Biogeochemical changes at the sediment–water interface during redox transitions in an acidic reservoir: exchange of protons,acidity and electron donors and acceptors

Redox transitions induced by seasonal changes in water column O₂ concentration can have important effects on solutes exchange across the sediment–water interface in systems polluted with acid mine drainage (AMD), thus influencing natural attenuation and bioremediation processes. The effect of such transitions was studied in a mesocosm experiment with water and sediment cores from an acidic reservoir (El Sancho, SW Spain). Rates of aerobic organic matter mineralization and oxidation of reduced inorganic compounds increased under oxic conditions (OX). Anaerobic process, like Fe(III) and sulfate reduction, also increased due to higher O₂ availability and penetration depth in the sediment, resulting in higher regeneration rates of their corresponding anaerobic e^? acceptors. The contribution of the different processes to oxygen uptake changed considerably over time. pH decreased due to the precipitation of schwertmannite and the release of H⁺ from the sediment, favouring the dissolution of Al-hydroxides and hydroxysulfates at the sediment surface. The increase in dissolved Al was the main contributor to water column acidity during OX. Changes in organic matter degradation rates and co-precipitation and dissolution of dissolved organic carbon and nitrogen with redox-sensitive Fe(III) compounds affected considerably C and N cycling at the sediment–water interface during redox transitions. The release of NO₂^? and NO₃^? during the hypoxic period could be attributed to ammonium oxidation coupled to ferric iron reduction (Feammox). Considering the multiple effects of redox transitions at the sediment–water interface is critical for the successful outcome of natural attenuation and bioremediation of AMD impacted aquatic environments.     

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     

Initial Test of the Benchmark Chemical Approach for Predicting Microbial Transformation Rates in Aquatic Environments

Using 2,4-dichlorophenoxyacetic acid methyl ester (2,4-DME) as a benchmark chemical, we determined relative pseudo-first-order rate coefficients for the butoxyethyl ester of 2,4-dichlorophenoxyacetic acid (2,4-DBE), methyl parathion, and methyl-3-chlorobenzoate in a diversity of microbial samples, including water, sediment, biofilm, and floating microbial mats collected from a laboratory mesocosm as well as from streams, lakes, and wetlands in Georgia and Florida. The decreasing order of reactivity for relative microbial transformation rates was 2,4-DBE > 2,4-DME > methyl-3-chlorobenzoate > methyl parathion. Half-lives of the chemicals varied about 60-fold depending on the chemical and microbial sample. Relative rate coefficients, however, typically varied only about threefold for field-collected samples. Relative rate coefficients determined with samples from a laboratory mesocosm were consistently low compared with the field sample data. Overall, the data indicated that microbial transformation rates of a chemical can be satisfactorily inferred for a wide variety of microbial habitats—such as water, biofilm, or a sediment—on the basis of its transformation rate relative to that of an appropriate benchmark chemical by using a single type of microbial sample.     

Contribution of quinone-reducing microorganisms to the anaerobic biodegradation of organic compounds under different redox conditions

The capacity of two anaerobic consortia to oxidize different organic compounds, including acetate, propionate, lactate, phenol and p-cresol, in the presence of nitrate, sulfate and the humic model compound, anthraquinone-2,6-disulfonate (AQDS) as terminal electron acceptors, was evaluated. Denitrification showed the highest respiratory rates in both consortia studied and occurred exclusively during the first hours of incubation for most organic substrates degraded. Reduction of AQDS and sulfate generally started after complete denitrification, or even occurred at the same time during the biodegradation of p-cresol, in anaerobic sludge incubations; whereas methanogenesis did not significantly occur during the reduction of nitrate, sulfate, and AQDS. AQDS reduction was the preferred respiratory pathway over sulfate reduction and methanogenesis during the anaerobic oxidation of most organic substrates by the anaerobic sludge studied. In contrast, sulfate reduction out-competed AQDS reduction during incubations performed with anaerobic wetland sediment, which did not achieve any methanogenic activity. Propionate was a poor electron donor to achieve AQDS reduction; however, denitrifying and sulfate-reducing activities carried out by both consortia promoted the reduction of AQDS via acetate accumulated from propionate oxidation. Our results suggest that microbial reduction of humic substances (HS) may play an important role during the anaerobic oxidation of organic pollutants in anaerobic environments despite the presence of alternative electron acceptors, such as sulfate and nitrate. Methane inhibition, imposed by the inclusion of AQDS as terminal electron acceptor, suggests that microbial reduction of HS may also have important implications on the global climate preservation, considering the green-house effects of methane.     

