Studies of heavy metal sorption by Trenton Channel (Detroit River) sediments

Sediment resuspension plays a dominant physical role in downstream transport of sediment-bound, or in-place pollutants. During resuspension, however, numerous sorption reactions may alter contaminant phase distributions. Previous field resuspension studies on heavily contaminated sediments (Theis et al., 1988, J. Great Lakes Res. 14, 216) showed parallel trends in metal partitioning with pH and time for each of 7 metals (Cd, Co, Cr, Cu, Ni, Pb, Zn), when pH was < 7.5 during resuspension. To improve our ability to interpret follow-up laboratory partitioning experiments using sediments from the field sites, we conducted an evaluation of sediment sample storage as a potential factor leading to field-laboratory partitioning differences. Although metal sorption observed in the laboratory differed substantially from that observed in the field, sample storage effects, reported as holding time and changes in solid phase metal fractionation, gave minimal support for the hypothesis that sample storage caused the differences. It appears, rather, than our in vitro batch equilibrium systems incompletely replicated those attributes of a sediment-water system that are relevant to adsorption and desorption of heavy metals during a resuspension event. Accordingly, we conclude that a general improvement in the understanding of contaminant partitioning would result if future studies would assign greater importance to evaluating the effects of relevant physical phenomena on partitioning (e.g. particle interaction and shear stress), in addition to such widely studied chemical determinants as pH, time, and metal species.     

Errors in pH measurement with colorimetric indicators in low alkalinity waters

A series of laboratory and field studies were conducted to evaluate the reliability of colorimetric pH indicators in dilute, acidic surface waters in New England. Laboratory studies consisted of comparing colorimetric and electrometric pH results in synthetic solutions with a range of ionic strength and buffering capacity. Field studies involved simultaneous colorimetric and electrometric pH determinations in 52 New England lakes with differing water chemistry. Colorimetric indicators deviated from electrometric pH results at low alkalinities and near the end of their operating range. They agreed with electrometric values, within 0.2 unit, at alkalinities of 20 µeq 1^-1 and greater, or near the center of their operating range. Changing ionic strength without changing buffering capacity did not affect results. When indicators were selected that bracketed sample pH, colorimetric determinations of pH in New England lakes agreed well with electrometric determinations.     

Inhibition of Methanosarcina barkeri strain 227 by cadmium

Abstract The effect of cadmium (Cd) on methane formation from methanol and/or H₂–CO₂ by Methanosarcina barkeri was examined in a defined growth medium and in a simplified buffer system containing 50 mM Tes with or without 2 mM dithiothreitol (DTT). No inhibition of methanogenesis by high concentrations of cadmium was observed in growth medium. Similarly, little inhibition of methanogenesis by whole cells in the Tes buffer system was observed in the presence of 430 μM Cd or 370 μM mercury (Hg) with 2 mM DTT. When the concentration of DTT was reduced to 0.4 mM, almost complete inhibition of methanogenesis from H₂–CO₂ and methanol by 600 μM Cd was observed. In the absence of DTT, 150 μM Cd inhibited methanogenesis from H₂–CO₂ completely and from methanol by 97%. Methanogenesis from H₂–CO₂ was more sensitive to Cd than that from methanol.     

Ecophysiological adaptations of anaerobic bacteria to low pH: analysis of anaerobic digestion in acidic bog sediments.

