allows for production and degradation of organic matter by both photic and aphotic processes with transformation of dissolved organic carbon (DOC) to increasingly refractory forms. The proposed scheme was compared to 10 months of experimental observations of the turnover and fate of particulate and dissolved organic matter in stream water from a peat‐covered catchment. The scheme was able to explain average decline in DOC concentration of 65% over 70 h with a 13% mean average percentage error based on turnover in three types of organic matter (particulate, labile dissolved, and refractory dissolved) although the order and rate of reactions did change between sets of experimental observations. The modeling suggests that activation energies are low for all except the most refractory forms of DOC in turn, suggesting that processes are not sensitive to temperature change. Application of the modeling scheme to organic matter turnover in the River Tees, northern England, showed that annual removal of total organic carbon was equivalent to between 13 and 33 t C/km2/yr from an at source export of between 22 and 56 t C/km2/yr giving a total in‐stream loss rate of between 53 and 62% over a median in‐stream residence time of 35 h.

Enhanced bacterial decomposition with increasing addition of autochthonous to allochthonous carbon without any effect on bacterial community composition

Abstract. Dissolved organic carbon (DOC) concentrations – mainly of terrestrial origin – are increasing worldwide in inland waters. Heterotrophic bacteria are the main consumers of DOC and thus determine DOC temporal dynamics and availability for higher trophic levels. Our aim was to study bacterial carbon (C) turnover with respect to DOC quantity and chemical quality using both allochthonous and autochthonous DOC sources. We incubated a natural bacterial community with allochthonous C (13C-labeled beech leachate) and increased concentrations and pulses (intermittent occurrence of organic matter input) of autochthonous C (phytoplankton lysate). We then determined bacterial C consumption, activities, and community composition together with the C flow through bacteria using stable C isotopes. The chemical analysis of single sources revealed differences in aromaticity and low- and high-molecular-weight substance fractions (LMWS and HMWS, respectively) between allochthonous and autochthonous C sources. Both DOC sources (allochthonous and autochthonous DOC) were metabolized at a high bacterial growth efficiency (BGE) around 50%. In treatments with mixed sources, rising concentrations of added autochthonous DOC resulted in a further, significant increase in bacterial DOC consumption of up to 68% when nutrients were not limiting. This rise was accompanied by a decrease in the humic substance (HS) fraction and an increase in bacterial biomass. Changes in DOC concentration and consumption in mixed treatments did not affect bacterial community composition (BCC), but BCC differed in single vs. mixed incubations. Our study highlights that DOC quantity affects bacterial C consumption but not BCC in nutrient-rich aquatic systems. BCC shifted when a mixture of allochthonous and autochthonous C was provided simultaneously to the bacterial community. Our results indicate that chemical quality rather than source of DOC per se (allochthonous vs. autochthonous) determines bacterial DOC turnover.

The rate of loss of dissolved organic carbon (DOC) through a catchment

Given observed trends in the concentration of DOC in surface waters in the northern hemisphere the degradation of DOC to CO2 could represent a major and increasing source of greenhouse gas to the atmosphere. However, studies of DOC turnover in rivers have been predominantly based upon mass balance studies and empirical studies have focused upon lakes and estuaries which have far longer residence times than would be the case for transit via rivers. The study measured DOC loss in unfiltered river water samples across an 818 km2 catchment every month for a year and considered total loss, photo and aphotic degradation as well as the rates of each process. The study found: Rate of total DOC change in daylight varied from loss of 30.1 mg C/l/day to an increase of 3.5 mg C/l/day: the average loss was 73% over 10 days. Rate of change due to photic processes varied from decrease of 19.4 mg C/l/day to an increase of 6 mg C/l/day, i.e. net photo-induced production was possible. Activation energy of the degradation was estimated as 2.6 ± 1.2 kJ/g C. The apparent quantum yield varied from 9.6 to −1.7 mmol C/mol photons. Coupling models of total loss of DOC with estimates of in-stream residence times showed that annual loss rates of DOC across the 818 km2 catchment would be between 48% and 69%, in line with estimates from mass balance studies, implying that in-stream DOC degradation represents a large, indirect source of CO2 emissions from peats and other organic soils. Annual rate of removal was increasing in line with increasing loss of DOC at source, implying that observed DOC trends are leading to increased CO2 emissions.

