Organic Carbon Degradation Rate Research Articles

Redox Conditions Alter Biodegradation Rates and Microbial Community Dynamics of Hydraulic Fracturing Fluid Organic Additives in Soil–Groundwater Microcosms

One of the environmental risks associated with use of hydraulic fracturing stimulation technologies for oil and natural gas recovery is the potential release of used fluids into surface waters, soils, and groundwater that could contaminate drinking water resources. To better characterize biodegradability of organic additives, we developed a synthetic fracturing fluid (SFF) based on industry-disclosed formulas, compared its organic carbon composition to fluids used in Pennsylvania's Marcellus shale, and amended agricultural soil–groundwater microcosms with three different SFF concentrations to determine organic carbon degradation rates, changes in system biogeochemistry, and microbial community dynamics under aerobic and anaerobic conditions. Microorganisms indigenous to soils and groundwater were able to degrade between 70% and 92% of the amended dissolved organic carbon within 39 days, suggesting significant mineralization, transformation, or biomass assimilation of organic additives across anaerobic and aerobic redox conditions. Sequencing analysis of the 16S rRNA gene revealed a greater abundance of Pseudomonas in aerobic treatments and a higher relative portion of Desulfovibrio in anaerobic treatments amended with SFF, indicating that these taxa may be involved in SFF biodegradation processes under specific redox conditions. Results provide insight into biodegradability of hydraulic fracturing fluid organic additives in shallow agricultural soils and groundwater and biogeochemical processes that may attenuate their migration if accidentally released or spilled at the surface during hydraulic fracturing activities.

Arsenic release and attenuation in a multilayer aquifer in the Po Plain (northern Italy): Reactive transport modeling

Groundwater As concentrations >WHO limit (10 μg/L) are frequently found in the Po Plain (N. Italy). Although several hypotheses on As mobilization exist (i.e., reductive dissolution driven by peat degradation), the mechanisms of As release and subsequent attenuation acting in the multilayer aquifer in the Po Plain were poorly understood.The present work aims at implementing a reactive transport modeling of the aquifer system in Cremona, affected by As <183 μg/L, in order to quantify and test the feasibility of As release by the reductive dissolution of Fe-oxides driven by the degradation of peat contained in leaky aquitards and As attenuation downstream by the co-precipitation in iron sulfides.The model, based on a partial equilibrium approach, revealed that the observed As, Fe and Mn chemistry could be mostly explained by the simultaneous equilibrium between Fe-oxide and sulfate reduction and FeS precipitation and by the equilibrium of rhodochrosite precipitation/dissolution. Model results, together with litholog analysis, supported the assumption of peat as the likely source of organic matter driving As release. The model fitted to measured data showed that the peak in the organic carbon degradation rate at 20–40 m below surface (average of 0.67 mM/y), corresponding to the shallow peaty aquitard and the upper portion of the underlying semiconfined aquifer, is associated with the peak of net release of As (average of 0.32 μM/y) that is followed just downstream by a net precipitation in iron sulfides at 40–60 m below surface (average of 0.30 μM/y). These results support the assumptions of peaty aquifers as drivers of As release and iron sulfides as As traps. The model also outlined the following aspects that could have a broad applicability in other alluvial As affected aquifers worldwide: (a) shallow peaty aquitards may have a greater role in driving the As release since they likely have young and more reactive organic matter; (b) the occurrence of Fe-oxide reduction and FeS precipitation, that represent the As source and sink, together with sulfate reduction occurring simultaneously close to equilibrium may restrict the As mobility limiting the extent of contamination just downstream the source of organic matter that drives its release.

