Rates and controls of anaerobic microbial respiration across spatial and temporal gradients in saltmarsh sediments

This study was undertaken to determine the rates and controls ofanaerobic respiration reactions coupled to organic matter mineralization as afunction of space and time along a transect from a bioturbated creekbank to themidmarsh in Georgia saltmarsh sediments. Sulfate reduction rates (SRR) weremeasured at 3 sites during 5 sampling periods throughout the growth season. Thesites differed according to hydrologic regime and the abundance of dominantplants and macrofauna. SRR and pore water / solid phase geochemistry showedevidence of enhanced sediment oxidation at sites exposed to intensebioturbation. Iron(III) reduction rates (FeRR) were directly determined insaltmarsh sediments for the first time, and in agreement with measured SRR,higher rates were observed at the bioturbated, unvegetated creekbank (BUC) andbioturbated, vegetated levee (BVL) sites in comparison to a vegetated mid-marsh(MM) site. An unexpected result was the fact that SRR varied nearly as muchbetween sites (2–3 x) as it did with temperature or season (3–4 x).The BVL site, vegetated by the tall form of Spartinaalterniflora, always exhibited the highest SRR and carbon oxidationrates (> 4000 nmol cm^–3 d^–1) with high activity levels extending deep ( 50 cm)into the sediment, while the MM site, dominated by the short form ofSpartina, always exhibited the lowest SRR which werelocalized to the top 15 cm of sediment. SRR and FeRR at BUC wereintermediate between those measured at the BVL and MM. Acetate was the mostabundant microbial fermentation product (concentrations up to > 1mM) in marsh porewaters, and its distribution reflectedrespirationactivity. Chemical exchange, caused by bioturbation, appeared to be the primarycontrol explaining trends in rates of sulfate and Fe(III) reduction withmacrophytes and carbon source acting as secondary controls.     

Quantification and characterization of dissolved organic carbon and iron in sedimentary porewater from Green Bay,WI, USA

Both dissolved organic carbon (DOC) and iron play an important role in biogeochemical processes in lacustrine benthic environments. Moreover, recent evidence has shown that both substances can act as active reductants in the redox transformation of organic pollutants. This paper examines the nature and abundance of DOC and dissolved ferrous iron (Fe^II) in porewaters from a sediment core collected in Green Bay, WI, USA. The concentration of dissolved Fe^II and the abundance, absorbance at 280 nm (A _{280 nm}), molar absorptivities (_{280 nm}), molecular weights, and polydispersities of DOC were measured as a function of depth in porewaters. Dissolved Fe^II concentrations increased from 3.6 M near the sediment–water interface to 163 M at a depth of 11 cm, then gradually declined. The DOC distribution varied with sediment depth, with the greatest variation in porewater DOC content and properties occurring in the transitional zone between oxic and suboxic conditions. The down-core porewater DOC profile was characterized by an increase in DOC concentration with depth from 0.64 mM OC at 1 cm to 1.23 mM OC at 13 cm, below which it remained relatively constant. A strong correlation was observed between Fe^II and DOC concentrations, suggesting that these constituents co-accumulate in these porewaters. The correlation between the DOC concentration of the porewaters and A _{280 nm} was significant, making this parameter a good predictor for DOC concentrations in these waters. The molecular weight distributions of the porewater DOC were primarily monomodal, with relatively low polydispersivities. Weight-average molecular weights ranged from 1505 to 1949 Da. This data set is unique in that it is the first detailed study of a relatively highly resolved DOC profile of benthic porewater in surficial sediment from the Laurentian Great Lakes.     

