Internal wave-induced redox shifts affect biogeochemistry and microbial activity in sediments: a simulation experiment

Internal waves (seiches) are well-studied physical processes in stratified lakes, but their effects on sediment porewater chemistry and microbiology are still largely unexplored. Due to pycnocline oscillations, sediments are exposed to recurrent changes between epilimnetic and hypolimnetic water. This results in strong differences of environmental conditions, which should be reflected in the responses of redox-sensitive biogeochemical processes at both, the sediment–water interface and deeper sediment layers. We tested in a series of mesocosm experiments the influence of seiche-induced redox changes on porewater chemistry and bacterial activity in the sediments under well controlled conditions. Thereby, we excluded effects of changes in current and temperature regimes. For a period of 10 days, intact sediment cores from oligotrophic Lake Stechlin were incubated under constant (either oxic or anoxic) or alternating redox conditions. Solute concentrations were measured as porewater profiles in the sediment, while microbial activity was determined in the upper 0.5 cm of sediment. Oxic and alternating redox conditions resulted in similar ammonium, phosphate, and methane porewater concentrations, while concentrations of each analyte were considerably higher in anoxic cores. Microbial activity was clearly lower in the anoxic cores than in the oxic and the alternating cores. In conclusion, cores with intermittent anoxic phases of up to 24 hours do not differ in biogeochemistry and microbial activities from static oxic sediments. However, due to various physical processes seiches cause oxygen to penetrate deeper into sediment layers, which affects sediment redox gradients and increase microbial activity in seiche-influenced sediments.     

Oxygen concentration profiles and exchange in sediment cores with circulated overlying water

SUMMARY. 1. The overlying water of intact sediment cores was constantly stirred with an impeller at a rate sufficient to mix turbulently the water column and maintain the diffusive boundary layer at a determined thickness. The system allowed standardization of water circulation in laboratory sediment core experiments.
2. Both oxygen concentration and oxygen penetration depth in the sediments decreased, the former by 70% and the latter from 4.2 mm to 2.0 mm, when the overlying water was not stirred for 24 h, as measured with oxygen microelectrodes in a lake sediment core.
3. Oxygen profiles measured in sediment cores in the laboratory were similar to those measured in situ when the overlying water was stirred with an impeller at such a rate that a similar thickness of the diffusive boundary layer at the sediment-water interface developed in the laboratory as that in situ.
4. Sediment oxygen consumption was calculated from: (1) measured oxygen profiles in the diffusive boundary layer and the molecular diffusion coefficient for oxygen in water; (2) the measured oxygen decrease in the top of the sediments and the estimated diffusion coefficient in the sediment; and (3) by oxygen differences in the overlying water after incubation of sediment cores.     

Contrasting dynamics of ciliate communities in sandy and silty sediments of an estuarine intertidal flat

During a 1-year study of the ciliate faunas of a silty and a sandy site on an intertidal flat in the Westerschelde estuary, a total number of 107 taxa were recorded belonging to at least 52 genera and 15 orders. Our results suggest that physical properties of the sediment were more important in regulating ciliate abundance, diversity and community composition than food availability, predation, temperature or oxygen concentration. Ciliate abundance and diversity were positively related to sediment grain size and the ciliate community of silty sediments was found to be a subset of that of sandy sediments. At the sandy site, where the sediment composition was stable, seasonal changes in the ciliate community were related to changes in food availability and/or temperature. At both the sandy and silty sites, a clear vertical gradient in the ciliate community was observed that appeared to be linked to gradients in food availability and oxygen concentration. These vertical gradients in ciliate community composition, however, were less steep than the measured oxygen gradients, probably due to the presence of oxic microniches in the anoxic zone.     

Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments,with emphasis on the role of burrowing animals

The present paper reviews the current knowledge on diagenetic carbon transformations at the oxic/anoxic interface in coastal marine sediments. Oxygen microelectrodes have revealed that most coastal sediments are covered only by a thin oxic surface layer. The penetration depth of oxygen into sediments is controlled by the balance between downward transport and consumption processes. Consumption of oxygen is directly or indirectly caused by respiration of benthic organisms. Aerobic organisms have the enzymatic capacity for complete oxidation of organic carbon. Anaerobic decay occurs stepwise, involving several types of bacteria. Large organic molecules are first fermented into small moieties. These are then oxidized completely by anaerobic respirers using a sequence of electron acceptors: Mn⁴⁺, NO₃ ^-, Fe³⁺, SO₄ ^2- and CO₂. The quantitative role of each electron acceptor depends on the sediment type and water depth. Since most of the sediment oxygen uptake is due to reoxidation of reduced metabolites, aerobic respiration is of limited importance. It has been suggested that sediments contain three major organic fractions: (1) fresh material that is oxidized regardless of oxygen conditions; (2) oxygen sensitive material that is only degraded in the presence of oxygen; and (3) totally refractory organic matter. Processes occurring at the oxic/anoxic boundaries are controlled by a number of factors. The most important are: (1) temperature, (2) organic supply, (3) light, (4) water currents, and (5) bioturbation. The role of bioturbation is important because the infauna creates a three-dimensional mosaic of oxic/anoxic interfaces in sediments. The volume of oxic burrow walls may be several times the volume of oxic surface sediment. The infauna increases the capacity, but not the overall organic matter decay in sediments, thus decreasing the pool of reactive organic matter. The increase in decay capacity is partly caused by injection of oxygen into the sediment, and thereby enhancing the decay of old, oxygen sensitive organic matter several fold. Finally, some future research directions to improve our understanding of diagenetic processes at the oxic/anoxic interface are suggested.     

Small-scale distribution of interstitial nitrite in freshwater sediment microcosms: the role of nitrate and oxygen availability,and sediment permeability

The spatial distribution of interstitial NO2(-) concentrations was studied in NO3(-)-exposed freshwater sediment microcosms, using pore water extractions as well as ion-selective microsensors. Porewater extractions revealed ecotoxicologically critical NO2(-) concentrations in hypoxic and anoxic sediment layers in which significant NO3(-) consumption took place. In contrast, the use of ion-selective microsensors demonstrated the high capacity of the thin oxic surface layer of the sediments to consume NO2(-) and to produce NO3(-). Two modes of NO3(-) supply to the sediments were compared: In treatments with NO3(-) supply to the overlying water, a subsurface maximum of NO2(-) concentration was observed, coinciding with the site of maximum NO3(-) consumption. When NO3(-) was perfused up through the sediment cores, however, NO2(-) accumulated throughout the entire sediment column. Such spatially extensive NO2(-) accumulations were only observed in sediments poor in organic matter with a relatively high permeability. By manipulating the O2 content of the overlying water, the release of NO2(-) from the sediments could be influenced: In treatments with air-saturated overlying water, the sediments did not release detectable amounts of NO2(-) into the water phase. When kept hypoxic (25% air saturation) instead, significant NO2(-) accumulations were recorded in the overlying water. These findings suggest that in treatments with air-saturated overlying water, NO2(-) that was produced in deeper sediment layers (denitrifying conditions) was completely consumed at the oxic sediment surface (nitrifying conditions) before it could reach the overlying water.     

Benthic pH gradients across a range of shelf sea sediment types linked to sediment characteristics and seasonal variability

This study used microelectrodes to record pH profiles in fresh shelf sea sediment cores collected across a range of different sediment types within the Celtic Sea. Spatial and temporal variability was captured during repeated measurements in 2014 and 2015. Concurrently recorded oxygen microelectrode profiles and other sedimentary parameters provide a detailed context for interpretation of the pH data. Clear differences in profiles were observed between sediment type, location and season. Notably, very steep pH gradients exist within the surface sediments (10–20 mm), where decreases greater than 0.5 pH units were observed. Steep gradients were particularly apparent in fine cohesive sediments, less so in permeable sandier matrices. We hypothesise that the gradients are likely caused by aerobic organic matter respiration close to the sediment–water interface or oxidation of reduced species at the base of the oxic zone (NH₄ ⁺, Mn²⁺, Fe²⁺, S^?). Statistical analysis suggests the variability in the depth of the pH minima is controlled spatially by the oxygen penetration depth, and seasonally by the input and remineralisation of deposited organic phytodetritus. Below the pH minima the observed pH remained consistently low to maximum electrode penetration (ca. 60 mm), indicating an absence of sub-oxic processes generating H⁺ or balanced removal processes within this layer. Thus, a climatology of sediment surface porewater pH is provided against which to examine biogeochemical processes. This enhances our understanding of benthic pH processes, particularly in the context of human impacts, seabed integrity, and future climate changes, providing vital information for modelling benthic response under future climate scenarios.     

