Response of the microflora in outdoor experimental streams to pentachlorophenol: compartmental contributions

Outdoor artificial streams were treated continuously with pentachlorophenol (PCP) for 88 days during the summer of 1983. The contributions of different stream compartments (microbial habitats) to microbial degradation of PCP were determined in a stream treated with 144 micrograms of PCP per liter. The 488-m long stream was composed of mud-bottomed pools alternating with gravel riffles. PCP loss in the stream attributable to microbial degradation after an adaptation period was in the range of 55 to 74%. Contributions to PCP loss were determined for rock surface (epilithic), macrophyte surface (epiphytic), sedimentary, and water column communities by measuring rates of PCP disappearance in stream water, containing ambient concentrations of PCP, in contact with representative compartmental samples. The specific capability, in units of micrograms of PCP per hour per square meter of stream cross-sectional area (macrophytes at maximum plant density, water column at mean depth, upper 10-cm layer of gravel), followed the order rock surface much greater than macrophytes greater than sediment approximately equal to water column. The compartmental contribution to total stream losses in units of grams per hour followed the same order, although the differences were smaller. The rate of PCP disappearance in the water column above sediment cores followed the order oxygen-rich greater than oxygen-poor approximately equal to anaerobic greater than sorption-only conditions. The large difference in specific capability between the rock surface and sediment compartments could be attributed to oxygen deficiency (because of chemical and biological oxygen demand) in the sediments. Free-floating and particle-attached organisms in the water column were important to PCP biodegradation.     

Response of the microflora in outdoor experimental streams to pentachlorophenol: environmental factors

The 2nd year of a 2-year study of the fate of pentachlorophenol in outdoor artificial streams focused on details of microbial degradation by a combination of in situ and laboratory measurements. Replicate streams were dosed continuously at pentachlorophenol concentrations of 0, 48, and 144 micrograms/L, respectively, for an 88-d period during the summer of 1983. Pentachlorophenol was degraded both aerobically and anaerobically. Aerobic degradation was more rapid than anaerobic degradation. Mineralization of pentachlorophenol was concommitant with pentachlorophenol disappearance under aerobic conditions, but lagged behind loss of the parent molecule under anaerobic conditions. Biodegradation in the streams, or in specific stream compartments such as the sediment or water column, was characterized by an adaptation period (3-5 weeks for the stream as a whole, and reproducible from the previous year), which was inversely dependent on the concentration of pentachlorophenol and microbial biomass. The adaptation in the streams could be attributed to the time necessary for selective enrichment of an initially low population of pentachlorophenol degraders on surface compartments. The extent of biodegradation in the streams (percent loss of initial concentration of pentachlorophenol) increased with increasing pentachlorophenol input, which was explicable by an increase in the pentachlorophenol degrader population with increasing pentachlorophenol concentration. The sediment zone most significant to overall pentachlorophenol biodegradation was the top 0.5- to 1-cm layer as shown by pentachlorophenol migration rates and depth profiles of degrader density within the sediment. Pentachlorophenol profiles in sediment cores taken during and after the adaptation period for degradation showed that diffusion of pentachlorophenol into the sediment was rate limiting to degradation in this compartment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Microbial Transformation of Polycyclic Aromatic Hydrocarbons in Pristine and Petroleum-Contaminated Sediments

To determine rates of microbial transformation of polycyclic aromatic hydrocarbons (PAH) in freshwater sediments, ¹⁴C-labeled PAH were incubated with samples from both pristine and petroleum-contaminated streams. Evolved ¹⁴CO₂ was trapped in KOH, unaltered PAH and polar metabolic intermediate fractions were quantitated after sediment extraction and column chromatography, and bound cellular ¹⁴C was measured in sediment residues. Large fractions of ¹⁴C were incorporated into microbial cellular material; therefore, measurement of rates of ¹⁴CO₂ evolution alone would seriously underestimate transformation rates of [¹⁴C]naphthalene and [¹⁴C]anthracene. PAH compound turnover times in petroleum-contaminated sediment increased from 7.1 h for naphthalene to 400 h for anthracene, 10,000 h for benz(a)anthracene, and more than 30,000 h for benz(a)pyrene. Turnover times in uncontaminated stream sediment were 10 to 400 times greater than in contaminated samples, while absolute rates of PAH transformation (micrograms of PAH per gram of sediment per hour) were 3,000 to 125,000 times greater in contaminated sediment. The data indicate that four- and five-ring PAH compounds, several of which are carcinogenic, may persist even in sediments that have received chronic PAH inputs and that support microbial populations capable of transforming two- and three-ring PAH compounds.     