Seasonal cycling of Fe in saltmarsh sediments

This study combines an analysis of porewater chemistry with new, solid phase wet chemical extractions to examine the seasonal cycling of Fe in vegetated and unvegetated (cyanobacterial mat) saltmarsh sediments. Saltmarsh sediments are shown to contain more solid phase reactive Fe than other marine sediments studied so far. From the partitioning and speciation of solid Fe, and solid/soluble reduced S analysis in 10 sediment cores, we have observed that a majority of solid Fe in these sediments is cycled rapidly and completely between oxidized reactive Fe and reduced Fe as pyrite. Vegetated porewaters showed a lower pH and much higher Fe(II) concentrations on average than unvegetated porewaters in the top 10 cm, whereas sulfate, alkalinity, and sulfide concentrations were similar in the two environments. The amorphous Fe(III) oxide fraction showed a high negative correlation to solid and soluble reduced S (r ² = –0.86 and –0.71, respectively) in surface vegetated sediments whereas the crystalline Fe(III) oxide fraction showed a high negative correlation (r ² = –0.96) to sulfide only at depth. Though reactive Fe was observed in unvegetated sediments, no seasonal trend was apparent and the speciation of solid Fe revealed that most of it was reduced. Solid phase and porewater chemistry support the dominant role of the biota (Spartina alterniflora and bacteria) in controlling the reactivity of Fe and suggest that the current definition of solid phase, reactive Fe should be expanded to include crystalline Fe(III) minerals which are available for pyrite formation in saltmarsh sediments. In support of previous saltmarsh studies, we present evidence that the redox cycle of solid Fe is controlled by sulfate reduction and sediment oxidation which respond to both annual cycles (light, temperature) and to short-term, episodic effects such as weather and tides.     

Spectroscopic characterization and molecular weight distribution of dissolved organic matter in sediment porewaters from Lake Erhai, Southwest China

Dissolved organic matter (DOM) in sediment porewaters from Lake Erhai, Southwest China was investigated using dissolved organic carbon (DOC) concentration, UV absorbance, fluorescence and molecular weight distribution. DOC exhibited a high concentration at the sediment–water interface with a rapid decrease to the oxic–anoxic interface at approximately 7 cm, and then increased with depth. Similar trends were also found for the UV absorption coefficients at 254 and 280 nm in the porewaters. DNA in the sediment was also measured, which confirmed the high abundance of aerobic bacteria in the upper layer of the sediment. Both humic-like (peaks A and C) and protein-like (peaks B and D) fluorescence were observed in the porewater DOM, and their fluorescence intensities exhibited a similar porewater profile as DOC concentration. A strong correlation was found between the peak fluorescence intensity ratio r(A, C) and r(D, B). Both the fluorescence index and UV absorption coefficient at 254 nm suggested a dramatic increase in aromaticity of porewater DOM across the oxic–anoxic interface. Porewater DOM exhibited a multimodal distribution of molecular weight with a relatively low polydispersity. The results of this study offer significant insight into the nature and properties of DOM in freshwater ecosystems.     

Thermophilic anaerobes in Arctic marine sediments induced to mineralize complex organic matter at high temperature

Marine sediments harbour diverse populations of dormant thermophilic bacterial spores that become active in sediment incubation experiments at much higher than in situ temperature. This response was investigated in the presence of natural complex organic matter in sediments of two Arctic fjords, as well as with the addition of freeze‐dried Spirulina or individual high‐molecular‐weight polysaccharides. During 50°C incubation experiments, Arctic thermophiles catalysed extensive mineralization of the organic matter via extracellular enzymatic hydrolysis, fermentation and sulfate reduction. This high temperature‐induced food chain mirrors sediment microbial processes occurring at cold in situ temperatures (near 0°C), yet it is catalysed by a completely different set of microorganisms. Using sulfate reduction rates (SRR) as a proxy for organic matter mineralization showed that differences in organic matter reactivity determined the extent of the thermophilic response. Fjord sediments with higher in situ SRR also supported higher SRR at 50°C. Amendment with Spirulina significantly increased volatile fatty acids production and SRR relative to unamended sediment in 50°C incubations. Spirulina amendment also revealed temporally distinct sulfate reduction phases, consistent with 16S rRNA clone library detection of multiple thermophilic Desulfotomaculum spp. enriched at 50°C. Incubations with four different fluorescently labelled polysaccharides at 4°C and 50°C showed that the thermophilic population in Arctic sediments produce a different suite of polymer‐hydrolysing enzymes than those used in situ by the cold‐adapted microbial community. Over time, dormant marine microorganisms like these are buried in marine sediments and might eventually encounter warmer conditions that favour their activation. Distinct enzymatic capacities for organic polymer degradation could allow specific heterotrophic populations like these to play a role in sustaining microbial metabolism in the deep, warm, marine biosphere.     