Distribution and Rate of Methane Oxidation in Sediments of the Florida Everglades

Rates of methane emission from intact cores were measured during anoxic dark and oxic light and dark incubations. Rates of methane oxidation were calculated on the basis of oxic incubations by using the anoxic emissions as an estimate of the maximum potential flux. This technique indicated that methane oxidation consumed up to 91% of the maximum potential flux in peat sediments but that oxidation was negligible in marl sediments. Oxygen microprofiles determined for intact cores were comparable to profiles measured in situ. Thus, the laboratory incubations appeared to provide a reasonable approximation of in situ activities. This was further supported by the agreement between measured methane fluxes and fluxes predicted on the basis of methane profiles determined by in situ sampling of pore water. Methane emissions from peat sediments, oxygen concentrations and penetration depths, and methane concentration profiles were all sensitive to light-dark shifts as determined by a combination of field and laboratory analyses. Methane emissions were lower and oxygen concentrations and penetration depths were higher under illuminated than under dark conditions; the profiles of methane concentration changed in correspondence to the changes in oxygen profiles, but the estimated flux of methane into the oxic zone changed negligibly. Sediment-free, root-associated methane oxidation showed a pattern similar to that for methane oxidation in the core analyses: no oxidation was detected for roots growing in marl sediment, even for roots of Cladium jamaicense, which had the highest activity for samples from peat sediments. The magnitude of the root-associated oxidation rates indicated that belowground plant surfaces may not markedly increase the total capacity for methane consumption. However, the data collectively support the notion that the distribution and activity of methane oxidation have a major impact on the magnitude of atmospheric fluxes from the Everglades.     

Mesocosm experiments: mimicking seasonal developments of microbial variables in North Sea sediments

This study investigated the suitability of mesocosms for studying the seasonal development of microbial variables in the benthic system of the North Sea. Undisturbed sediment cores were taken from two locations in the North Sea, one with sandy sediment (28 m depth) and the other with silty sediment (38 m depth) and installed in mesocosms in January–April 1989. Cores were kept as in situ temperature in the dark until December 1989. One set of sandy and silty sediments was starved and the other set received a supply of organic matter in May–June, simulating the settlement of the spring bloom of Phaeocystis pouchetii. Seasonal developments in bacterial production (methyl ³H-thymidine incorporation), abundance and biomass of bacteria and nanoflagellates and oxygen consumption were compared between the mesocosms and the field in surface sediments every 1.5 to 2.5 months. Effects of seasonal temperature variations (range 6–17.5 °C) on microbial variables in starved mesocosms were limited, which possibly indicates a subordinate role of temperature in microbial processes in North Sea sediments. Organic matter produced a direct response in bacterial production and oxygen consumption in mesocosms. Bacterial and protozoan abundance also increased. The effect of the organic input disappeared within 2 months and values of enhanced variables declined to initial levels. The organic matter enrichment in mesocosms apparently did not provide sufficient energy to keep the microbenthos active at field levels through summer.These results suggest that in the silty sediments in the field, organic matter is available for bacterial production throughout summer. In sandy sediments, the major organic matter input, which sets the seasonal pattern, appears to be in June. Apparently the seasonal development of microbial variables can be mimicked in mesocosms with organic matter supplies. Differences between the field and mesocosms are further illustrated by carbon budgets. Recycling of bacterial biomass was required to meet the bacterial carbon demand in the budget.Publication No. 22 of the project Applied Scientific Research Neth. Inst. for Sea Res. (BEWON).     