The dynamics of anaerobic digestion were examined in the low-pH sediments of Crystal Bog in Wisconsin. The sediments (pH 4.9) contained 71% organic matter and the following concentrations of dissolved gases (micromoles per liter): CO2, 1,140; CH4, 490; and H2, 0.01. The rate of methane production was 6.2 mumol/liter of sediment per h, which is slower than eutrophic, neutral sediments. Microbial metabolic processes displayed the following pH optima: hydrolysis reactions, between 4.2 and 5.6; aceticlastic methanogenesis, 5.2; and hydrogen-consuming reactions, 5.6. The turnover rate constants for key intermediary metabolites were (h-1): glucose, 1.10; lactate, 0.277; acetate, 0.118; and ethanol, 0.089. The populations of anaerobes were low, with hydrolytic groups (10(6)/ml) several orders of magnitude higher than methanogens (10(2)/ml). The addition of carbon electron donors to the sediment resulted in the accumulation of hydrogen, whereas the addition of hydrogen resulted in the accumulation of fatty acids and the inhibition of hydrogen-producing acetogenic reactions. Strains of Lactobacillus, Clostridium, and Sarcina ventriculi were isolated from the bog, and their physiological attributes were characterized in relation to hydrolytic process functions in the sediments. The present studies provide evidence that the pH present in the bog sediments alter anaerobic digestion processes so that total biocatalytic activity is lower but the general carbon and electron flow pathways are similar to those of neutral anoxic sediments.     

Inhibition of Acetoclastic Methanogenesis in Crude Oil- and Creosote-Contaminated Groundwater

Results from a series of studies of methanogenic processes in crude oil- and creosote-contaminated aquifers indicate that acetoclastic methanogenesis is inhibited near non-aqueous sources. At a crude oil-contaminated site, numbers of acetoclastic methanogens found close to crude oil were one hundred times fewer than those of hydrogen- and formate-utilizing methanogens. In laboratory toxicity assays, crude oil collected from the site inhibited methane production from acetate but not from formate or hydrogen. Toxicity assays with aqueous creosote extract completely inhibited acetate utilization over the range of tested dilutions but only mildly affected formate and hydrogen utilization. The combined results from the laboratory and field studies suggest that in methanogenic contaminated aquifers, inhibition of acetoclastic methanogenesis may lead to a buildup of acetate relative to dissolved organic carbon.     

Inhibition of Acetoclastic Methanogenesis in Crude Oil- and Creosote-Contaminated Groundwater

Results from a series of studies of methanogenic processes in crude oil- and creosote-contaminated aquifers indicate that acetoclastic methanogenesis is inhibited near non-aqueous sources. At a crude oil-contaminated site, numbers of acetoclastic methanogens found close to crude oil were one hundred times fewer than those of hydrogen- and formate-utilizing methanogens. In laboratory toxicity assays, crude oil collected from the site inhibited methane production from acetate but not from formate or hydrogen. Toxicity assays with aqueous creosote extract completely inhibited acetate utilization over the range of tested dilutions but only mildly affected formate and hydrogen utilization. The combined results from the laboratory and field studies suggest that in methanogenic contaminated aquifers, inhibition of acetoclastic methanogenesis may lead to a buildup of acetate relative to dissolved organic carbon.     

Inhibition of Acetoclastic Methanogenesis in Crude Oil- and Creosote-Contaminated Groundwater

Results from a series of studies of methanogenic processes in crude oil- and creosote-contaminated aquifers indicate that acetoclastic methanogenesis is inhibited near non-aqueous sources. At a crude oil-contaminated site, numbers of acetoclastic methanogens found close to crude oil were one hundred times fewer than those of hydrogen- and formate-utilizing methanogens. In laboratory toxicity assays, crude oil collected from the site inhibited methane production from acetate but not from formate or hydrogen. Toxicity assays with aqueous creosote extract completely inhibited acetate utilization over the range of tested dilutions but only mildly affected formate and hydrogen utilization. The combined results from the laboratory and field studies suggest that in methanogenic contaminated aquifers, inhibition of acetoclastic methanogenesis may lead to a buildup of acetate relative to dissolved organic carbon.     

Inhibition of Acetoclastic Methanogenesis in Crude Oil- and Creosote-Contaminated Groundwater

Results from a series of studies of methanogenic processes in crude oil- and creosote-contaminated aquifers indicate that acetoclastic methanogenesis is inhibited near non-aqueous sources. At a crude oil-contaminated site, numbers of acetoclastic methanogens found close to crude oil were one hundred times fewer than those of hydrogen- and formate-utilizing methanogens. In laboratory toxicity assays, crude oil collected from the site inhibited methane production from acetate but not from formate or hydrogen. Toxicity assays with aqueous creosote extract completely inhibited acetate utilization over the range of tested dilutions but only mildly affected formate and hydrogen utilization. The combined results from the laboratory and field studies suggest that in methanogenic contaminated aquifers, inhibition of acetoclastic methanogenesis may lead to a buildup of acetate relative to dissolved organic carbon.     