Inoculation with nitrogen turnover bacterial agent appropriately increasing nitrogen and promoting maturity in pig manure composting

The nitrogen turnover bacterial (NTB) agent, which is closely related to nitrogen turnover, was comprised of a bacterial consortium of ammonifiers, nitrobacteria and Azotobacter in this study. The three constituents of the bacterial consortium were added to pig manure and wheat straw mixtures in different doses and at different times, and subsequently composted to investigate their effects on nitrogen transformation and maturity. Throughout the period, the total N loss was 35–56%, 10.7–22.7% of which consisted of NH3, and 18–35% of the initial organic carbon was degraded. Adding the NTB agent prolonged the thermophilic stage by one to six days compared to the control. The lowest N loss (35%), the highest degradation rate of organic carbon (35%) and the greatest increase in total nitrogen content (36.1%) occurred in the inoculation with 1% NTB agent at the beginning of composting. However, adding 1% NTB agent after the thermophilic stage and 3% NTB agent at the beginning of composting had no positive effect with respect to retaining nitrogen or accelerating the maturation process. Therefore, the inoculation with 1% NTB agent at the beginning of composting was effective for reducing N loss and promoting maturity.

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO2 uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO3) and silicate (Si(OH)4) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35mmolm−2d−1, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1millionkm2. We estimate that 0.15TmolCy−1 is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on Corg export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.

Water consumption in hydrocarbon generation and its significance to reservoir formation

Abstract The geochemical effects of water consumption during hydrocarbon generation were studied on the basis of evolution laws of source rocks and simulation experiments on hydrocarbon generation. Water consumption statistics were obtained in order to study the relationship between water consumption during hydrocarbon generation and hydrocarbon migration and reservoir formation. The simulation experiments of hydrocarbon generation were performed under hydrous and anhydrous conditions for correlation. The geochemical characteristics of organic evolution under these two conditions were analyzed and the variations of hydrocarbon generation potential and carbon transformation ratio were emphasized. The results show the effects that organic matter and water have on each other during hydrocarbon generation: part of unavailable carbon is activated in kerogen and hydrogen is increased in degraded products, which leads to the increase of total hydrocarbon generation potential. According to water consumption mechanisms, the quantitative evaluation method of water consumption in hydrocarbon generation was put forward and used in the studies of the main source rocks in the Dongying Sag. Both of the water consumption and the depth range of the Upper Es4 Member are larger, while those of the Lower and Middle Es3 Members are smaller. Water consumption affects hydrocarbon migration and accumulation by increasing organic carbon degradation rate to increase fluid volume. Pore fluid pressure and oil-bearing saturation are consequently increased. The matching relationship between water-consuming hydrocarbon generation intervals and water-consuming diagenesis intervals enhances the dynamic forces of hydrocarbon migration, which benefits the formation of self-generating and self-preserving reservoirs or lower-generating and upper-preserving reservoirs.

Paleoproductivity and paleoredox conditions during late Pleistocene accumulation of laminated diatom mats in the tropical West Pacific

Paleoproductivity and paleoredox conditions were reconstructed in a sediment core in the Parece Vela Basin of the eastern Philippine Sea. The core consists of three units, from youngest to oldest: (1) laminated diatom mats (LDM) formed by Ethmodiscus rex during the Last Glacial Maximum (~18–28kyr B.P.), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Elevated levels of export productivity during LDM deposition are indicated by high values for excess Ba, opal content, and TOC/Ti ratios. Estimated rates of organic carbon degradation (ca. 98%), opal mass accumulation (average 1322gm−2yr−1), and corrected organic carbon flux (average 248gm−2yr−1) are comparable to high-productivity regions of the modern ocean. The LDM is also characterized by moderate enrichment of redox-sensitive elements such as U, Mo, Cd, and Zn, highly 34S-depleted pyrite sulfur isotopic compositions (indicating bacterial sulfate reduction in a sulfate-unlimited system), and C–S–Fe systematics reflecting limitation of pyrite formation by organic matter rather than reactive Fe availability. These features suggest mainly suboxic conditions in bottom waters but development of sulfidic–anoxic conditions at or close to the sediment–water interface. Association of intensified anoxia with productivity maxima indicates that export production was a more important control on bottom water redox conditions than lateral ventilation. The DC and PC accumulated under oxic to suboxic conditions. Our observations suggest that redox environments during deposition of laminated marine sediments are more complicated and varied than previously thought, and, thus, the use of sediment lamination as an indicator of anoxic bottom water conditions must be approached cautiously.