Oxygen measurements in the burrows of freshwater insects

1. Thin-tipped micro-electrodes were used to measure oxygen concentrations in the burrows of two common aquatic insects, the mayfly Hexagenia limbata and the alderfly Sialis velata . Both species maintain their surroundings oxygenated by drawing water from above the sediment surface into their tubes. 2. The temporal pattern of oxygen in the burrows differed between the species. The constant high oxygen concentration (>75% of air saturation) measured in the tubes of the mayfly suggest that this animal pumps water almost continuously, which is consistent with its high oxygen requirements. In contrast, oxygen concentration in burrows of the alderfly fluctuated widely over time, suggesting that this animal irrigates only irregularly, probably because it can tolerate short periods of low oxygen concentration in its burrow. 3. The interval between pumping episodes by the alderfly decreased with increasing temperature, a result of increased oxygen consumption by the animal and by sediment at high temperature. 4. Based on the tube dimensions, oxygen penetration depth and animal density in lakes, we estimate that Hexagenia could create an oxic micro-environment equivalent to 3–35% of the volume of the surface oxidized sediment layer created by molecular diffusion. The mosaic of oxic micro-environments created by the burrowing and irrigation of freshwater animals could influence chemical and biological processes in sediments, the fluxes of materials between the sediment and the overlying water column, and the exposure of benthic animals to sedimentary contaminants.     

Microbial carbon monoxide consumption in salt marsh sediments

We have examined sediments from a fringing salt marsh in Maine to further understand marine CO metabolism, about which relatively little is known. Intact cores from the marsh emitted CO during dark oxic incubations, but emission rates were significantly higher during anoxic incubations, which provided evidence for simultaneous production and aerobic consumption in surface sediments. CO emission rates were also elevated when cores were exposed to light, which indicated that photochemical reactions play a role in CO production. A kinetic analysis of marsh surface sediments yielded an apparent K(m) of about 82 ppm, which exceeded values reported for well-aerated soils that consume atmospheric CO (65nM). Surface (0-0.2 cm depth interval) sediment slurries incubated under oxic conditions rapidly consumed CO, and methyl fluoride did not inhibit uptake, which indicated that neither ammonia nor methane oxidizers contributed to the observed activity. In contrast, aerobic CO uptake was inhibited by additions of readily available organic substrates (pyruvate, glucose and glycine), but not by cellulose. CO was also consumed by surface and sub-surface sediment slurries incubated under anaerobic conditions, but rates were less than during aerobic incubations. Molybdate and nitrate or nitrite, but not 2-bromoethanesulfonic acid, partially inhibited anaerobic uptake. These results suggest that sulfidogens and acetogens, but not dissimilatory nitrate reducers or methanogens, actively consume CO. Sediment-free plant roots also oxidized CO aerobically; rates for Spartina patens and Limonium carolinianum roots were significantly higher than rates for Spartina alterniflora roots. Thus plants may also impact CO cycling in estuarine environments.     

pmoA-based analysis of methanotrophs in a littoral lake sediment reveals a diverse and stable community in a dynamic environment

Diversity and community structure of aerobic methane-oxidizing bacteria in the littoral sediment of Lake Constance was investigated by cloning analysis and terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of the pmoA gene. Phylogenetic analysis revealed a high diversity of type I and type II methanotrophs in the oxygenated uppermost centimeter of the sediment. T-RFLP profiles indicated a high similarity between the active methanotrophic community in the oxic layer and the inactive community in an anoxic sediment layer at a 10-cm depth. There were also no major changes in community structure between littoral sediment cores sampled in summer and winter. By contrast, the fingerprint patterns showed substantial differences between the methanotrophic communities of littoral and profundal sediments.     

pmoA-Based Analysis of Methanotrophs in a Littoral Lake Sediment Reveals a Diverse and Stable Community in a Dynamic Environment

Diversity and community structure of aerobic methane-oxidizing bacteria in the littoral sediment of Lake Constance was investigated by cloning analysis and terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of the pmoA gene. Phylogenetic analysis revealed a high diversity of type I and type II methanotrophs in the oxygenated uppermost centimeter of the sediment. T-RFLP profiles indicated a high similarity between the active methanotrophic community in the oxic layer and the inactive community in an anoxic sediment layer at a 10-cm depth. There were also no major changes in community structure between littoral sediment cores sampled in summer and winter. By contrast, the fingerprint patterns showed substantial differences between the methanotrophic communities of littoral and profundal sediments.     