Effects of the emergent macrophyte Juncus effusus L. on the chemical composition of interstitial water and bacterial productivity

Release of oxygen from the roots ofaquatic macrophytes into anaerobic sediments canaffect the quantity of interstitial dissolved organicmatter and nutrients that are available to bacteria. Nutrient and dissolved organic carbon (DOC)concentrations were compared between subsurface(interstitial) waters of unvegetated sediments andsediments among stands of the emergent herbaceousmacrophyte Juncus effusus L. in a lotic wetlandecosystem. Concentrations of inorganic nitrogen(NH₄ ⁺, NO₃ ^-, and NO₂ ^-)were greater from sediments of the unvegetatedcompared to the vegetated zone. DOC concentrations ofinterstitial waters were greater in sediments of theunvegetated zone both in the winter and springcompared to those from the vegetated zone. AlthoughDOC concentrations in hydrosoils collected from bothzones increased from winter to spring, bacterialproductivity per mg DOC in spring decreased comparedto winter. Greater initial bacterial productivityoccurred on DOM collected from the vegetated comparedto the unvegetated zone in winter samples (days 1 and4), with increased bacterial productivity on samplescollected from the unvegetated zone at the end of thestudy (day 20). Bacterial productivity wassignificantly greater on all sampling days on DOM fromvegetated samples compared to unvegetated samples. In nutrient enrichment experiments, bacterialproductivity was significantly increased (p < 0.05)with phosphorus but not nitrogen only amendments.     

<Emphasis Type="Italic">Spartina maritima</Emphasis> influence on the dynamics of the phosphorus sedimentary cycle in a warm temperate estuary (Mondego estuary,Portugal)

During the last decades the Mondego estuary has been under severe ecological stress mainly caused by eutrophication. In this salt march system, Spartina maritima covers about 10.5 ha of the intertidal areas. The objective of the present study was to evaluate the effect of Spartina maritima marshes on the dynamics of phosphorus (P) binding in the surface sediment. We compare phosphate and oxygen fluxes, P-adsorption capacity, phosphate concentrations and total amount, and the extractable P forms in the upper 20 cm of sediment in vegetated sediment with adjacent mudflats without vegetation. Sediment pore-water profiles followed a clear trend, with lower P concentrations in more superficial layers, and increasing with depth. The vegetated mudflats presented lower concentrations of dissolved inorganic phosphorus than adjacent bare bottom mudflats, lower phosphate total amount, as well as higher P-adsorption capacity. Results from the extraction procedure show that the superficial layers are the most important for estuarine phosphorus dynamics, since maximum concentrations of labile P pools are present here. In contrast, higher proportions of refractory P pool are found in deeper layers. Spartina marsh sediments had less total P, less iron bound P, and less exchangeable P than adjacent bare bottom mudflats. Also the pool of loosely sorbed P is lower in the Spartina marsh. Phosphate regeneration from the sediment to the overlying water was only 11.8 kg ha⁻¹ year⁻¹ in vegetated sediment while 25.8 kg ha⁻¹ year⁻¹ in the bare mud flat. Plant uptake for growth combined with an enhanced P-adsorption capacity of the sediment, may explain these differences. Therefore, Spartina marshes are very important agents in the sedimentary P cycle worldwide, and can be considered a useful management tool in estuarine ecosystem recovery efforts.     