Potential methane reservoirs beneath Antarctica

Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14 kilometres thick and an estimated 21,000 petagrams (1 Pg equals 10(15) g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300 metres in West Antarctica and 700 metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.

Enhanced leachate recirculation and stabilization in a pilot landfill bioreactor in Taiwan

This study focused on the treatment of municipal solid waste (MSW) by modification and recirculation of leachate from a simulated landfill bioreactor. Hydrogen peroxide was added to recirculated leachate to maintain a constant oxygen concentration as the leachate passed again through the simulated landfill bioreactor. The results showed that leachate recirculation increased the dissolved oxygen concentration in the test landfill bioreactor. Over a period of 405 days, the biochemical oxygen demand (BOD(5)) in the collected leachate reduced by 99.7%, whereas the chemical oxygen demand (COD) reduced by 96%. The BOD(5)/COD ratio at the initial stage of 0.9 improved to 0.09 under aerobic conditions (leachate recirculation with added hydrogen peroxide) compared with the anaerobic test cell 0.11 (leachate recirculation alone without hydrogen peroxide). The pH increased from 5.5 to 7.6, and the degradation rate of organic carbon was 93%. Leachate recirculation brings about the biodegradation of MSW comparatively faster than the conventional landfill operation. The addition of a constant concentration of hydrogen peroxide was found to further increase the biodegradation. This increased biodegradation rate ultimately enables an MSW landfill to reach a stable state sooner and free up the land for further reuse.

Prokaryotic community structure and respiration during long‐term incubations

Despite the importance of incubation assays for studies in microbial ecology that frequently require long confinement times, few reports are available in which changes in the assemblage structure of aquatic prokaryotes were monitored during long-term incubations. We measured rates of dissolved organic carbon degradation and microbial respiration by consumption of dissolved oxygen (DO) in four experiments with Lake Kinneret near-surface water and, concomitantly, we analyzed the variability in prokaryotic community structure during long-term dark bottle incubations. During the first 24 h, there were only minor changes in bacterial community composition. Thereafter there were marked changes in the prokaryotic community structure during the incubations. In contrast, oxygen consumption rates (a proxy for both respiration and dissolved organic carbon degradation rates) remained stable for up to 10–23 days. This study is one of the first to examine closely the phylo-genetic changes that occur in the microbial community of untreated freshwater during long-term (days) incubations in dark, sealed containers. Novel information on the diversity of the main bacterial phylotypes that may be involved in dissolved organic matter degradation in lake Kinneret is also provided. Our results suggest that, under certain ecological settings, constant community metabolic rates can be maintained as a result of shifts in community composition.

The dark portion of the Mediterranean Sea is a bioreactor of organic matter cycling

Total prokaryotic abundance, prokaryotic heterotrophic production and enzymatic activities were investigated in epi‐, meso‐ and bathypelagic waters along a longitudinal transect covering the entire Mediterranean Sea. The prokaryotic production and enzymatic activities in deep waters were among the highest reported worldwide at similar depths, indicating that the peculiar physico‐chemical characteristics of the Mediterranean Sea, characterized by warm temperatures (typically 13°C also at abyssal depths), support high rates of organic carbon degradation and incorporation by prokaryotic assemblages. The higher trophic conditions in the epipelagic waters of the Western basin resulted in significantly higher prokaryotic production and enzymatic activities rates than in the Central‐Eastern basin. While all of the variables decreased significantly from epi‐ to meso‐ and bathypelagic waters, cell‐specific hydrolytic activity and cell‐specific carbon production significantly increased. In addition, the deep‐water layers were characterized by low half‐saturation constants (Km) of all enzymatic activities. These findings suggest that prokaryotic assemblages inhabiting the dark portion of the Mediterranean Sea are able to channel degraded carbon into biomass in a very efficient way, and that prokaryotic assemblages of the deep Mediterranean waters work as a “bioreactor” of organic matter cycling. Since prokaryotic production and enzymatic activities in deep water masses were inversely related with oxygen concentration, we hypothesize a tight link between prokaryotic metabolism and oxygen consumption. As climate change is increasing deep‐water temperatures, the predicted positive response of prokaryotic metabolism to temperature increases may accelerate oxygen depletion of deep Mediterranean waters, with cascade consequences on carbon cycling and biogeochemical processes on the entire deep basin.

Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone

Benthic fluxes of dissolved ferrous iron (Fe2+) and phosphate (TPO4) were quantified by in situ benthic chamber incubations and pore‐water profiles along a depth transect (11°S, 80‐1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom‐water O2 levels were &lt; 2 µmol L−1 down to 500‐m water depth, and increased to ∼ 40 µmol L−1 at 1000 m. Fe2+ fluxes were highest on the shallow shelf (maximum 316 mmol m−2 yr−1), moderate (15.4 mmol m−2 yr−1) between 250 m and 600 m, and negligible at deeper stations. In the persistent OMZ core, continuous reduction of Fe oxyhydroxides results in depletion of sedimentary Fe : Al ratios. TPO4 fluxes were high (maximum 292 mmol m−2 yr−1) throughout the shelf and the OMZ core in association with high organic carbon degradation rates. Ratios between organic carbon degradation and TPO4 flux indicate excess release of P over C when compared to Redfield stoichiometry. Most likely, this is caused by preferential P release from organic matter, dissolution of fish debris, and/or P release from microbial mat communities, while Fe oxyhydroxides can only be inferred as a major P source on the shallow shelf. The benthic fluxes presented here are among the highest reported from similar, oxygen‐depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean. The shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom‐water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of Fe2+ and TPO4.

Application of Multiwalled Carbon Nanotubes in a Wetted-Wall Corona-Discharge Reactor To Enhance Phenol Decomposition in Water

A wetted-wall corona-discharge reactor can be used to decompose toxic organic compounds in water using reactive radicals and ozone. In this study, multiwalled carbon nanotubes (MWCNTs) were synthesized directly on the surface of a cylindrical stainless steel anode in this type of reactor to improve its decomposition performance. The surface area of the anode including the synthesized MWCNTs became approximately 332 times larger than that of the bare stainless steel anode. This increase in surface area enhanced the rate at which the phenol concentration decreased, although the degradation rate of total organic carbon (TOC) did not increase. When cobalt nanoparticles were loaded onto these MWCNTs, the TOC degradation rate improved significantly. The synthesis of MWCNTs on the anode surface was necessary to realize this enhancement. This study discusses the mechanisms of these enhancement effects.

Benthic remineralization in the northwest European continental margin (northern Bay of Biscay)

We report a dataset of sediment characteristics and biogeochemical fluxes at the water–sediment interface at the northwest European continental margin (northern Bay of Biscay). Cores were obtained in June 2006, May 2007 and 2008, at 18 stations on the shelf break (120–180 m), and at 2 stations on the continental slope (520 and 680 m). Water–sediment fluxes of dissolved oxygen (O 2), total alkalinity (TA), nitrate ( NO 3 − ), and dissolved silicate (DSi) were measured at a total of 20 stations. Sediment characteristics include: grain size, chlorophyll- a (Chl- a) and phaeopigment (Phaeo) content, particulate organic (POC) and inorganic (PIC) carbon content, and lead-210 ( 210Pb) and thorium-234 ( 234Th) activities. Sediments were sandy (fine to coarse) with organic matter (OM) (1.0–4.0%) and Chl- a (0.01–0.95 μg g −1) contents comparable to previous investigations in the same region, and a relatively high PIC fraction (0.8–10.2%). Water–sediment O 2 fluxes (−2.4 to −8.4 mmol O 2 m −2 d −1) were low compared to other coastal environments and correlated well with OM and Chl- a content. 234Th activity profiles indicated that Chl- a sediment content was mainly controlled by physical mixing processes related to local hydrodynamics. The correlation between water–sediment fluxes of O 2 and NO 3 − indicated a close coupling of nitrification/denitrification and total benthic organic carbon degradation. Dissolution of biogenic silica (0.05–0.95 mmol m −2 d −1) seemed uncoupled from organic carbon degradation, as characterized by water–sediment O 2 fluxes. The link between water–sediment fluxes of TA and O 2 indicated the occurrence of metabolic driven dissolution of calcium carbonates (CaCO 3) in the sediments (∼0.33±0.47 mmol m −2 d −1), which represented ∼1% of the pelagic calcification rates due to coccolithophores measured during the cruises. These CaCO 3 dissolution rates were below those reported in sediments of continental slopes and of the deep ocean, probably due to the high over-saturation with respect to CaCO 3 of the water column overlying the continental shelf sediments of the northern Bay of Biscay. Rates of total benthic organic carbon degradation were low compared to water column rates of primary production and aphotic community respiration obtained during the cruises.