High mobilization of arsenic, metals and rare earth elements in seepage waters driven by respiration of old allochthonous organic carbon

Metal and metalloid mobilization processes within seepage water are of major concern in a range of water reservoir systems. The mobilization process of arsenic and heavy metals within a dam and sediments of a drinking water reservoir was investigated. Principle component analysis (PCA) on time series data of seepage water showed a clear positive correlation of arsenic with iron and DOC (dissolved organic carbon), and a negative correlation with nitrate due to respiratory processes. A relationship of reductive metal and metalloid mobilization with respiration of old carbon was shown. The system is influenced by sediment layers as well as a recent DOC input from degraded ombrotrophic peatbogs in the catchment area. The isotopic composition ((12)C, (13)C and (14)C) of DOC is altered along the path from basin to seepage water, but no significant changes in structural parameters (LC-OCD-OND, FT-IR) could be seen. DIC (dissolved inorganic carbon) in seepage water partly originates from respiratory processes, and a higher relationship of it with sediment carbon than with the DOC inventory of infiltrating water was found. This study revealed the interaction of respiratory processes with metal and metalloid mobilization in sediment water flows. In contrast to the presumption that emerging DOC via respiratory processes mainly controls arsenic and metal mobilization it could be shown that the presence of aged carbon compounds is essential. The findings emphasize the importance of aged organic carbon for DOC, DIC, arsenic and metal turnover.

The effect of rewetting drained fens with nitrate-polluted water on dissolved organic carbon and phosphorus release

Restoration of peatlands has been found to mitigate eutrophication due to the high and long-term capacity of peatlands to reduce nitrate (NO3−) fluxes. However, altered peat composition and/or contamination of the water used for rewetting can promote the release of dissolved substances, resulting in serious biogeochemical constraints on achievement of restoration goals. Elevated dissolved organic carbon (DOC) release can alter the turnover of organic matter in rewetted fens and adjacent aquatic ecosystems. Increasing availability of soluble reactive phosphate (SRP) can significantly elevate primary productivity, stimulating eutrophication and associated problems. We conducted a laboratory study to evaluate the effect of rewetting long-term drained fens with NO3− polluted water on DOC and SRP release. The effects of peat decomposition, NO3− loading rate and temperature were considered. Different peat substrates from one degraded, drained fen in northeast Germany were incubated under stagnant, inundated conditions. Two levels of peat decomposition (high and moderate), two NO3− loads (40kgNha−1 y−1 and 140kgNha−1 y−1) and two incubation temperatures (20°C and 5°C) were studied. Peat types were characterized by means of a sequential chemical extraction procedure to estimate the size of the DOC and SRP mobile pools. It was shown that peat decomposition controlled SRP and DOC release due to significant differences in mobile C and P pools. Low temperature slowed the rates of microbiologically mediated processes involved in SRP and DOC turnover. To some extent, the effect of temperature modified the effect of peat decomposition. Due to complete NO3− removal at the peat surface layer (<1cm), NO3− load did not influence SRP and DOC production in the peat, but did impact their release at the peat-water interface. Our study found that when highly decomposed peat is incubated at 20°C under low nitrate conditions, a substantial amount of DOC is released. Similarly, SRP release only occurred under those same conditions. Results from our experiments provide insight into the potential release of DOC and SRP that could occur in rewetted fens.

Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:7-19 (2012) - DOI: https://doi.org/10.3354/meps09677 Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment Gemma Vila-Reixach1,*, Josep M. Gasol2, Clara Cardelús2, Montserrat Vidal1 1Departament d’Ecologia, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Catalunya, Spain 2Departament de Biologia Marina i Oceanografia, Institut de Ciències del Mar-CSIC, 08003 Barcelona, Catalunya, Spain *Email: gemmavila@ub.edu ABSTRACT: To understand dissolved organic carbon (DOC) seasonal dynamics in a coastal oligotrophic site in the north-western Mediterranean Sea, we monitored DOC concentrations monthly over 3 yr, together with the meteorological data and the food-web-related biological processes involved in DOC dynamics. Additional DOC samples were taken in several inshore−offshore transects along the Catalan coast. We found DOC concentrations of ~60 µmol C l−1 in winter, with increasing values through the summer and autumn and reaching 100 to 120 µmol C l−1 in November. There was high inter-annual variability in this summer DOC accumulation, with values of 36, 69 and 13 µmol C l−1 for 2006, 2007 and 2008, respectively. The analysis of the microbial food-web processes involved in the DOC balance did not reveal the causes of this accumulation, since the only occasion on which we observed net DOC production (0.3 ± 1 µmol C l−1 d−1 on average) was in 2007, and the negative DOC balance of 2006 and 2008 did not prevent DOC accumulating. The DOC accumulation episodes coincided with low rates of water renewal (average 0.037 ± 0.021 d−1 from May to October) compared with those of winter to early spring (average 0.11 ± 0.048 d−1 from November to April). Indeed, the amount of DOC accumulated each year was inversely correlated with the average summer rainfall. We hypothesize that decreased DOC turn-over due to photochemical or biological processes—mostly active during the summer—and low water renewal rate combine to determine seasonal DOC accumulation and influence its inter-annual variability. KEY WORDS: Coastal DOC · Seasonality · Primary production · Bacterial metabolism · Physical transport Full text in pdf format PreviousNextCite this article as: Vila-Reixach G, Gasol JM, Cardelús C, Vidal M (2012) Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment. Mar Ecol Prog Ser 456:7-19. https://doi.org/10.3354/meps09677 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Dynamics of bacterial carbon metabolism at the entrance of a tropical eutrophic bay influenced by tidal oscillation

Bacterial metabolic rates were assessed at the entrance to Guanabara Bay, SE Brazil, during the summer (i.e. wet) season, in order to evaluate short-term effects of tidal oscillation on the magnitude of carbon flow through the bacterioplankton. Bacterial production (BP), respiration (BR) and abundance, phytoplankton production and biomass, particulate and dissolved organic matter (POM and DOM) concentrations and elemental composition, nutrient concentrations and hydrologi- cal profiles were estimated at a fixed station during 3 consecutive days with intervals of 3 h, in sur- face and bottom (20 m) layers. The study period covered the end of the spring tide and the beginning of the neap tide. Bacterial metabolic rates were highly variable, and a small fraction of the dissolved organic carbon (DOC) pool was used by bacterioplankton (DOC turnover: 23 to 71 d). The input of allochthonous DOC, inferred from DOM elemental composition (C:N:Psurface ~150:9:1; C:N:Pbottom ~245:6:1), and temperature were the main controlling factors of bacterial carbon metabolism. Although the magnitude of carbon flow through the bacterioplankton was highly variable, there were nevertheless consistent differences between layers. At the surface, the particulate organic carbon (POC) production (~14.3 µM C h -1 ), mainly due to phytoplankton, was higher than BR, and bacterioplankton acted equally as POC producers and DOM remineralizers (bacterial growth effi- ciency ~52%). Near the bottom, BR was equivalent to total POC production (~0.3 µM C h -1 ) and higher than BP (~0.05 µM C h -1 ), and thus bacteria acted mostly as a DOM sink. This study highlights the importance of short-term variations in carbon flow through bacteria for understanding the carbon cycle of estuarine systems.