Microelectrode Measurements of the Activity Distribution in Nitrifying Bacterial Aggregates

Microelectrodes for ammonium, oxygen, nitrate, and pH were used to study nitrifying aggregates grown in a fluidized-bed reactor. Local reactant fluxes and distribution of microbial activity could be determined from the microprofiles. The interfacial fluxes of the reactants closely reflected the stoichiometry of bacterial nitrification. Both ammonium consumption and nitrate production were localized in the outer shells, with a thickness of approximately 100 to 120 μm, of the aggregates. Under conditions in which ammonium and oxygen penetrated the whole aggregate, nitrification was restricted to this zone; oxygen was consumed in the central parts of the aggregates as well, probably because of oxidation of dead biomass. A sudden increase of the oxygen concentration to saturation (pure oxygen) was inhibitory to nitrification. The pH profiles showed acidification in the aggregates, but not to an inhibitory level. The distribution of activity was determined by the penetration depth of oxygen during aggregate development in the reactor. Mass transfer was significantly limited by the boundary layer surrounding the aggregates. Microelectrode measurements showed that the thickness of this layer was correlated with the diffusion coefficient of the species. Determination of the distribution of nitrifying activity required the use of ammonium or nitrate microelectrodes, whereas the use of oxygen microelectrodes alone would lead to erroneous results.     

Dissolved iron:phosphate ratio as an indicator of phosphate release to oxic water of the inner and outer coastal Baltic Sea

Pore water concentrations and benthic fluxes of dissolved Fe, P and N were measured at two coastal basins in the Gulf of Finland, northern Baltic Sea, during a seasonal cycle. The bioturbated inner coastal basin, where exchange of near-bottom water is efficient, had a better ability to retain P in sediments than the outer basin, where near-bottom water O₂ concentration decreases during summer. Under the presence of O₂ high pore water dissolved Fe:P ratio (>3.6 w:w) in surface layer of the sediment, measured especially in winter, indicated negligible or low P-release and high N:P ratio in the efflux. On the contrary, low Fe:P ratio (<3.6), measured in summer and autumn, indicated high efflux of P and low N:P flux ratio. The low dissolved Fe:P ratio suggested that there was not enough diffusing Fe to form Fe³⁺ oxide-rich layer in the oxic surface zone of the sediments or near-bottom water to bind the P diffusing from the sediment. However, in sediments bioturbated by the abundant bivalve Macoma baltica, small efflux of P were measured almost throughout the study period. Thus, the Fe:P ratio cannot alone explain the P-release in bioturbated sediments. The low N:P ratio in the efflux measured in summer and autumn partly explains the measured low N:P ratio in the near-bottom water and thus N limitation of primary production in the Gulf. Additionally, it is evident that the release of P in the Gulf itself is of great importance for the trophic state of the Gulf of Finland.     

Correlation between anammox activity and microscale distribution of nitrite in a subtropical mangrove sediment

The distribution of anaerobic ammonium oxidation (anammox) in nature has been addressed by only a few environmental studies, and our understanding of how anammox bacteria compete for substrates in natural environments is therefore limited. In this study, we measure the potential anammox rates in sediment from four locations in a subtropical tidal river system. Porewater profiles of NO(x)(-) (NO2- plus NO3-) and NO2- were measured with microscale biosensors, and the availability of NO2- was compared with the potential for anammox activity. The potential rate of anammox increased with increasing distance from the mouth of the river and correlated strongly with the production of nitrite in the sediment and with the average concentration or total pool of nitrite in the suboxic sediment layer. Nitrite accumulated both from nitrification and from NO(x)(-) reduction, though NO(x)(-) reduction was shown to have the greatest impact on the availability of nitrite in the suboxic sediment layer. This finding suggests that denitrification, though using NO2- as a substrate, also provides a substrate for the anammox process, which has been suggested in previous studies where microscale NO2- profiles were not measured.     