Biodegradation and photolysis of pentachlorophenol in artificial freshwater streams.

The biodegradation, photolysis, and adsorption of pentachlorophenol (PCP) in outdoor, aquatic environments were examined with man-made channels built by the U.S. Environmental Protection Agency at a field station on the Mississippi River near Monticello, Minn. Four channels were used, each channel being approximately 520 m long and receiving river water that flowed through the channels for about 10 h before reentering the river. The channels were dosed continuously during the summer of 1982 with various concentrations of PCP (approximately 0, 48, 144, and 432 micrograms/liter). We monitored the biotic and abiotic degradation of PCP in these channels for approximately 16 weeks. Photolysis of PCP was rapid at the water surface, but greatly attenuated with depth. Depending on sunlight conditions, photolysis accounted for a 5 to 28% decline in initial PCP concentration. Adsorption of PCP by sediment and uptake by biota accounted for less than 15% and probably less than 5% in unacclimated water. Microbial degradation of PCP became significant about 3 weeks after the initiation of dosing and eventually became the primary mechanism of PCP removal, accounting for a 26 to 46% (dose-dependent) decline in initial PCP. Most of the PCP-mineralizing microorganisms that developed in the channels were either attached to surfaces (e.g., rocks and macrophytes) or associated with surface sediments. Total bacterial numbers (direct microscopic counts) in the various channels were not affected significantly by PCP concentrations of micrograms per liter. Numerous strains of bacteria able to grow at the expense of PCP were isolated from the adapted channels. The experiments reported here will help predict the responses of flowing aquatic ecosystems to contamination by biocides such as pentachlorophenol.     

Biodegradation and photolysis of pentachlorophenol in artificial freshwater streams

The biodegradation, photolysis, and adsorption of pentachlorophenol (PCP) in outdoor, aquatic environments were examined with man-made channels built by the U.S. Environmental Protection Agency at a field station on the Mississippi River near Monticello, Minn. Four channels were used, each channel being approximately 520 m long and receiving river water that flowed through the channels for about 10 h before reentering the river. The channels were dosed continuously during the summer of 1982 with various concentrations of PCP (approximately 0, 48, 144, and 432 micrograms/liter). We monitored the biotic and abiotic degradation of PCP in these channels for approximately 16 weeks. Photolysis of PCP was rapid at the water surface, but greatly attenuated with depth. Depending on sunlight conditions, photolysis accounted for a 5 to 28% decline in initial PCP concentration. Adsorption of PCP by sediment and uptake by biota accounted for less than 15% and probably less than 5% in unacclimated water. Microbial degradation of PCP became significant about 3 weeks after the initiation of dosing and eventually became the primary mechanism of PCP removal, accounting for a 26 to 46% (dose-dependent) decline in initial PCP. Most of the PCP-mineralizing microorganisms that developed in the channels were either attached to surfaces (e.g., rocks and macrophytes) or associated with surface sediments. Total bacterial numbers (direct microscopic counts) in the various channels were not affected significantly by PCP concentrations of micrograms per liter. Numerous strains of bacteria able to grow at the expense of PCP were isolated from the adapted channels. The experiments reported here will help predict the responses of flowing aquatic ecosystems to contamination by biocides such as pentachlorophenol.     

Measurements of phosphorus uptake by macrophytes and epiphytes from the LaPlatte River (VT) using 32P in stream microcosms

1. Phosphorus (P) uptake by macrophytes and epiphytes from the LaPlatte River (VT) was examined in the laboratory by adding ³²PO₄‐P to recirculating stream microcosms.
2. Water, plugs of sediment and plants were removed from the river and placed into the microcosms. ³²PO₄‐P was then added either to the water or the sediment, and its incorporation into plants and epiphytes was monitored over 3 days. Uptake was examined at both ambient (5 μg L^–1) and increased (50 μg L^–1) soluble reactive phosphorus (SRP) concentrations. A computer program was developed to fit curves to the radiotracer data and calculate rate constants for the simultaneous transfer of ³²P among compartments.
3. Both macrophytes and epiphytes removed P from the water, but epiphyte uptake of P was more rapid. Phosphate enrichment stimulated P uptake by both macrophytes and epiphytes. Macrophytes also obtained P from the sediment. The relative contribution of P to macrophytes from the water vs. that from the sediment appeared to vary with SRP in the overlying water. Accurate estimates of rates of P uptake from sediments by macrophytes were difficult to obtain however, due to very low and highly variable unit rate constants for P uptake and uncertainty about the magnitude of the phosphate pool available for uptake.
4. SRP concentrations were greater in the overlying water than in the sediment pore water of stream microcosms in the present study. Numerous reports in the literature have suggested that this condition favours uptake by macrophyte stems and leaves rather than by roots.
5. Phosphate uptake from the water by macrophytes in shallow streams may be more common than for macrophytes in lakes.     

