Aerobic Microbial Respiration Research Articles

Lignite reduces carbon and nitrogen loss from litter in commercial broiler housing

The loss of carbon and nitrogen from broiler litter limits nutrient recycling and is damaging to the environment. This study investigated lignite, a low-rank brown coal, as an amendment to reduce the loss of carbon and nitrogen from broiler litter over 3 consecutive grow-out cycles, November 2021 to May 2022, at a commercially operated farm in Victoria, Australia. Lignite-treated litter contained significantly more carbon and nitrogen, with an increase of 70.1 g/bird and 12.6 g/bird for carbon and nitrogen, respectively. Lignite also reduced aerobic microbial respiration, with a 46.0% reduction in CO2 flux recorded in week 7 of the study, resulting in reduced mass loss. It is expected that this is a key mechanism responsible for nutrient retention in litter following treatment with lignite. Furthermore, lignite treatment lowered litter moisture content by 7, 6 and 3 percentage points for grow-out 1, 2 and 3, respectively. These findings present lignite as a beneficial litter amendment for increasing the nutrient value of waste and reducing carbon dioxide emissions. The study highlights the potential of lignite to reduce the environmental impact of poultry production and presents an alternative use for lignite as an existing resource.

Films Floating on Water Surface: Coupled Redox Cycling of Iron Species (Fe(III)/Fe(II)) at Soil/Water and Water/Air Interfaces

Naturally occurring Fe(III) films with rainbow reflection iridescence have been observed floating on the water surface of various spots covered with shallow water (e.g., edges of wetlands and creeks, standing water over soils). This natural phenomenon has become a scenic attraction and stimulated much curiosity. We pursued an experimental inquiry aimed at probing this interesting, curious natural wonder. As the first critical task, floating Fe(III) films were successfully generated in an assessable, controllable setting in our laboratory. This enabled us to establish this phenomenon reproducibly under controlled conditions and characterize the phenomenon over the entire span of the formation and transformation of the Fe(III) films. Our film generation method requires a few things: fresh soil (source for Fe(III) and microbes), glucose (energy source), and water in a container. The floating Fe(III) films as observed in the field occurred in ~1–3 day(s) on the water surface of the inundated soil mixed with the sugar. The Fe(III) films then grew from initial very thin, colorless, somewhat transparent films with rainbow reflection iridescence to colored thicker films and then to orange/orange-red/red crusts over the time. A comprehensive mechanistic picture was formulated to depict the formation of the Fe(III) films. Several sequential processes are operative. First, the Fe(III) (oxides, oxyhydroxides) in the soil is reduced to Fe(II) by the Fe(III)-reducing microbes during their anerobic respiration with Fe(III) as the electron (e−) acceptor after depletion of dissolved O2 in the water as a result of aerobic microbial respiration with O2 as the e− acceptor. The Fe(II), being soluble, then diffuses to the water surface where it is oxidized to Fe(III). Subsequently, the Fe(III) hydrolyzes and various Fe(III) hydrolysis products polymerize to stabilize. A polymeric model was created to account for the Fe(III) film transformation. The Fe(III) films are considered to transform from the dimers and trimers and linear polymers of Fe(OH)3 to Fe(III) polymer sheets (e.g., Fe(OH)3, FeOOH), to 3D Fe(III) polymers, and eventually to Fe2O3 colloid particles. This floating Fe(III) film phenomenon boasts an environmental chemical drama of redox cycling of Fe(III)/Fe(II) at soil/water and water/air interfaces coupled with Fe(II) transport from the inundated soil to the water surface followed by ultimate mineralization of the Fe(III) polymers. Our Fe(III) film generation method can be readily scaled up to supply Fe(III) films of rich varieties in thickness, size, morphology, and structure over the entire span of various stages of their formation and transformation as desired for various uses. This setup offers a platform needed for further controlled studies on the kinetics, mechanism, and process of abiotic and biotic nature involved in the Fe(III) film phenomenon and for exploration of versatile roles of the Fe(III) films as nanofilms in Fe(III)/Fe(II)-surface catalyzed chemical and photochemical reactions involving various natural and synthetic compounds.