Ammonium adsorption in sediments of a tropical mangrove forest (Thailand) and a temperate Wadden Sea area (Denmark)

Dimensionless ammonium adsorption coefficients (K) were determined for tropical mangrove forest sediments and temperate Wadden Sea sediments. The K values were obtained from ammonium adsorption isotherms of KCl (2 M) extractable ammonium versus dissolved ammonium in the porewater; relationships that can be described by a linear model. Linearity was evident even at low porewater concentrations for mangrove sediment, according to isotherms based on KCl extractions on untreated sediment profiles. K-values were low in mangrove forest sediments (0.04–0.10), and higher in Wadden Sea sediments (0.17–1.12). The vertical range in K-values was larger at the vegetated sites, with highest values in subsurface sediments, which suggests differences in quantity and quality of the fine organic and inorganic fractions. The low ammonium adsorption in the mangrove sediments can be explained by a higher concentration of competitive cations, especially iron, in this iron-rich environment. The low adsorption of ammonium in mangrove sediments and vegetated surface sediment of the Wadden Sea was negatively related to the organic content of the sediments, which is in contrast to findings for other marine sediments. We suggest that organic material may have a diluting effect on the exchange capacity in fine-grained sediments, and that organic coatings may block ion exchange sites on clay surfaces. Thus, there may be a organic-rich ‘wetland’ versus organic-poor ‘sediment’ effect. This revised version was published online in July 2006 with corrections to the Cover Date.     

Activity of methanotrophic bacteria in Green Bay sediments

Abstract Sediment pore water samples obtained from a 19 m station in Green Bay in Lake Michigan were examined for levels of ambient dissolved methane and copper, and for the potential for in situ methane oxidation by methanotrophs found within surface sediments. The in situ methane concentration in the upper oxic sediment layer ranged from 20–150 μmol · 1⁻¹ at this station. The activity of methanotrophs and the kinetics of methane oxidation in these sediments were demonstrated by the uptake of radiolabeled methane. K_s values varied between 4.1–9.6 nmol · cm³ of sediment slurry. High V_max values (12.7–35.2 nmol · cm⁻³ · h⁻¹) suggest a large population of methanotrophs in the sediments. An average methane flux to the oxic sediments of 0.24 mol · m⁻² · year⁻¹ was calculated from the pore water methane gradients. Pore water concentrations of copper in the upper sediment layer ranged from 10–120 nmol · 1⁻¹. Based upon the copper concentration, other measured parameters, and equilibrium conditions defined by WATEQF4, an estimate for dissolved free Cu²⁺ concentration of 5–38 nmol · 1⁻¹ pore water was obtained. Several factors control the rate of methane oxidation, including oxygen, methane, and the bioavailability of free Cu²⁺.     

Microbial diversity and stratification of South Pacific abyssal marine sediments

Abyssal marine sediments cover a large proportion of the ocean floor, but linkages between their microbial community structure and redox stratification have remained poorly constrained. This study compares the downcore gradients in microbial community composition to porewater oxygen and nitrate concentration profiles in an abyssal marine sediment column in the South Pacific Ocean. Archaeal 16S rRNA clone libraries showed a stratified archaeal community that changed from Marine Group I Archaea in the aerobic and nitrate-reducing upper sediment column towards deeply branching, uncultured crenarchaeotal and euryarchaeotal lineages in nitrate-depleted, anaerobic sediment horizons. Bacterial 16S rRNA clone libraries revealed a similar shift on the phylum and subphylum level within the bacteria, from a complex community of Alpha-, Gamma- and Deltaproteobacteria, Actinobacteria and Gemmatimonadetes in oxic surface sediments towards uncultured Chloroflexi and Planctomycetes in the anaerobic sediment column. The distinct stratification of largely uncultured bacterial and archaeal groups within the oxic and nitrate-reducing marine sediment column provides initial constraints for their microbial habitat preferences.     

