Detection of microbial biomass by intact polar membrane lipid analysis in the water column and surface sediments of the Black Sea

The stratified water column of the Black Sea produces a vertical succession of redox zones, stimulating microbial activity at the interfaces. Our study of intact polar membrane lipids (IPLs) in suspended particulate matter and sediments highlights their potential as biomarkers for assessing the taxonomic composition of live microbial biomass. Intact polar membrane lipids in oxic waters above the chemocline represent contributions of bacterial and eukaryotic photosynthetic algae, while anoxygenic phototrophic bacteria and sulfate-reducing bacteria comprise a substantial amount of microbial biomass in deeper suboxic and anoxic layers. Intact polar membrane lipids such as betaine lipids and glycosidic ceramides suggest unspecified anaerobic bacteria in the anoxic zone. Distributions of polar head groups and core lipids show planktonic archaea below the oxic zone; methanotrophic archaea are only a minor fraction of archaeal biomass in the anoxic zone, contrasting previous observations based on the apolar derivatives of archaeal lipids. Sediments contain algal and bacterial IPLs from the water column, but transport to the sediment is selective; bacterial and archaeal IPLs are also produced within the sediments. Intact polar membrane lipid distributions in the Black Sea are stratified in accordance with geochemical profiles and provide information on vertical successions of major microbial groups contributing to suspended biomass. This study vastly extends our knowledge of the distribution of complex microbial lipids in the ocean.     

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.     

Sulfur Cycling in the Terrestrial Subsurface: Commensal Interactions, Spatial Scales, and Microbial Heterogeneity

Abstract Microbiological, geochemical, and isotopic analyses of sediment and water samples from the unconsolidated Yegua formation in east-central Texas were used to assess microbial processes in the terrestrial subsurface. Previous geochemical studies suggested that sulfide oxidation at shallow depths may provide sulfate for sulfate-reducing bacteria (SRB) in deeper aquifer formations. The present study further examines this possibility, and provides a more detailed evaluation of the relationship between microbial activity, lithology, and the geochemical environment on meter-to-millimeter scales. Sediment of varied lithology (sands, silts, clays, lignite) was collected from two boreholes, to depths of 30 m. Our findings suggest that pyrite oxidation strongly influences the geochemical environment in shallow sediments (∼5 m), and produces acidic waters (pH 3.8) that are rich in sulfate (28 mM) and ferrous iron (0.3 mM). Sulfur and iron-oxidizing bacteria are readily detected in shallow sediments; they likely play an indirect role in pyrite oxidation. In consistent fashion, there is a relative paucity of pyrite in shallow sediments and a low ³⁴S/³²S-sulfate ratio (0.2‰) (reflecting contributions from ³⁴S-depleted sulfides) in shallow regions. Pyrite oxidation likely provides a sulfate source for both oxic and anoxic aquifers in the region. A variety of assays and direct-imaging techniques of ³⁵S-sulfide production in sediment cores indicates that sulfate reduction occurs in both the oxidizing and reducing portions of the sediment profile, with a high degree of spatial variability. Narrow zones of activity were detected in sands that were juxtaposed to clay or lignite-rich sediments. The fermentation of organic matter in the lignite-rich laminae provides small molecular weight organic acids to support sulfate reduction in neighboring sands. Consequently, sulfur cycling in shallow sediments, and sulfate transport represent important mechanisms for commensal interaction among subsurface microorganisms by providing electron donors for chemoautotrophic bacteria and electron acceptors for SRB. The activity of SRB is linked to the availability of suitable electron donors from spatially distinct zones. Received: 10 November 1997; Accepted: 10 February 1998     

Distribution of Iron-Oxidizing Bacteria in the Nordic Uranium Tailings Deposit, Elliot Lake, Ontario, Canada

Iron-oxidizing bacteria are present within the top 2 m (but not always at the surface) and near the water table-capillary fringe of the vegetated Nordic uranium deposit, Elliot Lake, Ontario, Canada. They are distributed uniformly in the top 0.5 m of unvegetated tailings. The locations of these bacteria correlate with zones of pyrite oxidation as delineated in previous studies by the formation of soluble iron and sulfate. Heterotrophic bacteria are also present in the tailings, with greatest concentrations at the surface and near the water table-capillary fringe. Sulfate-reducing bacteria were detected in the soil and peat at the base of the tailings. The results of this study suggest that the establishment of vegetation directly upon the tailings surface does not arrest bacterial pyrite oxidation.     