A transfer function for the prediction of gas hydrate inventories in marine sediments

Abstract. A simple prognostic tool for gas hydrate (GH) quantification in marine sediments is presented based on a diagenetic transport-reaction model approach. One of the most crucial factors for the application of diagenetic models is the accurate formulation of microbial degradation rates of particulate organic carbon (POC) and the coupled formation of biogenic methane. Wallmann et al. (2006) suggested a kinetic formulation considering the ageing effects of POC and accumulation of reaction products (CH4, CO2) in the pore water. This model is applied to data sets of several ODP sites in order to test its general validity. Based on a thorough parameter analysis considering a wide range of environmental conditions, the POC accumulation rate (POCar in g/m2/yr) and the thickness of the gas hydrate stability zone (GHSZ in m) were identified as the most important and independent controls for biogenic GH formation. Hence, depth-integrated GH inventories in marine sediments (GHI in g of CH4 per cm2 seafloor area) can be estimated as: GHI = a · POCar · GHSZb · exp (– GHSZc/POCar/d) + e with a = 0.00214, b = 1.234, c = –3.339, d = 0.3148, e = –10.265. The transfer function gives a realistic first order approximation of the minimum GH inventory in low gas flux (LGF) systems. The overall advantage of the presented function is its simplicity compared to the application of complex numerical models, because only two easily accessible parameters need to be determined.

Organic matter release by the benthic upside-down jellyfish Cassiopea sp. fuels pelagic food webs in coral reefs

Recent studies have demonstrated that organic matter released by hermatypic corals can play an important role as carrier of energy, thereby initiating element cycles in coral reef systems. However, although another commonly occurring cnidarian, the scyphozoan upside-down jellyfish Cassiopea sp., can reach high abundances in such reef systems, its potential contribution to cycles of matter remains unresolved. Therefore this study aimed to quantify organic matter release by Cassiopea from the Northern Red Sea and evaluate whether this material is transferred to planktonic microbes and zooplankton. Mean mucoid particulate organic matter release was 21.2 ± 9.4 mg POC and 2.3 ± 1.1 PN m − 2 jellyfish surface area h − 1 , which exceeds release rates reported for hermatypic corals by factors of 2 to 15. Labelling experiments using stable N isotopes demonstrated uptake of Cassiopea-derived organic matter by the jellyfish-associated zooplanktonic mysids Idiomysis tsurnamali. Incubation experiments revealed that O 2 consumptions by microbes and zooplankton were 5.9- and 3.8-fold higher compared to seawater controls, respectively, when Cassiopea-derived organic matter was present. Total organic carbon (TOC) degradation rates increased 5.0-fold (0.27% h − 1 versus 1.38% h − 1 ), thereby indicating fast mineralization of Cassiopea-derived organic matter. These findings suggest that Cassiopea-derived organic matter may function as a newly discovered trophic pathway for organic matter from the benthic environment to pelagic food chains in coral reefs and other marine ecosystems.

