The effect of sulfate concentration on (sub)millimeter-scale sulfide δS in hypersaline cyanobacterial mats over the diurnal cycle

Substantial isotopic fractionations are associated with many microbial sulfur metabolisms and measurements of the bulk δ³⁴S isotopic composition of sulfur species (predominantly sulfates and/or sulfides) have been a key component in developing our understanding of both modern and ancient biogeochemical cycling. However, the interpretations of bulk δ³⁴S measurements are often non-unique, making reconstructions of paleoenvironmental conditions or microbial ecology challenging. In particular, the link between the μm-scale microbial activity that generates isotopic signatures and their eventual preservation as a bulk rock value in the geologic record has remained elusive, in large part because of the difficulty of extracting sufficient material at small scales. Here we investigate the potential for small-scale (∼100 μm-1 cm) δ³⁴S variability to provide additional constraints for environmental and/or ecological reconstructions. We have investigated the impact of sulfate concentrations (0.2, 1, and 80 mM SO₄) on the δ³⁴S composition of hydrogen sulfide produced over the diurnal (day/night) cycle in cyanobacterial mats from Guerrero Negro, Baja California Sur, Mexico. Sulfide was captured as silver sulfide on the surface of a 2.5 cm metallic silver disk partially submerged beneath the mat surface. Subsequent analyses were conducted on a Cameca 7f-GEO secondary ion mass spectrometer (SIMS) to record spatial δ³⁴S variability within the mats under different environmental conditions. Isotope measurements were made in a 2-dimensional grid for each incubation, documenting both lateral and vertical isotopic variation within the mats. Typical grids consisted of ∼400-800 individual measurements covering a lateral distance of ∼1 mm and a vertical depth of ∼5-15 mm. There is a large isotopic enrichment (∼10-20‰) in the uppermost mm of sulfide in those mats where [SO₄] was non-limiting (field and lab incubations at 80 mM). This is attributed to rapid recycling of sulfur (elevated sulfate reduction rates and extensive sulfide oxidation) at and above the chemocline. This isotopic gradient is observed in both day and night enrichments and suggests that, despite the close physical association between cyanobacteria and select sulfate-reducing bacteria, photosynthetic forcing has no substantive impact on δ³⁴S in these cyanobacterial mats. Perhaps equally surprising, large, spatially-coherent δ³⁴S oscillations (∼20-30‰ over 1 mm) occurred at depths up to ∼1.5 cm below the mat surface. These gradients must arise in situ from differential microbial metabolic activity and fractionation during sulfide production at depth. Sulfate concentrations were the dominant control on the spatial variability of sulfide δ³⁴S. Decreased sulfate concentrations diminished both vertical and lateral δ³⁴S variability, suggesting that small-scale variations of δ³⁴S can be diagnostic for reconstructing past sulfate concentrations, even when original sulfate δ³⁴S is unknown.     

Temporal Variability in the Concentration and Stable Carbon Isotope Composition of Dissolved Inorganic and Organic Carbon in Two Montana,USA Rivers

Here we report diel (24 h) and seasonal differences in the concentration and stable carbon isotope composition of dissolved inorganic (DIC) and organic carbon (DOC) in the Clark Fork (CFR) and Big Hole (BHR) Rivers of southwestern Montana, USA. In the CFR, DIC concentration decreased during the daytime and increased at night while DOC showed an inverse temporal relationship; increasing in the daytime most likely due to release of organic photosynthates and decreasing overnight due to heterotrophic consumption. The stable isotope composition of DIC (δ¹³C-DIC) became enriched during the day and depleted over night and the δ¹³C-DOC displayed the inverse temporal pattern. Additionally, the night time molar rate of decrease in the concentration of DOC was up to two orders of magnitude smaller than the rate of increase in the concentration of DIC indicating that oxidation of DOC was responsible for only a small part of the increase in inorganic carbon. In the BHR, in two successive years (late summer 2006 & 2007), the DIC displayed little diel concentration change, however, the δ¹³C-DIC did show a more typical diel pattern characteristic of the influences of photosynthesis and respiration indicating that the isotopic composition of DIC can change while the concentration stays relatively constant. During 2006, a sharp night time increase in DOC was measured; opposite to the result observed in the CFR and may be related to the night time increase in flow and pH also observed in that year. This night time increase in DOC, flow, and pH was not observed 1 year later at approximately the same time of year. An in-stream mesocosm chamber used during 2006 showed that the night time increase in pH and DOC did not occur in water that was isolated from upstream or hyporheic contributions. This result suggests that a “pulse” of high DOC and pH water was advected to the sampling site in the BHR in 2006 and a model is proposed to explain this temporal pattern.     