Influence of Algal Community Structure on Denitrification Rates in Periphyton Cultivated on Artificial Substrata

We conducted a field survey of periphyton cultivated on benthic mesh installations in freshwater aquatic systems, including two constructed wetlands and a pond, and also studied periphyton grown on a benthic mesh in laboratory mesocosms. The objectives of this study were to (1) determine if periphyton cultivated on benthic mesh denitrifies at higher rates than the underlying sediments and (2) determine if denitrification rates within periphyton vary with characteristics such as algal and bacterial community structure and biomass. We measured denitrification potential rates of field and laboratory periphyton by the acetylene inhibition method. We characterized algal community composition by algal identification and bacterial community composition by terminal restriction fragment length polymorphisms. Periphyton collected on benthic mesh from our field sites denitrified at significantly higher rates than the underlying sediments, regardless of sampling site or season. Results from both our field survey and laboratory studies indicated a significant, positive correlation between diatom presence and denitrification rate. In our laboratory studies, we found that periphyton with the highest diatom abundance showed the highest denitrification rates as well as a distinct bacterial community composition. These results suggest a synergistic relationship between diatoms and denitrifying bacteria that warrants further study.     

Environmental factors influencing methanogenesis in a shallow anoxic aquifer: a field and laboratory study

Summary The environmental factors influencing methanogenesis in a shallow anoxic aquifer were probed in a combined field and laboratory study. Field data collected over a year revealed that in situ rates of methane production were depressed in winter and elevated in summer. Over the same period, ground water pH values ranged from 6.0 to 7.8 while temperatures varied from 7–22°C. In situ methanogenesis was severely inhibited at temperatures < 13°C or by pH values < 7. The influence of these factors on microbial methane formation from both endogenous and exogenous substrates were tested in aquifer slurries adjusted to pH 5–9 and incubated at temperatures ranging from 5–45°C. Temperature optima for methane production from endogenous substrates varied as a function of pH, but the pH optimum was 8 at all temperatures. Optimal conditions for acetoclastic methanogenesis were found at pH 8 and 35°C. An analysis of variance revealed that pH, temperature, and a pH-temperature interaction are all significant variables influencing aquifer methanogenesis. In addition transient sulfate accumulations were also found to limit methane production in some areas. A comparison of field and laboratory methane production patterns suggest that pH, temperature, and sulfate accumultations are important, but not the only environmental variables influencing the mineralization of organic matter in shallow aquifers.     

Rates of methanogenesis and methanotrophy in deep-sea sediments

We use the carbon isotopic composition (δ¹³C) of the dissolved inorganic carbon (DIC) of pore fluids from Leg 175 of the Ocean Drilling Program (ODP) along the West African Margin to quantify rates of methane production (methanogenesis) and destruction via oxidation (methanotrophy) in deep‐sea sediments. Results from a model of diffusion and reaction in the sedimentary column show that anaerobic methane oxidation (AOM) occurs in the transition zone between the presence of sulfate and methane, and methanogenesis occurs below these depths in a narrow confined zone that ends at about 250 m below the sea‐sediments surface in all sediment profiles. Our model suggests that the rates of methanogenesis and AOM range between 6 · 10⁻⁸ and 1 · 10⁻¹⁰ mol cm⁻³ year⁻¹ at all sites, with higher rates at sites where sulfate is depleted in shallower depths. Our AOM rates agree with those based solely on sulfate concentration profiles, but are much lower than those calculated from experiments of sulfate reduction through AOM done under laboratory conditions. At sites where the total organic carbon (TOC) is less than 5% of the total sediment, we calculate that AOM is the main pathway for sulfate reduction. We calculate that higher rates of AOM are associated with increased recrystallization rates of carbonate minerals. We do not find a correlation between methanogenesis rates and the content of carbonate or TOC in the sediments, porosity, sedimentation rate, or the C:N ratio, and the cause of lack of methanogenesis below a certain depth is not clear. There does, however, appear to be an association between the rates of methanogenesis and the location of the site in the upwelling system, suggesting that some variable such as the type of the organic matter or the nature of the microbiological community may be important.     