Sinking krill carcasses as hotspots of microbial carbon and nitrogen cycling in the Arctic

Krill represent a major link between primary producers and higher trophic levels in polar marine food webs. Potential links to lower trophic levels, such as heterotrophic microorganisms, are less well documented. Here, we studied the kinetics of microbial degradation of sinking carcasses of two dominant krill species Thysanoessa raschii and Meganyctiphanes norvegica from Southwest Greenland. Degradation experiments under oxic conditions showed that 6.0-9.1% of carbon and 6.4-7.1% of nitrogen were lost from the carcasses after one week. Aerobic microbial respiration and the release of dissolved organic carbon were the main pathways of carbon loss from the carcasses. Ammonium release generally contributed the most to carcass nitrogen loss. Oxygen micro profiling revealed anoxic conditions inside krill carcasses/specimens, allowing anaerobic nitrogen cycling through denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification rates were up to 5.3 and 127.7 nmol N carcass-1 d-1 for T. raschii and M. norvegica, respectively, making krill carcasses hotspots of nitrogen loss in the oxygenated water column of the fjord. Carcass-associated DNRA rates were up to 4-fold higher than denitrification rates, but the combined activity of these two anaerobic respiration processes did not contribute significantly to carbon loss from the carcasses. Living krill specimens did not harbor any significant denitrification and DNRA activity despite having an anoxic gut as revealed by micro profiling. The investigated krill carcasses sink fast (1500-3000 m d-1) and our data show that only a small fraction of the associated carbon is lost during descent. Based on data on krill distribution, our findings are used to discuss the potential importance of sinking krill carcasses for sustaining benthic food webs in the Arctic.

Experimental evaluation of the role of inorganic phosphorus for terrestrial carbon degradation in stream hyporheic zones

Abstract The hyporheic zone (HZ) is a hotspot of carbon processing in stream ecosystems as a consequence of the mixing of organic matter and nutrients from ground water and surface waters. However, major knowledge gaps exist regarding the drivers of microbial activity and carbon processing in the HZ among stream ecosystems with different carbon sources and sediment properties. We investigated the impact of algal dissolved organic matter (DOMalgal) and inorganic phosphorus (P) on the degradation of soil DOM (DOMsoil) by hyporheic microorganisms in two laboratory experiments. In our first experiment, we explored the influence of different ratios of DOMalgal to DOMsoil with and without P additions on microbial respiration and DOM composition in laboratory hyporheic microcosms under oxic conditions. Here, we used glass beads colonised by stream microorganisms resembling a pristine stream system. As the addition of DOMalgal increased P concentrations, we added P to adjust the P concentrations to the same level of the pure DOMalgal in a second batch. In our second experiment, we determined the aerobic microbial respiration of HZ‐sediments from 20 streams along a land‐use gradient in Austria incubated with DOMsoil. Again, we performed the experiments with and without P additions to see whether effects on microbial respiration depended on the ambient P concentrations of the streams. Aerobic microbial respiration in the hyporheic microcosms decreased with increasing DOMsoil proportions. When P concentrations were adjusted to the P level of the DOMalgal, aerobic microbial respiration rates were similar between the different DOM mixtures in the microcosms, mainly stimulating the degradation of humic‐like DOM fractions. This highlights the stimulating effects of the P additions on hyporheic microbial respiration and humic‐like DOM degradation in pristine streams. However, P additions caused a significant increase in microbial respiration in only one of 20 natural HZ‐sediments, suggesting that aerobic microbial respiration rates rarely were controlled by P availability in the investigated streams. We conclude that nutrient pulses can, but do not necessarily, stimulate microbial activity and terrestrial carbon degradation in the HZ of streams. Nevertheless, at low ambient nutrient concentrations (i.e., in pristine streams) terrestrial carbon degradation in the HZ can be accelerated when nutrient pulses occur, which has consequences for CO2 outgassing and the organic matter quality in the stream and its export to downstream sections.

How low can they go? Aerobic respiration by microorganisms under apparent anoxia

Oxygen (O2) is the ultimate oxidant on Earth and its respiration confers such an energetic advantage that microorganisms have evolved the capacity to scavenge O2 down to nanomolar concentrations. The respiration of O2 at extremely low levels is proving to be common to diverse microbial taxa, including organisms formerly considered strict anaerobes. Motivated by recent advances in O2 sensing and DNA/RNA sequencing technologies, we performed a systematic review of environmental metatranscriptomes revealing that microbial respiration of O2 at nanomolar concentrations is ubiquitous and drives microbial activity in seemingly anoxic aquatic habitats. These habitats were key to the early evolution of life and are projected to become more prevalent in the near future due to anthropogenic-driven environmental change. Here, we summarize our current understanding of aerobic microbial respiration under apparent anoxia, including novel processes, their underlying biochemical pathways, the involved microorganisms, and their environmental importance and evolutionary origin.