Biogeochemistry of Fe(II) oxidation in a photosynthetic microbial mat: Implications for Precambrian Fe(II) oxidation

We studied the role of microbial photosynthesis in the oxidation of Fe(II) to Fe(III) in a high Fe(II) and high Mn(II) hot spring devoid of sulfide and atmospheric oxygen in the source waters. In situ light and dark microelectrode measurements of Fe(II), Mn(II) and O₂ were made in the microbial mat consisting of cyanobacteria and anoxygenic photosynthetic Chloroflexus sp. We show that Fe(II) oxidation occurred when the mat was exposed to varying intensities of sunlight but not near infrared light. We did not observe any Mn(II) oxidation under any light or dark condition over the pH range 5-7. We observed the impact of oxygenic photosynthesis on Fe(II) oxidation, distinct from the influence of atmospheric O₂ and anoxygenic photosynthesis. In situ Fe(II) oxidation rates in the mats and cell suspensions exposed to light are consistent with abiotic oxidation by O₂. The oxidation of Fe(II) to form primary Fe(III) phases contributed to banded iron-formations (BIFs) during the Precambrian. Both oxygenic photosynthesis, which produces O₂ as an oxidizing waste product, and anoxygenic photosynthesis in which Fe(II) is used to fix CO₂ have been proposed as Fe(II) oxidation mechanisms. Although we do not know the specific mechanisms responsible for all Precambrian Fe(II) oxidation, we assessed the relative importance of both mechanisms in this modern hot spring environment. In this environment, cyanobacterial oxygen production accounted for all the observed Fe(II) oxidation. The rate data indicate that a modest population of cyanobacteria could have mediated sufficient Fe(II) oxidation for some BIFs.     

Biomass layering and metabolism in mats of the macroalgaUlva lactuca L.

Studies of photosynthesis and respiration in the sheet-like green macroalgaUlva lactuca L. have typically made use of cut discs or individual thalli. Because this species accumulates in layered mats in the field, its structure has the potential to significantly alter metabolic rates compared to those measured for a single layer. The effect of biomass layering on photosynthesis and respiration inU. lactuca was assessed by incubating stacks of 1, 2, 4, and 8 thalli across a range of surface irradiance and in the dark. The photosynthesis-irradiance curve for a single thallus was hyperbolic in shape with maximum photosynthetic rates of 16–24.5 mg O₂ g^?1 dry weight h^?1, a photosynthetic efficiency of 0.18 mg O₂ g^?1 dry weight h^?1 (μE m^?2 s^?1)^?1, and dark respiration rates of 0.95–1.2 mg O₂ g^?1 dry weight h^?1. Weight-specific photosynthesis and respiration rates decreased as the amount of layering increased, with rates approximately twice as high in a single layer as for 8 layers. A simple model revealed that the reduced photosynthetic rates were due to attenuation of light through the stack. Each layer ofUlva reduced the light available to the next layer by approximately 55%, and variability in this absorption was directly related to thallus chlorophyll content. Model runs suggested that acclimation of the thalli by changes in chlorophyll content has the potential to reduce some, but not all of the effect of layering. Reduced respiration rates were atributed to the depletion of oxygen from the interstices between the layers. Results were incorporated into a model of anUlva mat, which predicted decreasing rates of weight-specific total mat production with increasing mat thickness as additional layers are added to the bottom of the mat that neither produce nor consume significant amounts of oxygen. The model predicted area-weighted mat production to increase up to a thickness of 10 layers, after which the rate was constant with increasing mat thickness for the same reason. SinceUlva has a tendency to accumulate in layers under natural conditions, these results improve upon metabolic measurements made with single thalli and will be useful for calculations of macroalgal production as well as in ecosystem models in whichUlva is an important primary producer.     