Localization of sulfate reduction in planted and unplanted rice field soil

Rates of in situ sulfate reduction (SRR) in planted and unplanted rice fieldsoil were measured by the ³⁵SO^2– ₄-radiotracermethod using soil microcosms. The concentration of ³⁵SO^2– ₄ decreased exponentially with time.However, time course experiments indicated that incubation times of10–30 min were appropriate for measurements of SRRusing a single time point in routine assays. Unplanted microcosmsshowed high SRR of 177 nmol cm^-3 d^-1 inthe uppermost centimeter where average sulfate concentrations were<33 µM. Fine scaled measurements (1 mmresolution) localized highest SRR (<100 nmol cm^-3d^-1) at the oxic/anoxic interface at 2–5 mmdepth. In planted rice field soil, SRR of <310 nmolcm^-3 d^-1 were observed at 0–2cm depth. Sulfate reduction rates were determined at a millimeter-scalewith distance to a two dimensional root compartment. The SRR was highestat 0–1.5 mm distance to the root layer with rates up to500 nmol cm^-3 d^-1, indicating a highstimulation potential of the rice roots. SRR seemed to be mainlydependent on the in situ sulfate porewater concentrations. At thesoil surface of unplanted microcosms sulfate concentration decreasedfrom <150 µM to <10 µM within the first 8 mm of depth. In planted microcosmssulfate concentration varied from 87–99 µMsulfate at the 0–3 mm distance to the root layer to48–62 µM sulfate at a root distance>4 mm from the roots.The depth distribution of inorganic sulfur compounds was determinedfor planted and unplanted rice field soil. Sulfate, acid volatilesulfide (AVS) and chromium reducible sulfide (CRS) were up to 20 foldhigher in planted than in unplanted microcosms. CRS was the majorinsoluble sulfur fraction with concentrations >1.7µmol cm^-3. Organic sulfur accounted for25–46% of the total sulfurpresent (269 µg/g dw) in an unplanted microcosm.The biogeochemical role of sulfate reduction forshort-term accumulation of inorganic sulfur compounds(FeS, FeS_{_2} and S°) in rice soil wasdetermined in a time course experiment with incubationperiods of 5, 10, 20, 30 and 60 min. The relativedistribution of CRS and AVS formation showedlittle depth dependence, whereas the formation of³⁵S° seemed to be the highest in themore oxidized upper soil layers and near the root surface.AV³⁵S was the first major product of sulfatereduction after 20–30 min, whereas CR³⁵Swas formed, as AV³⁵S and ³⁵S°decreased, at longer incubation periods of >30 min.     

Dissolved and particulate organic matter in contrasting Zostera marina (eelgrass) sediments

The influence of seagrass Zostera marina on sediment characteristics was examined in two contrasting sediments, one organic-rich and one organic-poor. The presence of plants leads to reduced sediment redox potential in both sediment types compared to bare sediment with the largest effects in the organic-poor sediment. Z. marina stimulated the sulfate reduction rates in organic-poor sediment with ∼50% and higher pools of dissolved organic carbon (DOC) were found. In contrast, sulfate reduction rates were lower in vegetated compared to bare sites in the organic-rich sediment. Despite a low contribution of dissolved carbohydrate (DCHO) to the DOC pool (<5%), the seagrass vegetation was responsible for an increase of ∼50% in DCHO pools with a peak in the root zone suggesting that Z. marina supplied DCHO to the pore waters. The Z. marina meadows also enhanced the contribution of particulate carbohydrate (PCHO) to sedimentary particulate organic carbon (POC) pools by 6-14% compared to bare sediment. Although the PCHO pools were higher in organic-rich than organic-poor sediments, the analyses of carbohydrate composition revealed that three groups of neutral sugars including glucose, galactose and mannose+xylose were the major compounds of PCHO and contributed with >60% to sedimentary carbohydrate pools at both sites. Only glucose showed depletion with depth in the vegetated sediments, whereas the percentage of ribose and rhamnose increased indicating a selective degradation of labile carbohydrates in the meadows. Galactose and mannose+xylose appeared to represent a refractory part of carbohydrate that remained after degradation of the more labile components. The sugar content was rather constant with depth at the bare organic-rich sediment indicating that only recalcitrant carbohydrate pools were buried. There was less difference in the PCHO composition profiles between vegetated and bare organic-poor sediments.     