Net dissolved inorganic nitrogen production in hyporheic mesocosms with contrasting sediment size distributions

The interstitial spaces within streambeds are recognized as an important location of dissolved inorganic nitrogen (DIN) transformations in streams. However, it remains uncertain how physical characteristics of streambeds affect the magnitude and net outcome of subsurface nitrogen transformations. We tested whether the size distribution of streambed sediments, in isolation from the influence of streambed topography and groundwater upwelling, could affect net DIN uptake or production along interstitial flow paths. Mesocosms constructed from PVC pipe (15 cm diameter × 1 m long) were filled with either coarse gravel/cobble or gravel/cobble mixed with finer sediments (5 mesocosms per sediment treatment). Mesocosms were submerged in a stream and oriented, so that surface water flowed through the sediments. After 2 months incubation, we measured DIN in interstitial water at 20 cm intervals and dissolved oxygen at 10 cm intervals along mesocosm flow paths. In both sediment types, DIN concentrations increased longitudinally along mesocosm flow paths in the direction of interstitial flow, indicating net DIN production. Although DIN increased to higher concentrations in mesocosms with fine sediments, greater exchange flow through coarse sediments resulted in similar rates of net DIN production and delivery to surface water. Production of DIN in both sediment types was concentrated within the first 10 cm of interstitial flow paths, with no significant production further along the flow paths. Coarse sediments had higher rates of oxygen consumption per unit sediment volume than the coarse–fine sediment mix, suggesting interstitial water velocity may be an important factor affecting hyporheic microbial metabolism.     

Microbial production and consumption of dimethyl sulfide (DMS) in a sea grass (Zostera noltii)-dominated marine intertidal sediment ecosystem (Bassin d'Arcachon, France)

The relation between net dimethyl sulfide (DMS) production and changes in near surface (0-5 mm) oxygen concentrations in a sea grass (Zostera noltii Hornem)-covered intertidal sediment ecosystem was examined during a diel cycle. Sediment covered with Zostera was found to be more oxygenated than uncovered sediment during the period of photosynthesis. This phenomenon was probably caused by radial oxygen loss of the Zostera root-rhizome system. The population sizes of the three functional groups of microbes mainly responsible for the concentration of DMS, the dimethylsulfoniopropionate (DMSP)-demethylating, DMSP-cleaving and DMS-oxidizing bacteria, were quantified by most probable number (MPN) methodologies. Sediments with Zostera supported substantially higher populations of both aerobic (149x10(6) cm(-3) DMSP-utilizing and 0.4x10(6) cm(-3) DMS-oxidizing) and anaerobic (43x10(6) cm(-3) DMSP-utilizing and 0.4x10(6) cm(-3) DMS-oxidizing) microorganisms than sediments without Zostera (DMSP-utilizing aerobes and anaerobes both 2x10(6) cm(-3) and DMS-oxidizing aerobes and anaerobes both 0.2x10(6) cm(-3)). Experiments conducted with sediment cores and sediment slurries suggested that the net production of DMS in these sediments was significantly lower during oxic periods than during anoxic periods. Intact sediment cores with and without Zostera produced DMS when incubated under anoxic/dark conditions (97.0 and 53.6 nmol DMS m(-2) h(-1), respectively), while oxic/light-incubated cores did not produce detectable amounts of DMS. In addition, kinetic parameter values (V(max) and K(m)) for DMSP degradation in cell suspensions of isolated DMSP-demethylating and DMSP-cleaving bacteria were measured and compared to documented values for other strains. Both V(max) and K(m) values for DMSP-demethylating organisms were found to be relatively low (14.4-20.1 nmol DMSP mg protein(-1) min(-1) and 4.1-15.5 μM, respectively) while these parameter values varied widely in the group of the DMSP-cleaving organisms (6.7-1000 nmol DMSP mg protein(-1) min(-1) and 2-2000 μM, respectively). It was hypothesized that a diel rhythm in DMS emission occurred, with a relatively low net production during the day and a high net production during the night. Environmental changes which result in increased anoxic conditions in coastal sediments, such as an increase in eutrophication, may therefore result in increased atmospheric DMS emission rates.