Bacteriohopanepolyols in a stratified cyanobacterial mat from Kiritimati (Christmas Island,Kiribati)

Bacteriohopanepolyols (BHPs) are lipids of distinct bacterial groups and are, along with geohopanoids as their diagenetic products, ubiquitous in microbial mats, sediments, soils and oil. Among BHP-producing bacteria, Cyanobacteria are of special interest since occurrences of C-2 methylated and other geohopanoids are interpreted as signals of oxygenic photoautotrophs in the early oceans. However, many questions, with respect to the source and function of hopanoids, remain open. Cyanobacterial mats are complex systems in terms of biogeochemical zones and hence, harbor phylogenetically and metabolically diverse photoautotrophic and heterotrophic microorganisms. Whereas Cyanobacteria are key microbial players for carbon fixation in these mat systems, most other bacterial and archaeal groups feed on their metabolic products and/or perform anoxygenic photosynthesis in deeper low-light regimes.We have analyzed the abundance and distribution of BHPs in distinct layers of a stratified mat from a hypersaline lake on Kiritimati. Suites of BHPs were observed, with the majority probably originating from (proteo-)bacteria thriving in the deeper layers of the mat and not in upper layers where mat-forming Cyanobacteria are prevalent. Interestingly, the BHPs in the deepest layer include C-2 methylated structures, which likely do not originate from Cyanobacteria, but rather from α-Proteobacteria thriving at the redoxcline of this stratified microbial system.     

Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico

Net fluxes of respiratory metabolites (O₂, dissolved inorganic carbon (DIC), NH₄ ⁺, NO₃ ^?, and NO₂ ^?) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of³⁵S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress.     

Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone

The coastal upwelling system off central Namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries. We present data on water column dissolved oxygen, sulfide, nitrate and nitrite, pore water profiles for dissolved sulfide and sulfate,³⁵S-sulfate reduction rates, as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope. The stations covered two transects and included the inner shelf with its anoxic and extremely oxygen-depleted bottom waters, the oxygen minimum zone on the continental slope, and the lower continental slope below the oxygen minimum zone. High concentrations of dissolved sulfide, up to 22 mM, in the near-surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide. The inner shelf break marks the seaward border of sulfidic bottom waters, and separates two different regimes of bacterial sulfate reduction. In the sulfidic bottom waters on the shelf, up to 55% of sulfide oxidation is mediated by the large nitrate-storing sulfur bacteria, Thiomargarita spp. The filamentous relatives Beggiatoa spp. occupy low-O₂ bottom waters on the outer shelf. Sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria. The data demonstrate the importance of large sulfur bacteria, which live close to the sediment-water interface and reduce the hydrogen sulfide flux to the water column. Modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column. However, the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment-water interface. Episodic advection of hydrogen sulfide, possibly triggered by methane eruptions, may contribute to hydrogen sulfide in the water column.     

Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas)

Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.     