Microbial Populations of the River-Recharged Fredericton Aquifer

The City of Fredericton, New Brunswick, Canada obtains its water from the Saint John River-recharged Fredericton aquifer. Water quality improves as it flows from the river into the aquifer in a process called bank filtration. Microorganisms contribute to water quality improvements during bank filtration by removing organic carbon. In the Fredericton aquifer, microbially catalyzed reductive dissolution of manganese oxide minerals negatively affects water quality. Aerobic and anaerobic microorganisms were cultured from Fredericton aquifer production well water, from associated bedrock groundwater, and from Saint John River sediment core and sediment groundwater samples. Aerobes were the largest culturable groups in all samples. The ratio of aerobes to other microbial groups, i.e., those reducing nitrate, Mn^{4 +}, Fe^{3 +}, or sulfate, did not vary significantly along the water flow path from the river to the aquifer. Analysis of microbial community composition along the flow path indicated an essentially identical community except in the immediate vicinity of the aquifer. This is in agreement with the absence of macroscale redox zones in the sediment below the Saint John River as determined by groundwater geochemical data. Bacteria isolated from groundwater samples, identified by 16S ribosomal RNA gene sequencing, were α -, β -, γ -, and δ -Proteobacteria, Actinobacteria, and Firmicutes. In contrast to groundwater samples, the majority of bacteria isolated from sediments were γ -Proteobacteria. Numbers of manganese-reducing bacteria, including Aeromonas spp., were small, however Mn^{4 +} reduction ability was widespread in bacteria enriched and isolated with other electron acceptors. The diverse Fredericton aquifer microbial community likely uses manganese oxide minerals as a sink for electrons derived from organic carbon oxidation.     

Effects of benthic filamentous algae on the sediment–water interface in an acidic mining lake

Natural alkalinity generation by microbial sulphate reduction in acidic lakes is usually inhibited by the low pH and a low primary production which results in a lack of suitable organic carbon sources. In some acidic mining lakes mass developments of filamentous benthic algae occur. The effects of this periphyton layer on the biogeochemistry of the sediment–water interface were investigated by in situ microsensor measurements and laboratory incubations in Mining Lake Grünewalder Lauch (Germany). Microsensor measurements showed that the oxic–anoxic boundary was located in the periphyton layer and was moving up and down depending on light triggered photosynthesis. The sediment itself was permanently anoxic. The diurnal redox shift and the maintenance of neutral conditions in the periphyton layer lead to an effective precipitation of iron and phosphorus. Under the periphyton layer very high sulphate reduction rates up to 265 nmol cm⁻³ d⁻¹ were measured in the sediment. These are the highest rates reported for mining lakes so far. The microbial activity was high enough to keep the pH in the surface sediment neutral and contributed to natural alkalinity production. Handling editor: L. Naselli-Flores     

Pore water nutrient profiles and dynamics in soft bottoms of the northern Baltic Sea

Chemical profiles of nutrients at the sediment–water interface were measured in the northern Baltic Sea. A whole core squeezer technique capable of mm-scale resolution was used to obtain the vertical profiles of NO₃ ^–, NO₂ ^–, o-P, NH₄ ⁺ and Si in the soft bottom sediments. The profiles were compared with nutrient flux and denitrification measurements. In the Gulf of Finland, the profiles revealed a marked chemical zonation in NO₃ ^– and NO₂ ^– distribution indicating strong potential of nitrification just under the sediment surface followed by a layer of denitrification down to a depth of 30 mm. Below the depth of 20 mm NO₃ ^– was usually absent, whereas other nutrients were increasing steadily in concentration. A distinct minimum of NO₃ ^– was observed at the sediment–water interface, suggesting NO₃ ^– uptake by a microbial biofilm and/or active denitrification at the suboxic microniches usually present in organic-rich sediments. At the deep stations in the Baltic Proper, the NO₃ ^– concentration in pore water, as well as denitrification, were very low. The concentrations of NH₄ ⁺, o-P and Si were usually increasing steadily with depth.     

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.     