Bioturbation and the role of microniches for sulfate reduction in coastal marine sediments

The effects of bioturbation in marine sediments are mainly associated with an increase in oxic and oxidized zones through an influx of oxygen‐rich water deeper into the sediment and the rapid transport of particles between oxic and anoxic conditions. However, macrofaunal activity also can increase the occurrence of reduced microniches and anaerobic processes, such as sulfate reduction. Our goal was to determine the two‐dimensional distribution of microniches associated with burrows of a ghost shrimp (Neotrypaea californiensis) and to determine microbial activities. In laboratory experiments, detailed measurements of sulfate reduction rates (SRR) were measured by injecting, in a 1 cm grid, radiolabelled sulfate directly into a narrow aquarium (40 cm × 30 cm × 3 cm) containing the complex burrow of an actively burrowing shrimp. Light‐coloured oxidized burrow walls, along with black reduced microniches, were clearly visible through the aquarium walls. Direct injection of radiotracers allowed for whole‐aquarium incubation to obtain two‐dimensional documentation of sulfate reduction. Results indicated SRR were up to three orders of magnitude higher (140–790 nmol SO₄^2? cm^?3 day^?1) in reduced microniches associated with burrows when compared with the surrounding sediment. Additionally, some of the subsurface sulfate‐reducing microniches associated with the burrow system appeared to be zones of dinitrogen fixation. Bioturbation may also lead to decreased sulfate reduction in other microniches and the sum of the activity in all microniches might not result in a total increase of sulfate reduction compared with non‐bioturbated control sediments.     

Nutrient cycling in shallow,oligotrophic Lake Kvie,Denmark

The importance of isoetids for the exchange of dissolved inorganic nitrogen (DIN) between sediment and water was studied in shallow Lake Kvie, Denmark. Vegetated sediments from the littoral zone (55% of lake area) were compared to unvegetated sediments from the littoral and profundal zone. Clear effects of the isoetids were found on DIN in the porewater. At the vegetated station, NH₄ ⁺ showed the highest concentrations just below the surface (< 40 µM) whereas NO₃ ^- was dominating below 5 cm depth with concentrations up to 100 µM during the spring. The unvegetated littoral sediment showed a distinct change between winter where NH₄ ⁺ dominated and summer where NO₃ ^- was most abundant. NH₄ ⁺ dominated in the profundal sediment and showed increasing concentration with depth. The E_h was high (> 400 mV) in the vegetated sediment, indicating isoetid release of O₂ in the rhizosphere. A low DIN uptake was observed at the vegetated station while, based on porewater data, a diffusive release from the sediment was expected. This difference was due to plant assimilation. In general a release of NH₄ ⁺ and an uptake of NO₃ ^- was seen in all sediments. The denitrification rate calculated from the mass balance for the entire lake was 0.4 mmol m - 2 d^-1 and accounted for removal of 77% of the annual N-input to Lake Kvie.     

Changes in Organic Matter Biodegradability Influencing Sulfate Reduction in an Aquifer Contaminated by Landfill Leachate

In situ experiments were conducted to measure sulfate reduction rates and identify rate-limiting factors in a shallow, alluvial aquifer contaminated with municipal landfill leachate. Single-well, push–pull tests conducted in a well adjacent to the landfill with >8 mM dissolved organic carbon (DOC) exhibited a sulfate reduction rate of 3.2 μmol SO₄ ⁻² (L sediment)⁻¹ day⁻¹, a value in close agreement with laboratory-derived estimates. Identical tests conducted in wells located 90 m downgradient where DOC levels remained high (>3 mM) showed no detectable sulfate consumption, and laboratory assays confirmed this observation. However, the rates of sulfate reduction in sediment samples obtained from this site were three times larger when they were amended with filter-sterilized groundwater from the upgradient location. The effect of various amendments on sulfate reduction rates was further examined in laboratory incubations using sediment collected from the downgradient site amended with ³⁵S sulfate. Unamended sediments showed only weak conversion of the tracer to ³⁵S sulfide (5 to 7 cpm/cm²), whereas the addition of Desulfovibrio cells increased ³⁵S sulfide production to 44 cpm/cm². However, the application of heat-killed Desulfovibrio had a similar stimulatory effect, as did a lactate amendment. Collectively, these findings indicate that the lack of measurable sulfate reduction at the downgradient site was not due to the absence of the necessary metabolic potential, the presence of lower sulfate concentration, or the quantity of electron donor, but by its biodegradability. The findings also indicate that field bioaugmentation attempts should be interpreted with caution.     