Ore transport and deposition in the Red Sea geothermal system: a geochemical model

Thermodynamic calculation of distribution of dissolved aqueous species in the Red Sea geothermal brine provides a model of ore transport and deposition in good agreement with observed accumulations of base metal sulfides, anhydrite, and barite.The Red Sea brine is recirculated seawater that acquires high salinity by low-temperature interaction with Miocene evaporites and is subsequently heated to temperatures in excess of 200°C by interaction with recent rift zone intrusive rocks. At temperatures up to 250°C, NaSO^?₄ and MgSO⁰₄ are the dominant sulfur-bearing species. H₂S forms by inorganic sulfate reduction at the higher temperatures but is maintained at a uniform concentration of about 2 ppm by the strength of the sulfate complexes.Chloride complexes solubilize metals at the higher temperatures, and thus sulfide and metals are carried together into the Atlantis II Deep. Below 150°C, the brine becomes supersaturated with respect to chalcopyrite, sphalerite, galena, and iron monosulfide due to chloride-complex dissociation. Sulfide precipitation rates, based on the rate of brine influx, are in good agreement with measured sedimentation rates. Anhydrite precipitates as crystalline fissure infillings from high-temperature inflowing brine. Barite forms from partial oxidation of sulfides at the interface between the lower hot brine and the transitional brine layer.     

Geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems

Mixing of hydrothermal fluids and seawater at the ocean floor, combined with slow reaction kinetics for oxidation/reduction reactions, provides a source of metabolic energy for chemolithotrophic microorganisms which are the primary biomass producers for an extensive submarine ecosystem that is essentially independent of photosynthesis. Thermodynamic models are used to explore geochemical constraints on the amount of metabolic energy potentially available from chemosynthetic reactions involving S, C, Fe, and Mn compounds during mixing of hydrothermal fluids with seawater. For the vent fluid used in the calculations (EPR 21 degrees N OBS), the model indicates that mixing environments are favorable for oxidation of H2S, CH4, Fe2+ and Mn2+ only below approximately 38 degrees C, with methanogenesis and reduction of sulfate or S degrees favored at higher temperatures, suggesting that environments dominated by mixing provide habitats for mesophilic (but not thermophilic) aerobes and thermophilic (but not mesophilic) anaerobes. A maximum of approximately 760 cal per kilogram vent fluid is available from sulfide oxidation while between 8 and 35 cal/kg vent fluid is available from methanotrophy, methanogenesis, oxidation of Fe or Mn, or sulfate reduction. The total potential for chemosynthetic primary production at deep-sea hydrothermal vents globally is estimated to be about 10(13) g biomass per year, which represents approximately 0.02% of the global primary production by photosynthesis in the oceans. Thermophilic methanogens and sulfate- and S degree-reducers are likely to be the predominant organisms in the walls of vent chimneys and in the diffuse mixing zones beneath warm vents, where biological processes may contribute to the high methane concentrations of vent fluids and heavy 34S/32S ratios of vent sulfide minerals. The metabolic processes taking place in these systems may be analogs of the first living systems to evolve on the Earth.     

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.     

黔中小流域水体C、S同位素特征及主要风化过程

选取贵州省红枫湖流域四条支流为研究对象,通过对水体中C、S元素浓度及稳定同位素特征的分析,探讨了典型碳酸盐小流域的主要风化过程对流域C、S循环的贡献。红枫湖流域河水中的C同位素组成在-7．47‰--11．68‰之间,硫酸根浓度为295．5～1315．6μmol／L,除受当地煤矿影响较大的区域外,其余部分水体中离子的δ^34S值在-3‰--7‰之内,反映了这些河段中硫化物矿物的氧化的影响。通过质量守恒的方法,得到硫化物的氧化、蒸发岩溶解以及大气输入对河水硫酸根离子的贡献率分别为63．8％、30．8％和5．4％。来自硫酸对碳酸盐岩、碳酸对碳酸盐岩以及碳酸对硅酸盐岩风化的DIC比例为1：3．58：1．37,三种不同的风化过程对区域二氧化碳的净吸收量为7．97×10^9mol／a。     