Multi-sensor measurement of O2, CO2 and reheating in triticale silage: An extended approach from aerobic stability to aerobic microbial respiration

The biochemical reactions of aerobic microbial respiration (AMR) suggest that silage temperature (Tsi) rise, oxygen (O2) consumption and carbon dioxide (CO2) emission may be equally useful as indicators of silage deterioration during feed-out, but only temperature has been used extensively to assess aerobic stability. Here we extend the study of aerobic stability to incorporate AMR of silage by developing a novel experimental cell integrated with multiple sensors. Silage samples, ensiled from a triticale crop, were made in twelve air-tight barrels (60 L), packed to bulk densities of 190 or 250 kg m−3 dry matter (DM). Tsi and O2 measurements were co-located at 15- and 30-cm behind the working face. CO2 was measured as flux across the working face. The experimental period of aerobic exposure was 7 days. We provide the first reports of: (i) distinct aerobic responses of these parameters, showing that Tsi varied with CO2 in phase but with O2 out-of phase; (ii) CO2 flux was dominated initially by anaerobic discharge and subsequently by aerobic products; (iii) linear relationships between aerobic reheating and both O2 consumption (0.994 ≥ R2 ≥ 0.815, P < 0.01) and CO2 flux (0.981 ≥ R2 ≥ 0.464, P < 0.01); and (iv) variable magnitude of daily aerobic production of CO2 per kg DM from 2.3 to 133.4 mmol kg d−1. These results demonstrate that the novel multi-sensor technique has powerful capacity to provide insight into AMR of silage and thus provide more detailed information to guide silage management than previous measurements of aerobic stability.

Accelerating biodegradation of an azo dye by using its inorganic intermediates

AbstractBACKGROUNDThe discharge of textile wastewater can cause various environment problems. How to remove these dyes in effluents effectively remains a major concern in wastewater treatment. In this work, the accelerating biodegradation effect of two inorganic intermediates generated by UV irradiation on a monoazo dye, Acid Orange 7 (AO7), was studied in a biofilm reactor.RESULTSThe biodegradation reaction fitted well with fractional‐order kinetics. The promotion effects of k trend for the two inorganic intermediates were NH4+ addition &gt; SO42− addition (&gt; AO7 biodegradation alone). NH4+ ions increased biochemical oxygen demand (BOD) gradually: compared with 0.5 mg L−1 addition protocol, the BOD of 3 mg L−1 addition protocol was enhanced about five‐fold. SO42− ions showed a positive effect at low concentration and a negative effect at high concentration. In addition, the effect of the two inorganic intermediates on aerobic microbial respiration rate (i.e. SOUR) was similar to BOD. Based on the analysis of high‐throughput sequencing and cluster of orthologous groups, a mechanism of accelerating azo dye biodegradation by using its available intermediates was proposed.CONCLUSIONThis work revealed that the main inorganic intermediates of the azo dye could accelerate its biodegradation rates. In addition, the bioreactor provided an integrated anaerobic/aerobic micro‐environment that was beneficial to promoting this acceleration effect. © 2021 Society of Chemical Industry (SCI).

Rainfall Alters Permafrost Soil Redox Conditions, but Meta-Omics Show Divergent Microbial Community Responses by Tundra Type in the Arctic

Soil anoxia is common in the annually thawed surface (‘active’) layer of permafrost soils, particularly when soils are saturated, and supports anaerobic microbial metabolism and methane (CH4) production. Rainfall contributes to soil saturation, but can also introduce oxygen, causing soil oxidation and altering anoxic conditions. We simulated a rainfall event in soil mesocosms from two dominant tundra types, tussock tundra and wet sedge tundra, to test the impacts of rainfall-induced soil oxidation on microbial communities and their metabolic capacity for anaerobic CH4 production and aerobic respiration following soil oxidation. In both types, rainfall increased total soil O2 concentration, but in tussock tundra there was a 2.5-fold greater increase in soil O2 compared to wet sedge tundra due to differences in soil drainage. Metagenomic and metatranscriptomic analyses found divergent microbial responses to rainfall between tundra types. Active microbial taxa in the tussock tundra community, including bacteria and fungi, responded to rainfall with a decline in gene expression for anaerobic metabolism and a concurrent increase in gene expression for cellular growth. In contrast, the wet sedge tundra community showed no significant changes in microbial gene expression from anaerobic metabolism, fermentation, or methanogenesis following rainfall, despite an initial increase in soil O2 concentration. These results suggest that rainfall induces soil oxidation and enhances aerobic microbial respiration in tussock tundra communities but may not accumulate or remain in wet sedge tundra soils long enough to induce a community-wide shift from anaerobic metabolism. Thus, rainfall may serve only to maintain saturated soil conditions that promote CH4 production in low-lying wet sedge tundra soils across the Arctic.