Methanogenesis under acidic pH conditions in a semi-continuous reactor system

Operating an anaerobic digester at low pH could offer several advantages over operation at neutral pH. Most wastewater streams targeted for anaerobic digestion are inherently acidic, requiring alkalinity supplementation (at added expense) to buffer the pH at neutral. Additionally, previously published work completed by the authors using batch systems suggested that lowering the system pH could increase methane production by as much as 30%. The goal of this research was to evaluate the feasibility of sustaining methanogenesis at low pH in a semi-continuous laboratory-scale fermentor. Significant methane production was achieved in a system ranging in pH from approximately 4.0-5.3. Results show that, if the consortium is allowed to sufficiently acclimate to acidic conditions, methanogenesis can be maintained under acidic pH conditions, resulting in overall chemical oxygen demand (COD) reduction and methane production comparable to that achieved in a neutral pH system.     

Epilimnetic sulfate reduction and its relationship to lake acidification

Sulfate reduction occurred from 0–3 cm below the surface of the epilimnetic sediments of three northwestern Ontario lakes, including L.223, which has been experimentally acidified by additions of sulfuric acid. Shallow water sites were conducive to SO₄ ^2– reduction because decomposition in these predominantly sandy sediments caused oxygen concentrations to decrease rapidly within mm below the interface. The occurrence of methanogenesis just below the depth of minimum SO₄ ^2- concentration demonstrated that availability of organic carbon was not a limiting factor for sulphate reduction.Laboratory studies showed that SO₄ ^2- reduction rates in mixed sediments were lower at pH 4 than at pH 6. However, sulfate gradients in sediments indicated that there was no effect of acidification on sulfate reduction in situ. This was probably because microbial H⁺ consumption in the epilimnetic sediments maintained steep pH gradients below the sediment-water interface. The pH increased from = 5.0 to 6.5 or higher by a depth of 3.0 cm into the sediments.     

Methane in maritime Antarctic freshwater lakes

Summary Methane was found to occur in all freshwater lakes, irrespective of trophic status, sampled during this preliminary investigation at Signy Island, South Orkney Islands, Antarctica. Methane accumulated in the water column of these lakes during the winter period when ice cover prevented wind-induced mixing. Maritime Antarctic lakes are usually subject to wind-induced complete mixing during the summer open-water period but two major exceptions to the rule were found during this study. Methanogenesis occurred in both littoral and profundal regions of oligotrophic Sombre Lake. The presence of a substantial algal mat stabilized the Eh status of underlying sediments at the littoral site. Methane production was confined to the sediments in both littoral and profundal sediments during the study period (December–March) but in winter probably migrated to the sediment surface at the profundal site. All Signy Island lakes sampled were sulphate-poor and addition of sulphate markedly inhibited methanogenesis. Radio-isotope studies indicated that the H₂/CO₂ pathway was probably the predominant route for methanogenesis in these sediments through the acetate pathway appeared equally important at the sediment surface. In the absence of sulphate, sulphate reducers probably acted as net hydrogen donors to the methanogens. The process rate was permanently limited by the consistent low temperature (annual range 1–3°C). Rates increased with increasing temperature over the range 4–32°C, but no evidence was found to suggest cold sensitivity or psychrophily. The optimum temperature for methanogenesis was in excess of 30°C, temperatures never experienced at Signy Island. Rates of methanogenesis during the study period (Dec–Mar) ranged from 0.29 to 0.45 mg of carbon m^-2 and on an annual basis methanogenesis was calculated equivalent to 13% of the organic carbon deposition rate.     