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.     

Factors controlling hyporheic respiration in a desert stream

1. Experimental manipulations were performed to determine the biological, chemical and physical attributes that govern sediment respiration in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream. 2. Hyporheic respiration per unit volume of sediment was inversely related to diameter of sediment particles, indicating that respiration is affected by availability of substrate for microbial colonization (i.e. sediment surfaces). Respiration rate per unit surface area on sediments was positively correlated with particle diameter, indicating greater metabolic activity of microbes on larger sediments. 3. Hyporheic respiration was more than twice as high in water collected from the surface flow than from subsurface flow. Further, hyporheic respiration was highest immediately following exposure of sediments to surface water and declined over time, presumably due to exhaustion of labile organic matter. 4. Microbial activity was stimulated by addition of algal leachate; however, amendments of leaf leachate had little effect. Respiration was also elevated with dextrose and leucine amendments, but not with inorganic nitrogen additions, indicating hyporheic respiration is carbon limited. 5. Water from the stream surface is probably enriched in labile organic matter derived from algae and stimulates respiration at points of hydrologic downwelling where surface water enters hyporheic sediments. The physical structure of sediments further affects metabolism by affecting the area available for microbial attachment.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Bioturbation: impact on the marine nitrogen cycle

Sediments play a key role in the marine nitrogen cycle and can act either as a source or a sink of biologically available (fixed) nitrogen. This cycling is driven by a number of microbial remineralization reactions, many of which occur across the oxic/anoxic interface near the sediment surface. The presence and activity of large burrowing macrofauna (bioturbators) in the sediment can significantly affect these microbial processes by altering the physicochemical properties of the sediment. For example, the building and irrigation of burrows by bioturbators introduces fresh oxygenated water into deeper sediment layers and allows the exchange of solutes between the sediment and water column. Burrows can effectively extend the oxic/anoxic interface into deeper sediment layers, thus providing a unique environment for nitrogen-cycling microbial communities. Recent studies have shown that the abundance and diversity of micro-organisms can be far greater in burrow wall sediment than in the surrounding surface or subsurface sediment; meanwhile, bioturbated sediment supports higher rates of coupled nitrification-denitrification reactions and increased fluxes of ammonium to the water column. In the present paper we discuss the potential for bioturbation to significantly affect marine nitrogen cycling, as well as the molecular techniques used to study microbial nitrogen cycling communities and directions for future study.     

Importance of advective mass transfer and sediment surface area for streambed microbial communities

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The disappearance of four leaf leachates in a hard and soft water stream in South Western Ontario,Canada

The disappearance of ‘dissolved’ organic carbon (D. O. C.) leached from maple, aspen, cedar and pine leaves was followed in hard and soft stream water and hard and soft stream water plus sediment. Differences were found in the rates and extent of the disappearance of D. O. C. between the leaves and the water types. These are discussed in relation to the productivity of two extreme stream types.     

Growth of macrophytes and ecosystem consequences in a lowland Danish stream

SUMMARY. 1. The River Suså is a small, nutrient-rich stream situated in an open landscape with clayish subsoil under intensive cultivation. Discharge was variable daily and seasonally due to low groundwater input. Above-ground development of submerged macrophytes was restricted to late May to November, when water velocity and depth were low. Dominant macrophytes were rooted Potamogeton pectinatus and Sparganium emersum and unrooted Cladophora . Biomass development was closet) related to light availability.
2. Growth rates of macrophytes were linearly related to light availability when self-shading was accounted for. Potamogeton pectinatus grew rapidly m May-June, concentrated the biomass at the water-surface during July-August, and then declined exponentially when the shoots became basally senescent. Sparganium emersum had linear, flexible leaves that were continuously replaced from a basal meristem. Sparganium emersum was less susceptible to high water velocities than Potamogeton pectinatus and the biomass declined later and at lower rates during autumn. Sparganium emersum also regrew after culling that left its meristem intact in the sediment. Unrooted Cladophora developed a high biomass during sunny periods and subsequently disappeared at high discharges. The summer biomass of rooted macrophytes was greater in years with high summer discharge, whereas the biomass of Cladophora and of the epiphytic microbial community was lower due to scouring.
3. Submerged macrophytes played a key role in structure and functioning of the ecosystem. They reduced water velocities two to four fold during summer and promoted extensive organic sedimentation. The biomass of benthic diatoms declined parallel to increased macrophyte shading and sedimentation. In addition, submerged macrophytes formed a large substratum for macroinvertebrates and for a microbial community.     