Occurrence of sulfate-reducing bacteria under a wide range of physico-chemical conditions in Au and Cu-Zn mine tailings

Sulfate-reducing bacteria (SRB) have been observed in mining environments, but their presence has not been linked to specific physico-chemical and mineralogical factors. The present study was undertaken to assess the presence of SRB in several Au and Cu-Zn mine tailings located near Timmins, Ont., Canada, and determine the factors responsible for their presence. Vegetated and non-vegetated mine tailings were sampled for SRB enumeration, pH, Eh, water content, total carbon content and sequential chemical extraction. Results first showed that SRB populations were present at all sites and that their distribution varied with depth. Populations were recovered from neutral pH and slightly anoxic tailings and from highly acidic (pH 2) and oxic tailings. The total carbon content of the tailings was generally low and not related to the presence of vegetation. In addition, the carbon content did not affect SRB population distribution and appeared to be more related to the type of tailings, i.e., oxidized and acidic Cu-Zn tailings contained on average more carbon than Au tailings. Results also indicated that the water content of the tailings varied greatly with depth and was not related to the presence of SRB populations. The sequential chemical extraction showed that the pyrite content of the tailings was lower in Au tailings than in Cu-Zn tailings, and that some oxidized Cu-Zn sites were depleted in pyrite due to microbial and chemical oxidation. Our results indicate that SRB could be cultured from a variety of sites and sample types, and that factors such as pH, Eh, water content and carbon content at the collection sites did not exert control on their presence.     

Cable bacteria extend the impacts of elevated dissolved oxygen into anoxic sediments

Profound biogeochemical responses of anoxic sediments to the fluctuation of dissolved oxygen (DO) concentration in overlaying water are often observed, despite oxygen having a limited permeability in sediments. This contradiction is indicative of previously unrecognized mechanism that bridges the oxic and anoxic sediment layers. Using sediments from an urban river suffering from long-term polycyclic aromatic hydrocarbons (PAHs) contamination, we analyzed the physicochemical and microbial responses to artificially elevated DO (eDO) in the overlying water over 9 weeks of incubation. Significant changes in key environmental parameters and microbial diversity were detected over the 0–6 cm sediment depth, along with accelerated degradation of PAHs, despite that eDO only increased the porewater DO in the millimeter subfacial layer. The dynamics of physicochemical and microbial properties coincided well with significantly increased presence of centimeter-long sulfide-oxidizing cable bacteria filaments under eDO, and were predominantly driven by cable bacteria metabolic activities. Phylogenetic ecological network analyses further revealed that eDO reinforced cable bacteria associated interspecific interactions with functional microorganisms such as sulfate reducers, PAHs degraders, and electroactive microbes, suggesting enhanced microbial syntrophy taking advantage of cable bacteria metabolism for the regeneration of SO₄²⁻ and long-distance electron transfer. Together, our results suggest cable bacteria may mediate the impacts of eDO in anaerobic sediments by altering sediment physiochemical properties and by reinforcing community interactions. Our findings highlight the ecological importance of cable bacteria in sediments.Subject terms: Freshwater ecology, Water microbiology, Community ecology     