An invasive macrophyte alters sediment chemistry due to suppression of a native isoetid

The submersed macrophyte Utricularia inflata (inflated bladderwort) is a recent invader of Adirondack Mountain lakes (NY, USA). A 15-week greenhouse experiment and a 7-week field experiment were conducted to test the hypothesis that this rootless species fundamentally changes sediment chemistry through its suppression of the native short-statured species, Eriocaulon aquaticum. E. aquaticum has an extensive root system that releases oxygen into the sediment. In greenhouse conditions, E. aquaticum raised the porewater redox potential of otherwise bare sediment from 25 to 324 mV, lowered the sediment porewater pH from 5.7 to 4.6, and depleted the dissolved inorganic carbon and ammonium concentrations in the sediment porewater by 68.4 and 96.0%, respectively (P<0.001 for all four parameters). A cover of U. inflata over E. aquaticum, however, greatly reduced the latter’s effect on redox potential (P<0.001), dissolved solutes (P<0.001), and pH (P<0.05). E. aquaticum biomass increased during the greenhouse experiment in the absence of U. inflata, but decreased in its presence (P<0.001). Redox and growth rate results from the field experiment paralleled those from the greenhouse experiment. Our data suggest that U. inflata may change nutrient cycling in Adirondack lake ecosystems by reducing the growth of native isoetid macrophytes, such as E. aquaticum, and consequently altering key features of sediment chemistry.     

Peat porewater dissolved organic carbon concentration and lability increase with warming: a field temperature manipulation experiment in a poor-fen

Studies conducted across northern Europe and North America have shown increases in dissolved organic carbon (DOC) in aquatic systems in recent decades. While there is little consensus as to the exact mechanisms for the increases in DOC, hypotheses converge on such climate change factors as warming, increased precipitation variability, and changes in atmospheric deposition. In this study, we tested the effects of warming on peat porewater composition by actively warming a peatland with infrared lamps mounted 1.24 m above the peat surface for 3 years. Mean growing season peat temperatures in the warmed plots (n = 5) were 1.9 ± 0.4 °C warmer than the control plots at 5 cm depth (t statistic = 5.03, p = 0.007). Mean porewater DOC concentrations measured throughout the growing season were 15 % higher in the warmed plots (73.4 ± 3.2 mg L^?1) than in the control plots (63.7 ± 2.1 mg L^?1) at 25 cm (t = 4.69, p < 0.001). Furthermore, DOC from the warmed plots decayed nearly twice as fast as control plot DOC in laboratory incubations, and exhibited lower aromaticity than control plot porewater (reduction in SUVA₂₅₄ in heated plots compared with control plots). Dissolved organic nitrogen (DON) concentrations tracked DOC patterns as expected, but the amount of dissolved N per unit C decreased with warming. Previous work has shown that warming increased net primary production at this site, and together with measured increases in the activities of chitinases and glucosidases we suggest that the increased DOC concentrations observed with warming were derived in part from microbial-plant interactions in the rhizosphere. We also detected more nitrogen containing compounds with higher double bond equivalents (DBE) unique to the warmed plots, within the pool of biomolecules able to deprotonate (16 % of all compounds identified using ultrahigh resolution ion electrospray mass spectrometry); we suggest these compounds could be the products of increased plant, microbial, and enzyme activity occurring with warming. With continued warming in peatlands, an increase in relatively labile DOC concentrations could contribute to dissolved exports of DOC in runoff, and would likely contribute to the pool of efficient electron donors (and acceptors) in the production of CO₂ and CH₄ in terrestrial and aquatic environments.     