Soil solution and stream water chemistry in a forested catchment II: Influence of organic matter

Differences between recharge and discharge areas in soil forming processes and nutrient conditions were observed in an earlier study of a catchment area in central Sweden (63°07′N, 16°70′E; Vestin, J.L.K., Norström, S.H., Bylund, D., Mellander, P-E., Lundström, U.S., submitted for publication to Geoderma. Soil solution and stream water chemistry in a forested catchment, I Dynamics.). To further examine the factors that determine the soil and stream water properties in the catchment area, the present study focused on the organic dynamics and the association of cations to different size fractions of organic matter. Six sampling plots were established in each of the recharge and discharge areas, respectively, with samples taken in June 2004. Dissolved organic carbon (DOC) and cation concentrations in the soil solution and stream water were determined. In the recharge area, low molecular mass (LMM) organics played an important role in transportation of several cations through the soil profile, inducing the podzolisation process by associating with Al and Fe. In the discharge area close to the stream, high molecular mass (HMM) organics appeared to play a crucial role in transportation of cations. Here the majority of recovered cations were associated with HMM organic matter, inhibiting the podzolisation process. The total concentration of C and DOC was higher than in the recharge area, and the concentration of cations increased with depth in the mineral soil. In the stream water, as in the discharge area soil solution, all carboxylic groups were associated to cations. Both Al and Fe were completely associated with the HMM DOC fraction, which indicated a rapid turnover of LMM DOC in stream water. Thus we conclude that DOC plays an important role in soil forming processes, and that its different size fractions have large effects on the transportation of elements in different soils and in stream water.

Rapid turnover of DOC in temperate forests accounts for increased CO2 production at elevated temperatures

The evidence for the contribution of soil warming to changes in atmospheric CO(2) concentrations and carbon stocks of temperate forest ecosystems is equivocal. Here, we use data from a beech/oak forest on concentrations and stable isotope ratios of dissolved organic carbon (DOC), phosphate buffer-extractable organic carbon, soil organic carbon (SOC), respiration and microbial gross assimilation of N to show that respired soil carbon originated from DOC. However, the respiration was not dependent on the DOC concentration but exceeded the daily DOC pool three to four times, suggesting that DOC was turned over several times per day. A mass flow model helped to calculate that a maximum of 40% of the daily DOC production was derived from SOC and to demonstrate that degradation of SOC is limiting respiration of DOC. The carbon flow model on SOC, DOC, microbial C mobilization/immobilization and respiration is linked by temperature-dependent microbial and enzyme activity to global warming effects of CO(2) emitted to the atmosphere.

A numerical simulation study of dissolved organic carbon accumulation in the northern Adriatic Sea

A mechanistic explanation for the accumulation of dissolved organic carbon (DOC), observed in coastal seas such as the Northern Adriatic Sea, is proposed here on the basis of numerical simulations of the marine ecosystem dynamics carried out with a coupled biogeochemical‐circulation model. The biogeochemical model is based on the European Regional Seas Ecosystem Model (ERSEM) upgraded with a more detailed representation of the DOC‐bacteria interactions and resolving different level of DOC lability/refractivity. The circulation model is the Adriatic Sea implementation of the Princeton Ocean Model. The analysis of simulations confirms the important role of the Po river nutrient input on the ecosystem dynamics and highlights the presence of a strong across‐shelf trophic gradient that, affecting the Bacterial Growth Efficiency (BGE), could be a key factor for the DOC accumulation. The simulations show the importance of circulation features in modulating the exchanges between areas having different trophic structure such as the western coastal strip, strongly influenced by the Po river runoff, and the open sea areas in the centre of the northern Adriatic sub‐basin. The DOC produced in the high energy system of the Po runoff coastal strip, characterized by high BGE, is transported toward the open areas, which is a more oligotrophic environment with lower BGE. In this area the DOC turnover time is strongly increased giving rise to the DOC accumulation.