Biogeochemical phenomena induced by bacteria within sulfidic mine tailings

Summary Mill tailings resulting from mining and metallurgical processes are usually disposed of into open-air impoundments, where they become subjected to chemical or microbial leaching. At the surface of the tailings, where oxic conditions prevail, acidophilic bacteria, such as thiobacilli, can oxidize sulfidic minerals (e.g. pyrite and pyrrhotite) and generate acidic metal-rich leachates as by-products of their metabolism. This, combined with chemical oxidation, leads to acid mine drainage (AMD). Biomineralization, whereby a proportion of the metal leachate is precipitated, can also occur in the oxidized tailings, often as a result of a close metal-bacteria interaction. Iron-rich precipitates are usually found on bacterial cell walls, and are thought to serve as nucleation sites for further mineralization within the tailings impoundments. As depth increases in mine tailings, oxygen depletion and the presence of water-saturated pores usually lead to anoxic conditions. Under such redox and chemical conditions, populations of sulfate-reducing bacteria (SRBs) can colonize the tailings. As a result of their metabolic activity, sulfate is reduced to hydrogen sulfide, which in turn can react with dissolved metals to form metal sulfide precipitates. Microbial sulfate reduction also generates alkalinity, although chemical dissolution of carbonate and oxide minerals probably also play an important role in the generation of alkaline conditions in mine tailings.     

Phosphate exchange across the sediment-water interface when shifting from anoxic to oxic conditions an experimental comparison of freshwater and brackish-marine systems

Comparative, experimental studies on sediment cores from freshwater andbrackish-marine conditions reveal major differences in the benthic exchangeof phosphate across the sediment-water interface when shifting from anoxicto oxic conditions. The flux of phosphate to the sediment during this shiftwas found to be mediated mainly by scavenging from newly formed colloidalferric oxohydroxide. The capacity of the iron-rich particles to scavengephosphorus depended on the stoichiometric ratio between dissolved iron andphosphorus built up in the supernatant water during reducing conditions. Thefreshwater system was characterized by high iron to phosphorus ratios in thedissolved phase and thus most of the phosphate was incorporated into thecolloidal iron oxohydroxide during the oxygenation. In contrast, the marinesystems reached lower iron to phosphorus ratios during the anoxic period whichresulted in less efficient phosphate scavenging. Consequently, significantamounts of phosphate remained dissolved in the marine systems after the changeto oxic conditions, possibly increasing the proportion of phosphate recycledto the euphotic zone. Manganese showed a consistent redox-dependent behaviourin all the investigated systems, but interacted neither with phosphate norwith iron.     

Associations between redox‐sensitive trace metals and microbial communities in a Proterozoic ocean analogue

Constraints on Precambrian ocean chemistry are dependent upon sediment geochemistry. However, diagenesis and metamorphism can destroy primary biosignatures, making it difficult to consider biology when interpreting geochemical data. Modern analogues for ancient ecosystems can be useful tools for identifying how sediment geochemistry records an active biosphere. The Middle Island Sinkhole (MIS) in Lake Huron is an analogue for shallow Proterozoic waters due to its low oxygen water chemistry and microbial communities that exhibit diverse metabolic functions at the sediment–water interface. This study uses sediment trace metal contents and microbial abundances in MIS sediments and an oxygenated Lake Huron control site (LH) to infer mechanisms for trace metal burial. The adsorption of trace metals to Mn‐oxyhydroxides is a critical burial pathway for metals in oxic LH sediments, but not for the MIS mat and sediments, consistent with conventional understanding of Mn cycling. Micronutrient trace metals (e.g., Zn) are associated with organic matter regardless of oxygen and sulfide availability. Although U and V are conventionally considered to be organically complexed in suboxic and anoxic conditions, U and organic covary in oxic LH sediments, and Mn‐oxyhydroxide cycling dominates V deposition in the anoxic MIS sediments. Significant correlations between Mo and organic matter across all redox regimes have major implications for our interpretations of Mo isotope systematics in the geologic record. Finally, while microbial groups vary between the sampling locales (e.g., the cyanobacteria in the MIS microbial mat are not present in LH sediments), LH and MIS ultimately have similar relationships between microbial assemblages and metal burial, making it difficult to link trace metal burial to microbial metabolisms. Together, these results indicate that bulk sediment trace metal composition does not capture microbiological processes; more robust trace metal geochemistry such as isotopes and speciation may be critical for understanding the intersections between microbiology and sediment geochemistry.     