Sulfate reduction and sediment metabolism in Tomales Bay,California

Sulfate reduction rates (SRR) in subtidal sediments of Tomales Bay, California, were variable by sediment type, season and depth. Higher rates were measured in near-surface muds during summer (up to 45 nmol cm^-3 h^-1), with lower rates in sandy sediments, in winter and deeper in the sediment. Calculations of annual, average SRR throughout the upper 20 cm of muddy subtidal sediments (about 30 mmol S m^-2 d^-1) were much larger than previously reported net estimates of SRR derived from both benthic alkalinity flux measurements and bay wide, budget stoichiometry (3.5 and 2.6 mmol m^-2 d^-1, respectively), indicating that most reduced sulfur in these upper, well-mixed sediments is re-oxidized. A portion of the net alkalinity flux across the sediment surface may be derived from sulfate reduction in deeper sediments, estimated from sulfate depletion profiles at 1.5 mmol m^-2 d^-1. A small net flux of CO₂ measured in benthic chambers despite a large SRR suggests that sediment sinks for CO₂ must also exist (e.g., benthic microalgae).     

The photoprotective role of humus-DOC for Selenastrum and Daphnia

Cell numbers and fluorescence of the green algae Selenastrum capricornutuum and survival of Daphnia magna exposed to simulated sun-light was assessed along a gradient of DOC (0, 1, 5 10 and 50 mg C l^–1). When exposed to UV-doses and spectral distribution (295–750 nm) closely resembling surface solar radiation during mid summer, Selenastrum showed major losses of cell fluorescence. In the absence of DOC, fluorescence was severely depressed, with successively decreasing effects with increased DOC. Surviving cells also required an extensive recovery period (10–12 d) for regrowth after exposure, while an almost immediate recovery was observed at concentrations above 1 mg DOC l^–1. For Daphnia, survival was reduced to less than 10% after 4 h exposure, and almost zero after 8 h exposure in the absence of humus DOC, while no effects were observed in treatments with 10 and 50 mg C l^–1. Selenastrum and Daphnia that were not directly irradiated, but exposed to UV-irradiated water with the same concentrations of DOC did not reveal negative effects. This indicates negligible indirect effects mediated by long-lived free radicals or other toxic compounds. Irradiation of Daphnia under increased oxygen concentration (200% saturation) did not indicate acute effects, suggesting that effects of ambient radicals and oxidants would be of minor importance relative to intracellular photoproducts.     

Characterization of the indigenous PAH-degrading bacteria of <Emphasis Type="Italic">Spartina</Emphasis> dominated salt marshes in the New York/New Jersey Harbor

The aerobic polyaromatic hydrocarbon (PAH) degrading microbial communities of two petroleum-impacted Spartina-dominated salt marshes in the New York/New Jersey Harbor were examined using a combination of microbiological, molecular and chemical techniques. Microbial isolation studies resulted in the identification of 48 aromatic hydrocarbon-degrading bacterial strains from both vegetated and non-vegetated marsh sediments. The majority of the isolates were from the genera Paenibacillus and Pseudomonas. Radiotracer studies using ¹⁴C-phenanthrene and ¹⁴C-pyrene were used to measure the PAH-mineralization activity in salt marsh sediments. The results suggested a trend towards increased PAH mineralization in vegetated sediments relative to non-vegetated sediments. This trend was supported by the enumeration of PAH-degrading bacteria in non-vegetated and vegetated sediment using a Most Probable Numbers (MPN) technique, which demonstrated that PAH-degrading bacteria existed in non-vegetated and vegetated sediments at levels ranging from 10² to 10⁵ cells/g sediment respectively. No difference between microbial communities present in vegetated versus non-vegetated sediments was found using terminal restriction fragment length polymorphism (of the 16S rRNA gene) or phospholipid fatty acid analysis. These studies provide information on the specific members and activity of the PAH-degrading aerobic bacterial communities present in Spartina-dominated salt marshes in the New York/New Jersey Harbor estuary.     