Effect of Salinity on Mercury-Methylating Activity of Sulfate-Reducing Bacteria in Estuarine Sediments

The biomethylation of mercury was measured in anoxic estuarine sediments that ranged in salinity from 0.03 to 2.4% with or without added molybdate, an inhibitor of sulfate reducers. Mercury methylation was inhibited by molybdate by more than 95%, regardless of sediment salinity. In the absence of inhibitor, high-salinity sediments methylated mercury at only 40% of the level observed in low-salinity sediments. In response to molybdate inhibition of sulfate reducers, methanogenesis increased up to 258% in high-salinity sediments but only up to 25% in low-salinity sediments. In contrast to an earlier low-salinity isolate, a Desulfovibrio desulfuricans strain from high-salinity sediment required 0.5 M sodium for optimal growth and mercury methylation activity. The formation of negatively charged mercuric chloride complexes at high salinity did not noticeably interfere with the methylation process. Results of these studies demonstrate that sulfate reducers are responsible for mercury methylation in anoxic estuarine sediments, regardless of the prevailing salinity.     

Effect of sulfate on carbon and electron flow during microbial methanogenesis in freshwater sediments.

The effect of sulfate on methane production in Lake Mendota sediments was investigated to clarify the mechanism of sulfate inhibition of methanogenesis. Methanogenesis was shown to be inhibited by the addition of as little as 0.2 mM sulfate. Sulfate inhibition was reversed by the addition of either H2 or acetate. Methane evolved when inhibition was reversed by H2 additions was derived from 14CO2. Conversely, when acetate was added to overcome sulfate inhibition, the evolved methane was derived from [2-14C]acetate. A competition for available H2 and acetate was proposed as the mechanism by which sulfate inhibited methanogenesis. Acetate was shown to be metabolized even in the absence of methanogenic activity. In the presence of sulfate, the methyl position of acetate was converted to CO2. The addition of sulfate to sediments did not result in the accumulation of significant amounts of sulfide in the pore water. Sulfate additions did not inhibit methanogenesis unless greater than 100 mug of free sulfide per ml was present in the pore water. These results indicate that carbon and electron flow are altered when sulfate is added to sediments. Sulfate-reducing organisms appear to assume the role of methanogenic bacteria in sulfate-containing sediments by utilizing methanogenic precursors.     

Relationship of formate to growth and methanogenesis by Methanococcus thermolithotrophicus

Methanococcus thermolithotrophicus is a methanogenic archaebacterium that can use either H2 or formate as its source of electrons for reduction of CO2 to methane. Growth and suspended-whole-cell experiments show that H2 plus CO2 methanogenesis was constitutive, while formate methanogenesis required adaptation time; selenium was necessary for formate utilization. Cells grown on formate had 20 to 100 times higher methanogenesis rates on formate than cells grown on H2-CO2 and transferred into formate medium. Enzyme assays with crude extracts and with F420 or methyl viologen as the electron acceptor revealed that hydrogenase was constitutive, while formate dehydrogenase was regulated. Cells grown on formate had 10 to 70 times higher formate dehydrogenase activity than cells grown on H2-CO2 with Se present in the medium; when no Se was added to H2-CO2 cultures, even lower activities were observed. Adaptation to and growth on formate were pH dependent, with an optimal pH for both about one pH unit above that optimal for H2-CO2 (pH 5.8 to 6.5). When cells were grown on H2-CO2 in the presence of formate, formate (greater than or equal to 50 mM) inhibited both growth and methanogenesis at pH 5.8 to 6.2, but not at pH greater than 6.6. Both acetate and propionate produced similar inhibition. Formate inhibition was also observed in Methanospirillum hungatei.     

Relationship of formate to growth and methanogenesis by Methanococcus thermolithotrophicus.