Metabolically active microbial communities in marine sediment under high-CO2 and low-pH extremes

Sediment-hosting hydrothermal systems in the Okinawa Trough maintain a large amount of liquid, supercritical and hydrate phases of CO₂ in the seabed. The emission of CO₂ may critically impact the geochemical, geophysical and ecological characteristics of the deep-sea sedimentary environment. So far it remains unclear whether microbial communities that have been detected in such high-CO₂ and low-pH habitats are metabolically active, and if so, what the biogeochemical and ecological consequences for the environment are. In this study, RNA-based molecular approaches and radioactive tracer-based respiration rate assays were combined to study the density, diversity and metabolic activity of microbial communities in CO₂-seep sediment at the Yonaguni Knoll IV hydrothermal field of the southern Okinawa Trough. In general, the number of microbes decreased sharply with increasing sediment depth and CO₂ concentration. Phylogenetic analyses of community structure using reverse-transcribed 16S ribosomal RNA showed that the active microbial community became less diverse with increasing sediment depth and CO₂ concentration, indicating that microbial activity and community structure are sensitive to CO₂ venting. Analyses of RNA-based pyrosequences and catalyzed reporter deposition-fluorescence in situ hybridization data revealed that members of the SEEP-SRB2 group within the Deltaproteobacteria and anaerobic methanotrophic archaea (ANME-2a and -2c) were confined to the top seafloor, and active archaea were not detected in deeper sediments (13–30 cm in depth) characterized by high CO₂. Measurement of the potential sulfate reduction rate at pH conditions of 3–9 with and without methane in the headspace indicated that acidophilic sulfate reduction possibly occurs in the presence of methane, even at very low pH of 3. These results suggest that some members of the anaerobic methanotrophs and sulfate reducers can adapt to the CO₂-seep sedimentary environment; however, CO₂ and pH in the deep-sea sediment were found to severely impact the activity and structure of the microbial community.     

The Effect of Tensile Stress on the Conformational Free Energy Landscape of Disulfide Bonds

Disulfide bridges are no longer considered to merely stabilize protein structure, but are increasingly recognized to play a functional role in many regulatory biomolecular processes. Recent studies have uncovered that the redox activity of native disulfides depends on their C–C–S–S dihedrals, and . Moreover, the interplay of chemical reactivity and mechanical stress of disulfide switches has been recently elucidated using force–clamp spectroscopy and computer simulation. The and angles have been found to change from conformations that are open to nucleophilic attack to sterically hindered, so–called closed states upon exerting tensile stress. In view of the growing evidence of the importance of C–C–S–S dihedrals in tuning the reactivity of disulfides, here we present a systematic study of the conformational diversity of disulfides as a function of tensile stress. With the help of force-clamp metadynamics simulations, we show that tensile stress brings about a large stabilization of the closed conformers, thereby giving rise to drastic changes in the conformational free energy landscape of disulfides. Statistical analysis shows that native TDi, DO and interchain Ig protein disulfides prefer open conformations, whereas the intrachain disulfide bridges in Ig proteins favor closed conformations. Correlating mechanical stress with the distance between the two –carbons of the disulfide moiety reveals that the strain of intrachain Ig protein disulfides corresponds to a mechanical activation of about 100 pN. Such mechanical activation leads to a severalfold increase of the rate of the elementary redox reaction step. All these findings constitute a step forward towards achieving a full understanding of functional disulfides.     

Microbial Diversity in Water and Sediment of Lake Chaka, an Athalassohaline Lake in Northwestern China

We employed culture-dependent and -independent techniques to study microbial diversity in Lake Chaka, a unique hypersaline lake (32.5% salinity) in northwest China. It is situated at 3,214 m above sea level in a dry climate. The average water depth is 2 to 3 cm. Halophilic isolates were obtained from the lake water, and halotolerant isolates were obtained from the shallow sediment. The isolates exhibited resistance to UV and gamma radiation. Microbial abundance in the sediments ranged from 10⁸ cells/g at the water-sediment interface to 10⁷ cells/g at a sediment depth of 42 cm. A major change in the bacterial community composition was observed across the interface. In the lake water, clone sequences affiliated with the Bacteroidetes were the most abundant, whereas in the sediments, sequences related to low G+C gram-positive bacteria were predominant. A similar change was also present in the archaeal community. While all archaeal clone sequences in the lake water belonged to the Halobacteriales, the majority of the sequences in the sediments were related to those previously obtained from methanogenic soils and sediments. The observed changes in the microbial community structure across the water-sediment interface were correlated with a decrease in salinity from the lake water (32.5%) to the sediments (approximately 4%). Across the interface, the redox state also changed from oxic to anoxic and may also have contributed to the observed shift in the microbial community.     