Sulfate reduction and thiosulfate transformations in a cyanobacterial mat during a diel oxygen cycle

Abstract Bacterial sulfate reduction and transformations of thiosulfate were studied with radiotracers in a Microcoleus chthonoplastes -dominated microbial mat growing in a hypersaline pond at the Red Sea. The study showed how a diel cycle of oxygen evolution affected respiration by sulfate-reducing bacteria and the metabolism of thiosulfate through oxidative and reductive pathways. Sulfate reduction occurred in both oxic and anoxic layers of the mat and varied diurnally, apparently according to temperature rather than to oxygen. Time course experiments showed that the radiotracer method underestimated sulfate reduction in the oxic zone due to rapid reoxidation of the produced sulfide. Extremely high reduction rates of up to 10 μmol cm⁻³ d⁻¹ were measured just below the euphotic zone. Although thiosulfate was simultaneously oxidized, reduced and disproportionated by bacteria in all layers of the mat, there was a shift from predominant oxidation in the oxic zone to predominant reduction below. Concurrent disproportionation of thiosulfate to sulfate and sulfide occurred in all zones and was an important pathway of the sulfur cycle in the mat.     

RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) Biodegradation in Aquifer Sediments under Manganese-Reducing Conditions

A shallow, RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine)-contaminated aquifer at Naval Submarine Base Bangor has been characterized as predominantly manganese-reducing, anoxic with local pockets of oxic conditions. The potential contribution of microbial RDX degradation to localized decreases observed in aquifer RDX concentrations was assessed in sediment microcosms amended with [U-¹⁴C] RDX. Greater than 85% mineralization of ¹⁴C-RDX to ¹⁴CO₂ was observed in aquifer sediment microcosms under native, manganese-reducing, anoxic conditions. Significant increases in the mineralization of ¹⁴C-RDX to ¹⁴CO₂ were observed in anoxic microcosms under NO₃-amended or Mn(IV)-amended conditions. No evidence of ¹⁴C-RDX biodegradation was observed under oxic conditions. These results indicate that microbial degradation of RDX may contribute to natural attenuation of RDX in manganese-reducing aquifer systems.     

Arsenic(V) Reduction in Relation to Iron(III) Transformation and Molecular Characterization of the Structural and Functional Microbial Community in Sediments of a Basin-Fill Aquifer in Northern Utah

Basin-fill aquifers of the Southwestern United States are associated with elevated concentrations of arsenic (As) in groundwater. Many private domestic wells in the Cache Valley Basin, UT, have As concentrations in excess of the U.S. EPA drinking water limit. Thirteen sediment cores were collected from the center of the valley at the depth of the shallow groundwater and were sectioned into layers based on redoxmorphic features. Three of the layers, two from redox transition zones and one from a depletion zone, were used to establish microcosms. Microcosms were treated with groundwater (GW) or groundwater plus glucose (GW+G) to investigate the extent of As reduction in relation to iron (Fe) transformation and characterize the microbial community structure and function by sequencing 16S rRNA and arsenate dissimilatory reductase (arrA) genes. Under the carbon-limited conditions of the GW treatment, As reduction was independent of Fe reduction, despite the abundance of sequences related to Geobacter and Shewanella, genera that include a variety of dissimilatory iron-reducing bacteria. The addition of glucose, an electron donor and carbon source, caused substantial shifts toward domination of the bacterial community by Clostridium-related organisms, and As reduction was correlated with Fe reduction for the sediments from the redox transition zone. The arrA gene sequencing from microcosms at day 54 of incubation showed the presence of 14 unique phylotypes, none of which were related to any previously described arrA gene sequence, suggesting a unique community of dissimilatory arsenate-respiring bacteria in the Cache Valley Basin.     