桂林漓江水体溶解无机碳迁移与水生光合碳固定研究

河流溶解无机碳含量昼夜变化主要受碳酸盐反向沉积、水生光合利用和脱气作用控制,被水生光合利用的溶解无机碳是岩溶碳汇的组成部分,脱气作用比例的大小是影响碳汇稳定性的决定因素。本文以漓江中游省里—冠岩之间15 km长河段为研究对象,开展昼夜高分辨率水化学自动化监测与高频取样,分析水生植物光合作用利用HCO3-1及相关钙沉降过程。结果表明,监测河段水生光合利用的无机碳转化通量为859 kgC?d-1,单位流程光合作用溶解无机碳转化量和钙沉降量分别为2.06 t?（d?km）-1和0.78 t?（d?km）-1。光合作用与钙沉降消耗DIC约占总转化量的70 %,以光合有机碳和CaCO3形式储存于河床,成为岩溶碳汇组成部分。无机碳转化量约占输入DIC总量的6.0 %（其中1.7%以CO2形式返回大气）,说明夏季低水位期间强烈的水生植物光合利用溶解无机碳,可有效遏制白天水气界面CO2脱气过程发生,低脱气比例证实漓江水体的溶解无机碳还是比较稳定的。      

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.     

Thermodynamic and kinetic control on anaerobic oxidation of methane in marine sediments

The free energy yield of microbial respiration reactions in anaerobic marine sediments must be sufficient to be conserved as biologically usable energy in the form of ATP. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SRR) has a very low standard free energy yield of ΔG^° = −33 kJ mol⁻¹, but the in situ energy yield strongly depends on the concentrations of substrates and products in the pore water of the sediment. In this work ΔG for the AOM-SRR process was calculated from the pore water concentrations of methane, sulfate, sulfide, and dissolved inorganic carbon (DIC) in sediment cores from different sites of the European continental margin in order to determine the influence of thermodynamic regulation on the activity and distribution of microorganisms mediating AOM-SRR. In the zone of methane and sulfate coexistence, the methane-sulfate transition zone (SMTZ), the energy yield was rarely less than −20 kJ mol⁻¹ and was mostly rather constant throughout this zone. The kinetic drive was highest at the lower part of the SMTZ, matching the occurrence of maximum AOM rates. The results show that the location of maximum AOM rates is determined by a combination of thermodynamic and kinetic drive, whereas the rate activity mainly depends on kinetic regulation.     

Benthic metabolism and nutrient cycling in Boston Harbor, Massachusetts

To gain insight into the importance of the benthos in carbon and nutrient budgets of Boston Harbor and surrounding bays, we measured sediment-water exchanges of oxygen, total carbon dioxide (DIC), nitrogen (ammonium, nitrate+nitrite, urea, N₂O), silicate, and phosphorus at several stations in different sedimentary environments just prior to and subsequent to cessation of sewage sludge disposal in the harbor. The ratio of the average annual DIC release to O₂ uptake at three primary stations ranged from 0.84 to 1.99. Annual average DIC:DIN flux ratios were consistently greater than predicted from the Redfield ratio, suggesting substantial losses of mineralized N. The pattern was less clear for P: some stations showed evidence that the sediments were a sink for P while others appeared to be a net source to the water column over the study period. In general, temporal and spatial patterns of respiration, nutrient fluxes, and flux ratios were not consistently related to measures of sediment oxidation-reduction status such as Eh or dissolved sulfide. Sediments from Boston Harbor metabolize a relatively high percentage (46%) of the organic matter inputs from phytoplankton production and allochthonous inputs when compared to most estuarine systems. Nutrient regeneration from the benthos is equivalent to 40% of the N, 29% of the P, and more than 60% of the Si demand of the phytoplankton. However, the role of the benthos in supporting primary production at the present time may be minor as nutrient inputs from sewage and other sources exceed benthic fluxes of N and P by 10-fold and Si by 4-fold. Our estimates of denitrification from DIC:DIN fluxes suggests that about 45% of the N mineralized in the sediments is denitrified, which accounts for about 17% of the N inputs from land.     