Microbial Community Structure and Arsenic Biogeochemistry in an Acid Vapor-Formed Spring in Tengchong Geothermal Area,China

Arsenic biogeochemistry has been studied extensively in acid sulfate-chloride hot springs, but not in acid sulfate hot springs with low chloride. In this study, Zhenzhuquan in Tengchong geothermal area, a representative acid sulfate hot spring with low chloride, was chosen to study arsenic geochemistry and microbial community structure using Illumina MiSeq sequencing. Over 0.3 million 16S rRNA sequence reads were obtained from 6-paired parallel water and sediment samples along its outflow channel. Arsenic oxidation occurred in the Zhenxhuquan pool, with distinctly high ratios of arsenate to total dissolved arsenic (0.73–0.86). Coupled with iron and sulfur oxidation along the outflow channel, arsenic accumulated in downstream sediments with concentrations up to 16.44 g/kg and appeared to significantly constrain their microbial community diversity. These oxidations might be correlated with the appearance of some putative functional microbial populations, such as Aquificae and Pseudomonas (arsenic oxidation), Sulfolobus (sulfur and iron oxidation), Metallosphaera and Acidicaldus (iron oxidation). Temperature, total organic carbon and dissolved oxygen significantly shaped the microbial community structure of upstream and downstream samples. In the upstream outflow channel region, most microbial populations were microaerophilic/anaerobic thermophiles and hyperthermophiles, such as Sulfolobus, Nocardia, Fervidicoccus, Delftia, and Ralstonia. In the downstream region, aerobic heterotrophic mesophiles and thermophiles were identified, including Ktedonobacteria, Acidicaldus, Chthonomonas and Sphingobacteria. A total of 72.41–95.91% unassigned-genus sequences were derived from the downstream high arsenic sediments 16S rRNA clone libraries. This study could enable us to achieve an integrated understanding on arsenic biogeochemistry in acid hot springs.     

硫酸盐对淡水养殖池塘表层底泥微生物的影响

硫酸盐引起的生态学效应已得到了越来越多的关注,但目前关于硫酸盐对养殖池塘底泥微生物的影响还知之甚少。【目的】探究不同浓度硫酸盐对养殖池塘底泥微生物的影响规律及可能的机制。【方法】本研究利用采集自养殖池塘的底泥和表层水构建了试验系统,研究了加入约0 mg/L （对照组）、30 mg/L （T1处理组）、150 mg/L （T2处理组）、500 mg/L （T3处理组） Na₂SO₄后表层底泥微生物的丰度、多样性、组成和共生网络的变化规律,并分析了环境影响因素。【结果】孵育第30天前,各实验组底泥微生物变化不大;但到第50天时,T2和T3处理组微生物丰度和多样性相比对照组均明显下降。相比其他实验组,T1处理组酸杆菌门（Acidobacteriota）、拟杆菌门（Bacteroidota）相对丰度出现显著升高（P<0.05）,T3处理组变形菌门（Proteobacteria）和放线菌门（Actinobacteriota）相对丰度出现显著升高（P<0.05）。与对照组相比,T1处理组增加了较多差异类群（62种）,而T3处理组差异类群大量减少（45种）。共生网络图分析显示硫酸盐浓度的增加引起了底泥微生物网络复杂性的增加,说明微生物群落可能通过自身的调节来响应硫酸盐引起的环境改变。冗余分析（redundant analysis,RDA）和相关性分析揭示底泥总有机碳、总氮和氧化还原电位是影响底泥微生物的主要环境因素,提示底泥微生物可能受到硫酸盐和有机质作用的影响。【结论】较长时间的高浓度硫酸盐会对池塘底泥微生物群落造成重要影响,微生物群落自身的转变和硫酸盐引起的有机质分解改变可能是造成微生物群落变化的关键因素。     

Dominance of sulfur-fueled iron oxide reduction in low-sulfate freshwater sediments

A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant pathway in iron reduction. This process was mediated by (re)cycling of sulfur upon reaction of sulfide and iron oxides to support continued sulfur-based respiration—a cryptic sulfur cycle involving generation and consumption of sulfur intermediates. Although canonical iron respiration was not observed in the sediments amended with the more crystalline iron oxides, iron respiration did become dominant in the presence of ferrihydrite once sulfate was consumed. Thus, despite more favorable energetics, ferrihydrite reduction did not precede sulfate reduction and instead an inverse redox zonation was observed. These findings indicate that sulfur (re)cycling is a dominant force in iron cycling even in low-sulfate systems and in a manner difficult to predict using the classical thermodynamic ladder.     

Considerations on the mechanism of photosynthetic water oxidation – dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water

Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O–O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O–O formation step. Water oxidation in the O–O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.     