Supercritical water oxidation of ion exchange resins: Degradation mechanisms

Spent ion exchange resins are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation could offer a viable treatment alternative to destroy the organic structure of resins and contain radioactivity. IER degradation experiments were carried out in a continuous supercritical water reactor. Total organic carbon degradation rates in the range of 95–98% were obtained depending on operating conditions. GC–MS chromatography analyses were carried out to determine intermediate products formed during the reaction. Around 50 species were identified for cationic and anionic resins. Degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol and acetic acid are the main compounds. However, other products are detected in appreciable yields such as phenolic species or heterocycles, for anionic IERs degradation. Intermediates produced by intramolecular rearrangements are also obtained. A radical degradation mechanism is proposed for each resin. In this overall mechanism, several hypotheses are foreseen, according to HOO radical attack sites.

Photo-induced environmental depletion processes of β-blockers in river waters

In order to improve the understanding of the fate and behaviour of pharmaceuticals in the environment there is a need to investigate in-stream depletion mechanisms, e.g. phototransformation of active pharmaceutical ingredients (APIs) in natural surface waters. In this study, abiotic and biotic degradation of selected beta-blockers was measured simultaneously in non-sterilised and sterilised river waters and deionised water (DIW) under simulated sunlight (lambda: 295-800 nm) and dark conditions, and at environmentally relevant concentrations, i.e.<or= ppb levels. Results suggested that the overall degradation followed pseudo first order kinetics under the solar simulation conditions and was between two and ten times faster in river waters than in DIW. There was a significant correlation (p < 0.07) between dissolved organic carbon (DOC) and overall first order degradation rate constants for the tested beta-blockers (n = 4-6), suggesting coloured DOC triplet-induced or reactive transient mediated oxidation mechanisms in river waters. Phototransformation was the main depletion mechanism for the beta-blockers tested over a 2 to 7 day period. Slow hydrolysis was observed for metoprolol only. Loss due to biodegradation in river waters was not observed for propranolol but was found for metoprolol and atenolol at a very slow rate within the study period. However, biodegradation of metoprolol was accelerated under the light conditions, implying that photo-induced intermediates could be more easily biodegraded in river waters.

Colonization patterns along the equatorial West African margin: Implications for functioning and diversity maintenance of bathyal and abyssal communities

In the framework of the deep-sea environmental programme BIOZAIRE (Ifremer-Total), colonization trays were deployed for 283–433 days at three sites along the equatorial West African margin: ZA at 1300-m depth, ZC at 4000-m depth far from the Congo canyon and ZD at 4000-m depth close to the Congo canyon. The experiments aimed at determining the influence of depth and local environmental settings on macrofaunal colonization patterns and organic carbon degradation rates. The trays were filled with glass beads and this artificial substrate was enriched with ground particulate organic matter in a gradient of 0%, 0.34%, 1.02% and 3.43% organic carbon. The highest rates of organic carbon degradation ranged, according to the duration of the experiments, from 1.59 to 2.36 gC m −2 day −1 but were independent of depth or location. Colonization rates, conversely, varied by one order of magnitude between bathyal and abyssal experiments. The influence of experimental treatments on the structure of the colonizing macrofauna also varied according to location and depth. At ZA, colonization patterns were highly predictable and driven by a shift in dominance of opportunistic taxa along the enrichment gradient. To a lesser extent, this was also true at ZD, near the Congo canyon, while at ZC the treatments had no significant effect on the composition of the colonizing fauna. At abyssal depth, high rates of organic matter degradation associated with low rates of colonization suggested that pulse of organic matter would mainly benefit the resident community. At bathyal depth, high colonization rates of a specialized fauna might conversely play an important role in the functioning of the ecosystem. The regional and local coexistence of an opportunistic fauna via a spatial storage effect associated with dispersal might significantly contribute to the maintenance of high diversity on continental margins.