Distribution of viral abundance in the reef environment of Key Largo, Florida.

The distribution of viral and microbial abundance in the Key Largo, Fla., reef environment was measured. Viral abundance was measured by transmission electron microscope direct counts and plaque titer on specific bacterial hosts in water and sediment samples from Florida Bay (Blackwater Sound) and along a transect from Key Largo to the outer edge of the reef tract in Key Largo Sanctuary. Water column viral direct counts were highest in Blackwater Sound of Florida Bay (1.2 x 10(7) viruses per ml), decreased to the shelf break (1.7 x 10(6) viruses per ml), and were inversely correlated with salinity (r = -0.97). Viral direct counts in sediment samples ranged from 1.35 x 10(8) to 5.3 x 10(8)/cm(3) of sediment and averaged nearly 2 orders of magnitude greater than counts in the water column. Viral direct counts (both sediment and water column measurements) exceeded plaque titers on marine bacterial hosts (Vibrio natriegens and others) by 7 to 8 orders of magnitude. Water column viral abundance did not correlate with bacterial direct counts or chlorophyll a measurements, and sediment viral parameters did not correlate with water column microbial, viral, or salinity data. Coliphage, which are indicators of fecal pollution, were detected in two water column samples and most sediment samples, yet their concentrations were relatively low (<2 to 15/liter for water column samples, and <2 to 108/cm(3) of sediment). Our findings indicate that viruses are abundant in the Key Largo environment, particularly on the Florida Bay side of Key Largo, and that processes governing their distribution in the water column (i.e., salinity and freshwater input) are independent of those governing their distribution in the sediment environment.     

A landscape perspective of surface–subsurface hydrological exchanges in river corridors

1. River corridors can be visualised as a three‐dimensional mosaic of surface–subsurface exchange patches over multiple spatial scales. Along major flow paths, surface water downwells into the sediment, travels for some distance beneath or along the stream, eventually mixes with ground water, and then returns to the stream. 2. Spatial variations in bed topography and sediment permeability result in a mosaic of patch types (e.g. gravel versus sandy patches) that differ in their hydrological exchange rate with the surface stream. Biogeochemical processes and invertebrate assemblages vary among patch types as a function of the flux of advected channel water that determines the supply of organic matter and terminal electron acceptors. 3. The overall effect of surface–subsurface hydrological exchanges on nutrient cycling and biodiversity in streams not only depends on the proportion of the different patch types, but also on the frequency distribution of patch size and shape. 4. Because nutrients are essentially produced or depleted at the downwelling end of hyporheic flow paths, reach‐scale processing rates of nutrients should be greater in stretches with many small patches (e.g. short compact gravel bars) than in stretches with only a few large patches (e.g. large gravel bars). 5. Based on data from the Rhône River, we predict that a reach with many small bars should offer more hyporheic refugia for epigean fauna than a reach containing only a few large gravel bars because benthic organisms accumulate preferentially in sediments located at the upstream and downwelling edge of bars during floods. However, large bars are more stable and may provide the only refugia during severe flood events. 6. In river floodplain systems exhibiting pronounced expansion/contraction cycles, hyporheic assemblages within newly created patches not only depend on the intrinsic characteristics of these patches but also on their life span, hydrological connection with neighbouring patches, and movement patterns of organisms. 7. Empirical and theoretical evidence illustrate how the spatial arrangement of surface–subsurface exchange patches affects heterogeneity in stream nutrient concentration, surface water temperature, and colonisation of dry reaches by invertebrates. 8. Interactions between fluvial action and geomorphic features, resulting from seasonal and episodic flow pulses, alter surface–subsurface exchange pathways and repeatedly modify the configuration of the mosaic, thereby altering the contribution of the hyporheic zone to nutrient transformation and biodiversity in river corridors.     