Context-Specific Bioturbation Mediates Changes to Ecosystem Functioning

Species are often grouped according to their biological or functional traits to better understand their contribution to ecosystem functioning. However, it is becoming clear that a single species can perform different roles in different habitats. Austrohelice crassa, a burrow-building mud crab shifts its primary bioturbational role to that of a vertical mixer in non-cohesive sediments as frequent burrow collapse greatly enhances sediment reworking. We conducted in situ crab density manipulations in two sediment environments (a non-cohesive sand and a cohesive muddy-sand) to examine if the context-specific functional roles were linked to changes in solute fluxes across the sediment–water interface. Across both habitats, we show that A. crassa regulated nutrient cycling, creating strong density driven effects on solute exchanges. Increasing crab density increased sediment O₂ demand and the flux of NH₄ ⁺ from the sediment, indicating much of the response was physiologically driven. Clear interactions between A. crassa and microphytobenthos were also detected in both habitats. Despite lowering microphyte standing stock through deposit feeding, A. crassa increased benthic primary production per unit of chlorophyll a. Our experiment also revealed important context-specific differences, most notably for NH₄ ⁺ fluxes, which were higher where burrows and their associated microbial communities were most stable (muddy-sand). This study highlights the need to integrate interactions between organism behavior and habitat type into functional group studies to broaden conceptual frameworks and avoid oversimplification of highly complex organism–sediment interactions.     

Nitrate uptake rate in anoxic profundal sediments from a eutrophic reservoir

There is renewed interest in the use of nitrate to treat the profundal zone of lakes to inhibit anaerobic biogeochemical processes that result in the degradation of bottom water quality (e.g., sediment phosphorus release, mercury methylation). In this study we used experimental sediment–water interface chambers to quantify the rate of sediment nitrate uptake (SNU) in profundal sediments from Lake Perris, a eutrophic raw water reservoir in Southern California. Deoxygenated chamber water was spiked with nitrate, and nitrate concentration was monitored over time under quiescent conditions, followed by mixed conditions with average water velocities of 1 cm/s. Key findings included: (1) SNU decreased with decreasing nitrate concentration, (2) SNU was higher under mixed versus quiescent conditions by nearly 50%, and (3) nitrate uptake as a function of nitrate concentration followed a conventional sediment oxygen demand model in which nitrate uptake was proportional to the square root of nitrate concentration. The probable mechanism for elevated SNU under mixed conditions was an increased diffusional concentration gradient combined with a decrease in the diffusional boundary layer at the sediment–water interface, both of which enhanced the flux of nitrate from overlaying water into sediment. Managers planning to implement lake nitrate addition should account for induced nitrate demand when determining dosing rates. For example, based on our modeling efforts from this data set, SNU in Lake Perris could range by an order of magnitude, from around 12 mg N/m²/d under quiescent, low nitrate conditions (0.1 mg N/l) to around 120 mg N/m²/d under mixed, high nitrate conditions (5 mg N/l). Handling editor: L. Naselli-Flores     

Anaerobic Mineralization of Toluene by Enriched Sediments with Quinones and Humus as Terminal Electron Acceptors

The anaerobic microbial oxidation of toluene to CO₂ coupled to humus respiration was demonstrated by use of enriched anaerobic sediments from the Amsterdam petroleum harbor (APH) and the Rhine River. Both highly purified soil humic acids (HPSHA) and the humic quinone moiety model compound anthraquinone-2,6-disulfonate (AQDS) were utilized as terminal electron acceptors. After 2 weeks of incubation, 50 and 85% of added uniformly labeled [¹³C]toluene were recovered as ¹³CO₂ in HPSHA- and AQDS-supplemented APH sediment enrichment cultures, respectively; negligible recovery occurred in unsupplemented cultures. The conversion of [¹³C]toluene agreed with the high level of recovery of electrons as reduced humus or as anthrahydroquinone-2,6-disulfonate. APH sediment was also able to use nitrate and amorphous manganese dioxide as terminal electron acceptors to support the anaerobic biodegradation of toluene. The addition of substoichiometric amounts of humic acids to bioassay reaction mixtures containing amorphous ferric oxyhydroxide as a terminal electron acceptor led to more than 65% conversion of toluene (1 mM) after 11 weeks of incubation, a result which paralleled the partial recovery of electron equivalents as acid-extractable Fe(II). Negligible conversion of toluene and reduction of Fe(III) occurred in these bioassay reaction mixtures when humic acids were omitted. The present study provides clear quantitative evidence for the mineralization of an aromatic hydrocarbon by humus-respiring microorganisms. The results indicate that humic substances may significantly contribute to the intrinsic bioremediation of anaerobic sites contaminated with priority pollutants by serving as terminal electron acceptors.