Dynamics of Redox Changes of Iron Caused by Light–dark Variations in Littoral Sediment of a Freshwater Lake

Depth profiles of oxygen concentration and the redox status of acid-extractable iron were measured in littoral sediment cores of Lake Constance incubated under a light–dark regimen of 12 h. While oxygen penetrated to 3.4±0.2 mm depth in the dark, photosynthetic oxygen production shifted the oxic–anoxic interface down to 4.0±0.2 mm or 5.9±1.6 mm depth, at low or high light intensity, respectively, and caused a net oxygen efflux into the water column. After a light–dark or dark–light transition, the oxygen concentration at the sediment surface reached a new steady state within about 20 min. The redox state of the bioavailable iron was determined in 1-mm slices of sediment subcores. After a dark period of 12 h, 85% of the acid-extractable iron (10.5 μmol cm⁻³ total) in the uppermost 8 mm was in the reduced state. Within 12 h at low or high light intensity, the proportion of ferrous iron decreased to 82 or 75%, respectively, corresponding to net rates of iron oxidation in the range of 244 and 732 nmol cm⁻³ h⁻¹, respectively. About 55 or 82% of the iron oxidation at low or high light intensity occurred in the respective oxic zone of the sediment; the remaining part was oxidized in the anoxic zone, probably coupled to nitrate reduction. The areal rates of iron oxidation in the respective oxic layer (21 or 123 nmol cm⁻² h⁻¹ at low or high light intensity, respectively) would account for 4 and 23% of the total electron flow to oxygen, respectively. Light changes caused a rapid migration of the oxic–anoxic interface in the sediment, followed by a slow redox reaction of biologically available iron, thus providing temporal niches for aerobic iron oxidizers and anaerobic iron reducers.     

Next‐Generation Sequencing Assessment of Eukaryotic Diversity in Oil Sands Tailings Ponds Sediments and Surface Water

Tailings ponds in the Athabasca oil sands (Canada) contain fluid wastes, generated by the extraction of bitumen from oil sands ores. Although the autochthonous prokaryotic communities have been relatively well characterized, almost nothing is known about microbial eukaryotes living in the anoxic soft sediments of tailings ponds or in the thin oxic layer of water that covers them. We carried out the first next‐generation sequencing study of microbial eukaryotic diversity in oil sands tailings ponds. In metagenomes prepared from tailings sediment and surface water, we detected very low numbers of sequences encoding eukaryotic small subunit ribosomal RNA representing seven major taxonomic groups of protists. We also produced and analysed three amplicon‐based 18S rRNA libraries prepared from sediment samples. These revealed a more diverse set of taxa, 169 different OTUs encompassing up to eleven higher order groups of eukaryotes, according to detailed classification using homology searching and phylogenetic methods. The 10 most abundant OTUs accounted for > 90% of the total of reads, vs. large numbers of rare OTUs (< 1% abundance). Despite the anoxic and hydrocarbon‐enriched nature of the environment, the tailings ponds harbour complex communities of microbial eukaryotes indicating that these organisms should be taken into account when studying the microbiology of the oil sands.     

Microbial metabolism and adaptations in Atribacteria-dominated methane hydrate sediments

Gas hydrates harbour gigatons of natural gas, yet their microbiomes remain understudied. We bioprospected 16S rRNA amplicons, metagenomes, and metaproteomes from methane hydrate-bearing sediments under Hydrate Ridge (offshore Oregon, USA, ODP Site 1244, 2–69 mbsf) for novel microbial metabolic and biosynthetic potential. Atribacteria sequences generally increased in relative sequence abundance with increasing sediment depth. Most Atribacteria ASVs belonged to JS-1-Genus 1 and clustered with other sequences from gas hydrate-bearing sediments. We recovered 21 metagenome-assembled genomic bins spanning three geochemical zones in the sediment core: the sulfate–methane transition zone, the metal (iron/manganese) reduction zone, and the gas hydrate stability zone. We found evidence for bacterial fermentation as a source of acetate for aceticlastic methanogenesis and as a driver of iron reduction in the metal reduction zone. In multiple zones, we identified a Ni-Fe hydrogenase-Na⁺/H⁺ antiporter supercomplex (Hun) in Atribacteria and Firmicutes bins and in other deep subsurface bacteria and cultured hyperthermophiles from the Thermotogae phylum. Atribacteria expressed tripartite ATP-independent transporters downstream from a novel regulator (AtiR). Atribacteria also possessed adaptations to survive extreme conditions (e.g. high salt brines, high pressure and cold temperatures) including the ability to synthesize the osmolyte di-myo-inositol-phosphate as well as expression of K⁺-stimulated pyrophosphatase and capsule proteins.     