The Structure of a Macroinvertebrate Community in a Northern German Lake Outlet (Lake Belau,Schleswig-Holstein) with Special Emphasis on Abundance,Biomass and Secondary Production

The structure of macroinvertebrate communities was studied at I I sampling sites of the outlet of Lake Belau in the lowlands of northern Germany. To describe the structures of macrobenthic animal communities three different units were examined: abundance, biomass, and secondary production. 112 taxa were collected from the entire stream. The numbers of species ranged from 31 (fine sand) to 70 (submerged macrophytes). For the stream, average macroinvertebrate density was 18.400 ind. M⁻¹. Density was highest at the macrophytes amounting to 35,630 individuals per m², and lowest in the pure sand with only 3,900 ind. M⁻². Average biomass (dry mass) was 194 g DM m⁻² varying from 9.8 (peat) to 381 g DM m⁻² (gravel with mollusk shells near the upstream lake). For the stream, average annual production was 129 g DM m⁻² varying from 15 (peat) to 286 g DM m⁻² (macrophytes). The highest values for each unit were found in stream sections with gravel and submerged macrophytes. Lower values occured in sections that contained peat and sand. Usually, a single structure of the macroinvertebrate community was dominated by less than ten taxa, which varied at each sampling site depending on the units observed.     

Regiospecific dechlorination of pentachlorophenol by dichlorophenol-adapted microorganisms in freshwater, anaerobic sediment slurries.

The reductive dechlorination of pentachlorophenol (PCP) was investigated in anaerobic sediments that contained nonadapted or 2,4- or 3,4-dichlorophenol (DCP)-adapted microbial communities. Adaptation of sediment communities increased the rate of conversion of 2,4- or 3,4-DCP to monochlorophenols (CPs) and eliminated the lag phase before dechlorination was observed. Both 2,4- and 3,4-DCP-adapted sediment communities dechlorinated the six DCP isomers to CPs. The specificity of chlorine removal from the DCP isomers indicated a preference for ortho-chlorine removal by 2,4-DCP-adapted sediment communities and for para-chlorine removal by 3,4-DCP-adapted sediment communities. Sediment slurries containing nonadapted microbial communities either did not dechlorinate PCP or did so following a lag phase of at least 40 days. Sediment communities adapted to dechlorinate 2,4- or 3,4-DCP dechlorinated PCP without an initial lag phase. The 2,4-DCP-adapted communities initially removed the ortho-chlorine from PCP, whereas the 3,4-DCP-adapted communities initially removed the para-chlorine from PCP. A 1:1 mixture of the adapted sediment communities also dechlorinated PCP without a lag phase. Dechlorination by the mixture was regiospecific, following a para greater than ortho greater than meta order of chlorine removal. Intermediate products of degradation, 2,3,5,6-tetrachlorophenol, 2,3,5-trichlorophenol, 3,5-DCP, 3-CP, and phenol, were identified by a combination of cochromatography (high-pressure liquid chromatography) with standards and gas chromatography-mass spectrometry.     

Key role of streambed moisture and flash storms for microbial resistance and resilience to long‐term drought

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Microbial methanogenesis and acetate metabolism in a meromictic lake.

Methanogenesis and the anaerobic metabolism of acetate were examined in the sediment and water column of Knaack Lake, a small biogenic meromictic lake located in central Wisconsin. The lake was sharply stratified during the summer and was anaerobic below a depth of 3 m. Large concentrations (4,000 mumol/liter) of dissolved methane were detected in the bottom waters. A methane concentration maximum occurred at 4 m above the sediment. The production of (14)CH(4) from (14)C-labeled HCOOH, HCO(3) (-), and CH(3)OH and [2-(14)C]acetate demonstrated microbial methanogenesis in the water column of the lake. The maximum rate of methanogenesis calculated from reduction of H(14)CO(3) (-) by endogenous electron donors in the surface sediment (depth, 22 m) was 7.6 nmol/h per 10 ml and in the water column (depth, 21 m) was 0.6 nmol/h per 10 ml. The methyl group of acetate was simultaneously metabolized to CH(4) and CO(2) in the anaerobic portions of the lake. Acetate oxidation was greatest in surface waters and decreased with water depth. Acetate was metabolized primarily to methane in the sediments and water immediately above the sediment. Sulfide inhibition studies and temperature activity profiles demonstrated that acetate metabolism was performed by several microbial populations. Sulfide additions (less than 5 mug/ml) to water from 21.5 m stimulated methanogenesis from acetate, but inhibited CO(2) production. Sulfate addition (1 mM) had no significant effect on acetate metabolism in water from 21.5 m, whereas nitrate additions (10 to 14,000 mug/liter) completely inhibited methanogenesis and stimulated CO(2) formation.