Surface microlayers on temperate lowland lakes

At the air–water interface material, organisms accumulate and form a thin layer of organic and inorganic material called the surface microlayer (SML). In order to investigate the development, composition, and metabolism of SML on lakes, samples were collected using a screen sampler along with subsurface water (SSW) in an eutrophic and a mesotrophic lake from April to September 2007. Wind, solar irradiance, and lake temperature were followed continuously. Samples were analyzed for organic and inorganic compounds as well as for photosynthesis and respiration. Most compounds were enriched in the SML relative to the SSW. Enrichment was small, however, probably because sampling was performed on nonslick areas. Most compounds correlated closely between the SML and the SSW, confirming the hypothesis that most SML material originates from the bulk water. Correlations were strongest in the eutrophic lake, probably because external sources had a greater effect on SML concentrations in the mesotrophic lake. Enrichment of compounds and metabolic rates in the SML had similar seasonality and dependency of climatic conditions in the two lakes, suggesting common regulating mechanisms of enrichment and production. Enrichment factors of several compounds were higher at low bulk water concentrations, suggesting that atmospheric deposition then contributed relatively more to concentrations in the SML. Increasing temperature significantly decreased SML enrichment of TOC (total organic carbon), related to changes in TOC composition and higher heterotrophic activity, while wind and solar irradiance had no pronounced enrichment effect on any compound. Net photosynthesis was significantly lower in the SML, experiencing photoinhibition in one-third of the samples. In contrast, respiration was much elevated in the SML. Nonetheless, respiration in the SML never contributed by more than 0.3% of water column respiration, but the combination of enhanced degradation rates of organic carbon in the SML and strong interaction with water below suggests that the SML, nonetheless, may play an important role in degradation of refractory organic carbon. Combining these results, we found that the SML of nonslicked areas on lakes are enriched in organic and inorganic pools and constitute a strong heterotrophic environment, albeit of minor importance for whole lake pelagic metabolism.

Si–C interactions during degradation of the diatom Skeletonema marinoi

Abstract While a relationship between ballast and carbon in sedimenting particles has been well-documented, the mechanistic basis of this interaction is still under debate. One hypothesis is that mineral ballast protects sinking organic matter from degradation. To test this idea, we undertook a laboratory experiment using the diatom Skeletonema marinoi to study in parallel the dissolution of one of the most common mineral ballasts, biogenic silica (bSiO2), and the associated degradation of organic matter. Three different models were applied to our results to help elucidate the mechanisms driving bSiO2 dissolution and organic compound degradation. Results of this modeling exercise suggest that the diatom frustule is made up of two bSiO2 phases that dissolve simultaneously, but at different rates. In our experiments, the first phase was more soluble ( k bSiO 2 = 0.27 d - 1 ) and made up 31% of the total bSiO2. In this phase, bSiO2 was mainly associated with membrane lipids and the amino acids glutamic acid, tyrosine, and leucine. The second phase was more refractory ( k bSiO 2 = 0.016 d - 1 ), and contained more neutral lipid alcohols and glycine. Until it dissolved, the first bSiO2 phase effectively protected much of the organic matter from degradation: particulate organic carbon (POC) degradation rate constants increased from 0.025 to 0.082 d−1 after the total dissolution of this phase, and particulate organic nitrogen (PON) degradation rate constants increased from 0.030 to 0.094 d−1. Similar to POC and PON, the total hydrolyzable amino acids (THAA) degradation rate constant increased from 0.054 to 0.139 d−1 after dissolution of the first bSiO2 phase. The higher THAA degradation rate constant is attributed to a pool of amino acids that was produced during silicification and enclosed between the two silica phases. This pool of amino acids might come from the incorporation of silica deposition vesicles into the diatom wall and might not be directly associated with bSiO2. In contrast, most lipid degradation was not prevented by association with the more soluble bSiO2 phase, as the average lipid degradation rate constant decreased from 0.048 to 0.010 d−1 after 17 d of degradation. This suggests that most lipids were associated with rather than protected by silica, except pigments that appeared resistant to degradation, independently from silica dissolution. When the only organic compounds remaining were associated with the second bSiO2 phase, degradation rate constants decreased greatly; concentrations changed only slightly after day 25.