Multiple stressors trigger ecological changes in tropical Lake La Tembladera (Ecuador)

We examined the relationship between cladoceran, limnological, geological and biological variables in a recent sediment sequence to assess the cladoceran community response to flood conditions and human impacts during the last century in shallow Lake La Tembladera, on the southern coast of Ecuador. We recovered three sediment cores from central and littoral locations to analyze cladoceran subfossils and plant macrofossils. Redundancy analysis identified three environmental variables controlling the assemblage composition: the presence of non-aquatic plants, As and Be concentrations in the sediment. Before AD 1925, the surface area of the lake was smaller than it is now and the most representative cladocerans were Euryalona at lower lake levels and Leydigiopsis at relatively higher lake levels. After ca. AD 1925, anthropogenic works increased the lake level and the expanse of the littoral zone, favoring phytophilous cladocerans such as Kurzia. Around ca. AD 1990, the disposal of mining tailings contaminated with arsenic increased cladoceran sexual reproduction (total chydorid ephippia) and favored certain taxa. Our study shows the influence of anthropogenic activities on hydrologic regime, lake stage and heavy metal contamination and our results indicate the environmental trajectory of this lake, as it shifted from more natural to more impacted conditions in the last century.     

Spatial variability of sulfate reduction in a shallow aquifer

The distribution and metabolic activity of sulfate-reducing bacteria (SRB) in a shallow, suboxic aquifer were studied. A radioimaging technique was used to visualize and quantify the activity of sulfate reducers in sediments at a centimetre-level scale. The distribution of SRB metabolic activity was heterogeneous with areas showing little activity far outnumbering areas with high activity. Variation in sulfate-reducing activity was not statistically correlated with variation in depth, bacterial numbers, or the following sediment properties: sediment type (sand, peat or silt), grain size, permeability and hydraulic conductivity. Sulfate-reducing bacteria activity did vary significantly with sediment porosity (multivariate analysis, r = 0.48). We hypothesized that the small pore sizes associated with sediments with low porosity restricted the ability of SRB to grow to high numbers as well as their access to nutrients. To further explore the relationship between pore size and microbial metabolic activity, columns with varying pore diameters were constructed. Sulfate-reducing bacteria in the columns with the smallest pore diameters had the lowest rates of metabolism and SRB metabolic rates increased as the pore diameter increased. For the aquifer studied, sediment porosities and pore sizes were the main factor controlling SRB activity.     

Differences in Hyporheic-Zone Microbial Community Structure along a Heavy-Metal Contamination Gradient

The hyporheic zone of a river is nonphotic, has steep chemical and redox gradients, and has a heterotrophic food web based on the consumption of organic carbon entrained from downwelling surface water or from upwelling groundwater. The microbial communities in the hyporheic zone are an important component of these heterotrophic food webs and perform essential functions in lotic ecosystems. Using a suite of methods (denaturing gradient gel electrophoresis, 16S rRNA phylogeny, phospholipid fatty acid analysis, direct microscopic enumeration, and quantitative PCR), we compared the microbial communities inhabiting the hyporheic zone of six different river sites that encompass a wide range of sediment metal loads resulting from large base-metal mining activity in the region. There was no correlation between sediment metal content and the total hyporheic microbial biomass present within each site. However, microbial community structure showed a significant linear relationship with the sediment metal loads. The abundances of four phylogenetic groups (groups I, II, III, and IV) most closely related to α-, β-, and γ-proteobacteria and the cyanobacteria, respectively, were determined. The sediment metal content gradient was positively correlated with group III abundance and negatively correlated with group II abundance. No correlation was apparent with regard to group I or IV abundance. This is the first documentation of a relationship between fluvially deposited heavy-metal contamination and hyporheic microbial community structure. The information presented here may be useful in predicting long-term effects of heavy-metal contamination in streams and provides a basis for further studies of metal effects on hyporheic microbial communities.