Molecular biogeochemistry of sulfate reduction, methanogenesis and the anaerobic oxidation of methane at Gulf of Mexico cold seeps

The anaerobic oxidation of methane in aquatic environments is a globally significant sink for a potent greenhouse gas. Significant gaps remain in our understanding of the anaerobic oxidation of methane because data describing the distribution and abundance of putative anaerobic methanotrophs in relation to rates and patterns of anaerobic oxidation of methane activity are rare. An integrated biogeochemical, molecular ecological and organic geochemical approach was used to elucidate interactions between the anaerobic oxidation of methane, methanogenesis, and sulfate reduction in sediments from two cold seep habitats (one brine site, the other a gas hydrate site) along the continental slope in the Northern Gulf of Mexico. The results indicate decoupling of sulfate reduction from anaerobic oxidation of methane and the contemporaneous occurrence of methane production and consumption at both sites. Phylogenetic and organic geochemical evidence indicate that microbial groups previously suggested to be involved in anaerobic oxidation of methane coupled to sulfate reduction were present and active. The distribution and isotopic composition of lipid biomarkers correlated with microbial distributions, although concrete assignment of microbial function based on biomarker profiles was complicated given the observed overlap of competing microbial processes. Contemporaneous activity of anaerobic oxidation of methane and bicarbonate-based methanogenesis, the distribution of methane-oxidizing microorganisms, and lipid biomarker data suggest that the same microorganisms may be involved in both processes.     

氮输入对陆地生态系统碳循环关键过程的影响

碳氮作为陆地生态系统最关键的两大生源要素,它们在自然界的循环过程中不仅各自对全球变暖做出重要贡献,而且两者的循环过程显著耦合,互相影响各自的作用和效果。从氮元素对植物光合作用、呼吸作用以及土壤呼吸作用影响的角度入手,综述了氮输入对陆地生态系统碳固定和碳排放这两个碳循环关键过程的影响特征和机理,分析了陆地生态系统碳源汇对氮素变化响应的不确定性,在此基础上对未来的相关重点研究方向进行了探讨和展望。     

Influence of a reservoir chain on the transport of riverine inorganic carbon in the karst area

The Wujiang River is an important tributary to the Changjiang River that has been intensively impounded for hydropower exploitation. To understand the potential impact of reservoir construction on the riverine inorganic carbon transport, seasonal longitudinal sampling was conducted in four reservoirs Hongjiadu (HJD), Dongfeng (DF), Suofengying (SFY) and Wujiangdu (WJD) along the Wujiang River from April 2006 to January 2007. Results indicated that damming the river induced an obvious discontinuity of water chemistry in the warmer seasons. δ ¹³C of dissolved inorganic carbon (DIC) ranged from ?3 to ?11.4 ‰, likely as the results of photosynthesis, respiration and carbonate weathering. During periods of thermal stratification, the addition of CO₂ from respiration to hypolimnion and the deep water release for hydropower generation led to higher pCO₂ downstream, as well as ¹³C depletion in DIC and undersaturated to calcite. An estimate of DIC budget indicated that only DF reservoir was the sink for DIC while reservoirs HJD, SFY and WJD were the sources for DIC. However, when the retained water was taken into account, for the reason of water storage occurring mainly in HJD and DF, all reservoirs became the sources for DIC with exporting rates of 26.68, 7.97, 6.22 and 11.80 % for HJD, DF, SFY and WJD, respectively.     

陆架边缘海沉积物有机碳矿化及其对海洋碳循环的影响

边缘海沉积物是海洋重要的碳储库,其内部的碳循环主要是由有机质矿化分解过程来驱动的。有机碳进入边缘海沉积物后,矿化分解为溶解无机碳(DIC)进入沉积物孔隙水并扩散到上层水柱,参与海洋系统碳循环;同时还有部分DIC与钙镁等离子结合形成自生碳酸盐,保存于沉积物碳库。从生物地球化学角度探讨有机质埋藏机制和效率,在此基础上重点综述沉积物硫酸盐还原、产甲烷和甲烷厌氧氧化过程的耦合机制,以及有机质矿化对自生碳酸盐形成的影响等方面的研究进展,以期加深对陆架边缘海沉积物在全球碳循环收支平衡中的作用及其气候环境效应的认识。