Production and cycling of natural microbial exopolymers (EPS) within a marine stromatolite

Extracellular polymeric secretions (EPS) that are produced by cyanobacteria represent potential structuring agents in the formation of marine stromatolites. The abundance, production, and degradation of EPS in the upper layers of a microbial mat forming shallow subtidal stromatolites at Highborne Cay, Bahamas, were determined using 14C tracer experiments and were integrated with measurements of other microbial community parameters. The upper regions of a Type 2 [Reid, R.P., Visscher, P.T., Decho, A.W., Stolz, J., Bebout, B., MacIntyre, I.G., Dupraz, C., Pinckney, J., Paerl, H., Prufert-Bebout, L., Steppe, T., Des Marais, D., 2000. The role of microbes in accretion, lamination and early lithification of modern marine stromatolites. Nature (London) 406, 989–992] stromatolite mat exhibited a distinct layering of alternating “green” cyanobacteria-rich layers (Layers 1 and 3) and “white” layers (Layers 2 and 4), and the natural abundance of EPS varied significantly depending on the mat layer. The highest EPS abundance occurred in Layer 2. The production of new EPS, as estimated by the incorporation of 14C-bicarbonate into EPS, occurred in all layers examined, with the highest production in Layer 1 and during periods of photosynthesis (i.e., daylight hours). A large pool (i.e., up to 49%) of the total 14C-bicarbonate uptake was released as low molecular-weight (MW) dissolved organic carbon (DOC). This DOC was rapidly mineralized to CO 2 by heterotrophic bacteria. EPS degradation, as determined by the conversion of 14C-EPS to 14CO 2, was slowest in Layer 2. Results of slurry experiments, examining O 2 uptake following additions of organic substrates, including EPS, supported this degradation trend and further demonstrated selective utilization by heterotrophs of specific monomers, such as acetate, ethanol, and uronic acids. Results indicated that natural EPS may be rapidly transformed post-secretion by heterotrophic degradation, specifically by sulfate-reducing bacteria, to a more-refractory remnant polymer that is relatively slow to accumulate. A mass balance analysis suggested that a layer-specific pattern in EPS and low-MW DOC turnover may contribute to major carbonate precipitation events within stromatolites. Our findings represent the first estimate of EPS turnover in stromatolites and support an emerging idea that stromatolite formation is limited by a delicate balance between evolving microbial activities and environmental factors.

Sedimentary sources of old high molecular weight dissolved organic carbon from the ocean margin benthic nepheloid layer

Average 14C ages of dissolved organic carbon (DOC) in the ocean are 3–6,000 years, and are influenced by old DOC from continental margins. However, sources of DOC from terrestrial, autochthonous, and sedimentary organic carbon seem to be too young to be responsible for the old DOC observed in the ocean. Since colloidal organic carbon (COC, i.e., high molecular weight DOC), which is chemically very similar to that of bulk DOC, can be effectively isolated from seawater using cross-flow ultrafiltration, it can hold clues to sources and pathways of DOC turnover in the ocean. Radiocarbon measurements on COC in the water column and benthic nepheloid layer (BNL) from two continental margin areas (the Middle Atlantic Bight and the Gulf of Mexico) and controlled laboratory experiments were carried out to study sources of old DOC in the ocean margin areas. Vertical distributions of suspended particulate matter (SPM), particulate organic carbon (POC), nitrogen (PON), and DOC in the water column and bottom waters near the sediment-water interface all demonstrate a well developed benthic nepheloid layer in both ocean margin areas. Δ 14C values of COC isolated from the BNL (ranging from −362 ± 4‰ to −456 ± 3‰) were distinctly lower than their counterparts in surface waters (with a range of >0‰ to −210 ± 5‰). In other words, COC from the BNL was much older than COC from the overlying water column. These results, together with strong concentration gradients of SPM, POC, PON, and DOC, suggest a sedimentary source for organic carbon species and possibly for old COC as well in BNL waters. This is confirmed by the results from controlled laboratory experiments which showed that COC isolated from sediment resuspension had Δ 14C values between −260‰ and −352‰, which are significantly lower than those of bulk sediment (−87 ± 6‰) or resuspended particles (−138 ± 8‰). The heterogeneity of Δ 14C signatures in bulk SOC thus points to a preferential release of old organic components from sediment resuspension, which can be the transport mechanism of the old benthic COC observed in ocean margin areas. Old COC from continental margin nepheloid layers may thus be a potential source of old DOC to the deep ocean.