亚热带常绿阔叶林和杉木人工林有机碳流失动态特征对降雨的响应

揭示亚热带森林土壤有机碳流失规律是制定相应措施以巩固和维持森林生态系统碳汇的关键。然而已有研究存在的监测对象单一、频率过低、时间过短等问题,导致对这一规律的认识仍然不足。选择亚热带典型的常绿阔叶林和杉木人工林为研究对象,每次降雨过后监测其径流量、泥沙量,分析径流和泥沙中的可溶性有机碳（Dissolved organic carbon,DOC）含量以及颗粒有机碳（Particle organic carbon,POC）含量。旨在比较两种森林DOC和POC流失量的差异,并分析二者与降雨量、降雨强度、5 min最大雨强和降雨侵蚀力四个降雨特征值的关联。拟验证以下两个问题：（1）杉木人工林的DOC和POC流失量是否高于常绿阔叶林;（2）降雨侵蚀力对DOC和POC的解释是否优于降雨量、降雨强度和5 min最大雨强。研究结果发现常绿阔叶林径流量、泥沙量、径流水中DOC浓度和POC浓度、DOC和POC流失量均显著高于杉木人工林。回归分析表明常绿阔叶林和杉木人工林DOC和POC流失量与降雨量、降雨强度和降雨侵蚀力呈显著的线性或幂函数相关,其中降雨量与DOC和POC流失量之间的拟合关系最优。常绿阔叶林产流和产沙量高于杉木人工林可能与前者的林下植被生物量较低有关,前者径流水中DOC浓度和POC浓度较高可归因于其较高的总生物量和土壤有机碳含量。在未来森林经营过程中应合理管理林下植被,尽量减少和避免林下植被的抚育伐,从而能够降低有机碳的水土流失,达到巩固和维持森林碳汇的目的。在未来气候变暖导致降水变化背景下,利用降雨量作为预测指标能够较好评估我国亚热带森林有机碳流失的风险。     

The role of bivalve molluscs as tools in estuarine sediment toxicity testing: a review

Estuarine sediments frequently are repositories and therefore potential sources of anthropogenic contaminants. Many organic and metallic chemical compounds released into aquatic systems bind to particulates and so accumulate in the sediments, thus, sediments become repositories of contaminants in estuaries. These may also cause contamination through diffusion of porewater, resuspension of particulates and dispersal of benthic fauna. There is a need to assess the biological affects of these anthropogenic contaminants because they may be toxic to infauna and bottomfish. Sediment toxicity bioassays are a means for carrying out such an assessment and primarily provide data on toxicity by measuring the effects on the test organism. Existing sediment toxicity bioassays rely on a battery of aquatic toxicity tests, which are based on the extraction of pore water, and elutriate from sediments and then subjecting these sediment phases to toxicity testing regimes. Two estuarine bivalve molluscs, Scrobicularia plana and Tapes semidecussatus were used to assess the ecotoxicity of field-collected sediments from estuarine and coastal areas around the Irish and English Coast over a 3-year study period. A variety of endpoints were measured during the study including survival in air, behaviour, animal condition, biochemistry, soft tissue metal concentrations, lysosomal membrane integrity and histopathology. Of these endpoints, the most sensitive were survival, survival in air, lysosomal membrane integrity, behaviour and histopathology.     

Relationship between porewater organic carbon content, sulphate reduction and nitrogen fixation (acetylene reduction) in the rhizosphere of Zostera noltii

Depth profiles of nitrogen fixation (acetylene reduction), sulphate reduction, NH ₄ ⁺ concentration and porewater volatile fatty acids concentrations were measured in Zostera noltii colonised sediments in the Bassin d'Arcachon, France in March 1994. Acetylene reduction activity (ARA) was detectable throughout sediment profiles. Addition of sodium molybdate (20 mmol l^–1) a specific inhibitor of sulphate reduction to slurries inhibited ARA by >75% inferring that sulphate-reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. The peak of ARA was coincident with that of sulphate reduction and a relatively constant relationship of 40 mole sulphate reduced per mole acetylene reduced was recorded throughout the profiles. From this ratio it was calculated that at least 17% of the ATP yield from sulphate reduction would be required to support the measured rates of nitrogen fixation (acetylene reduction).Acetate was the dominant constituent of the porewater volatile fatty acids pool, accounting for >90% of the total pool as measured by HPLC. Concentrations of porewater acetate recorded by HPLC were compared with those measured using an enzymatic technique and these data indicate that approximately 10% of the total porewater acetate pool was not available to microbial metabolism. Profiles of porewater acetate concentrations measured by both techniques were similar to those recorded for both ARA and sulphate reduction and thus acetate oxidation may fuel these activities.