Methanococcus thermolithotrophicus is a methanogenic archaebacterium that can use either H2 or formate as its source of electrons for reduction of CO2 to methane. Growth and suspended-whole-cell experiments show that H2 plus CO2 methanogenesis was constitutive, while formate methanogenesis required adaptation time; selenium was necessary for formate utilization. Cells grown on formate had 20 to 100 times higher methanogenesis rates on formate than cells grown on H2-CO2 and transferred into formate medium. Enzyme assays with crude extracts and with F420 or methyl viologen as the electron acceptor revealed that hydrogenase was constitutive, while formate dehydrogenase was regulated. Cells grown on formate had 10 to 70 times higher formate dehydrogenase activity than cells grown on H2-CO2 with Se present in the medium; when no Se was added to H2-CO2 cultures, even lower activities were observed. Adaptation to and growth on formate were pH dependent, with an optimal pH for both about one pH unit above that optimal for H2-CO2 (pH 5.8 to 6.5). When cells were grown on H2-CO2 in the presence of formate, formate (greater than or equal to 50 mM) inhibited both growth and methanogenesis at pH 5.8 to 6.2, but not at pH greater than 6.6. Both acetate and propionate produced similar inhibition. Formate inhibition was also observed in Methanospirillum hungatei.     

Spiked Sediment Toxicity Testing of Hydrophobic Organic Chemicals: Bioavailability,Technical Considerations,and Applications

Many evaluations estimating safe levels of hydrophobic organic chemicals in sediments do not account for confounding factors such as physical habitat quality or covariance among chemicals. Controlled experiments demonstrating cause and effect can be conducted with spiked sediment toxicity tests, but application of this methodology has been limited in part by concerns about chemical bioavailability and challenges in achieving target concentrations. Relevant literature was reviewed to assess the utility of standardizing sediment equilibration times; hydrophobicity, complex sediment characteristics, and temperature were identified as potentially equally important factors. Disequilibrium appears likely following limited equilibration time but should yield conservative toxicity test results relative to aged field sediments. Nominal and measured concentrations in over 20 published studies were compared to assess spiked chemical recovery (i.e., measured concentration/nominal concentration). Recovery varied substantially among studies and was not readily predictable based on spiking or extraction method, chemical properties, or measured sediment characteristics, although unmeasured differences between sediments appeared to be important. Factors affecting specific studies included chemical adsorption to glassware, biodegradation, and volatilization. Pre- and post-toxicity test analyses are recommended to confirm exposure concentrations. Studies with 2,3,7,8-tetrachloro-dibenzo-p-dioxin (2,3,7,8-TCDD) and hexachlorobenzene (HCB) exemplify the utility of verifying results of field studies using spiked sediment tests. Sediments spiked with these chemicals at concentrations greatly exceeding those in associated field studies caused no adverse effects in test organisms, demonstrating that other chemicals co-occurring in test sediment samples caused toxicity initially attributed to 2,3,7,8-TCDD and HCB in the field studies. Another key application of spiked sediment tests has been the investigation of TOC as the primary factor affecting bioavailability of hydrophobic organic chemicals. A review of LC50s for nine chemicals reported in 12 studies shows that comparable LC50s derived in different sediments generally agree within a factor of five when concentrations are normalized to a constant TOC. Additionally, use of spiked sediment toxicity testing to investigate toxicological interactions among chemicals provides a promising approach to improving the ability to predict sediment toxicity in the field.     

Thermophilic methanogenesis in a hot-spring algal-bacterial mat (71 to 30 degrees C).

Algal-bacterial mats which grow in the effluent channels of alkaline hot springs provided an environment suitable for studying natural thermophilic methane producing bacteria. Methane was rapidly produced in cores taken from the meat and appeared to be an end product of decomposition of the algal-bacterial organic matter. Formaldehyde prevented production of methane. Initial methanogenic rate was lower and methanogenesis became exponential when samples were permitted to cool before laboratory incubation. Methanogenesis occurred and methanogenic bacteria were present over a range of 68 to 30 degrees C, with optimum methanogenesis near 45 degrees C. The temperature distribution of methanogenesis in the mat is discussed relative to published results on standing crop, primary production, and decomposition in the thermal gradient. The depth distribution of methanogenesis was similar to that of freshwater sediments, with a zone of intense methanogenesis near the mat surface. Methanogenesis in deeper mat layers was very low or undetectable despite large numbers of viable methanogenic bacteria and could not be stimulated by addition of anoxic source water, sulfide, or a macronutrient solution.