Temperature effects on carbon mineralization of sinking copepod carcasses

Copepod carcasses are prevalent in marine ecosystems and might represent an important component of the sinking flux of particulate organic carbon in the ocean. The extent to which copepod carcasses contribute to the biological carbon pump is controlled by different environmental factors, including temperature. However, the effect of temperature on the longer-term kinetics of carbon mineralization of copepod carcasses is not well-studied. We conducted laboratory experiments to quantify the carbon mineralization associated with sinking carcasses of the cosmopolitan copepod Acartia tonsa through aerobic microbial respiration at 5 temperatures (20, 16, 12, 8, and 4°C). Microbial respiration rates associated with the carcasses were positively correlated with temperature and characterized by an initial short lag-phase, a rapid increase to a maximum rate, and a subsequent gradual decline in the rate of degradation. On average, 50% of the total carbon of the carcasses was mineralized within 6-12 d at 20°C, versus &gt;60 d at 4°C. During the incubations, most carbon mineralization occurred in the ambient seawater, likely fueled by dissolved organic carbon leaking from the carcasses into the surrounding seawater. Extrapolating measured carbon turnover and sinking rates suggests that at 20°C, the mineralization of sinking copepod carcasses is constrained to the surface ocean. In contrast, at 4°C, sinking copepod carcasses can reach the deep ocean before they have been completely degraded. Hence, in low-temperature regions, copepod carcasses may represent an important agent for carbon export through the biological carbon pump.

Hyporheic Zone Microbiome Assembly Is Linked to Dynamic Water Mixing Patterns in Snowmelt‐Dominated Headwater Catchments

AbstractTerrestrial and aquatic elemental cycles are tightly linked in upland fluvial networks. Biotic and abiotic mineral weathering, microbially mediated degradation of organic matter, and anthropogenic influences all result in the movement of solutes (e.g., carbon, metals, and nutrients) through these catchments, with implications for downstream water quality. Within the river channel, the region of hyporheic mixing represents a hot spot of microbial activity, exerting significant control over solute cycling. To investigate how snowmelt‐driven seasonal changes in river discharge affect microbial community assembly and carbon biogeochemistry, depth‐resolved pore water samples were recovered from multiple locations around a representative meander on the East River near Crested Butte, CO, USA. Vertical temperature sensor arrays were also installed in the streambed to enable seepage flux estimates. Snowmelt‐driven high river discharge led to an expanding zone of vertical hyporheic mixing and introduced dissolved oxygen into the streambed that stimulated aerobic microbial respiration. These physicochemical processes contributed to microbial communities undergoing homogenizing selection, in contrast to other ecosystems where lower permeability may limit the extent of mixing. Conversely, lower river discharge conditions led to a greater influence of upwelling groundwater within the streambed and a decrease in microbial respiration rates. Associated with these processes, microbial communities throughout the streambed exhibited increasing dissimilarity between each other, suggesting that the earlier onset of snowmelt and longer periods of base flow may lead to changes in the composition (and associated function) of streambed microbiomes, with consequent implications for the processing and export of solutes from upland catchments.

On the role of soil water retention characteristic on aerobic microbial respiration

Abstract. Soil water status is one of the most important environmental factors that control microbial activity and rate of soil organic matter (SOM) decomposition. Its effect can be partitioned into effect of water energy status (water potential) on cellular activity, effect of water volume on cellular motility, and aqueous diffusion of substrate and nutrients, as well as the effect of air content and gas-diffusion pathways on concentration of dissolved oxygen. However, moisture functions widely used in SOM decomposition models are often based on empirical functions rather than robust physical foundations that account for these disparate impacts of soil water. The contributions of soil water content and water potential vary from soil to soil according to the soil water characteristic (SWC), which in turn is strongly dependent on soil texture and structure. The overall goal of this study is to introduce a physically based modeling framework of aerobic microbial respiration that incorporates the role of SWC under arbitrary soil moisture status. The model was tested by comparing it with published datasets of SOM decomposition under laboratory conditions.

Model-based evaluation of selenate and nitrate reduction in hydrogen-based membrane biofilm reactor

A biofilm model was developed to describe the simultaneous NO3− and SeO42− reduction in a H2-based membrane biofilm reactor (MBfR). Model calibration and validation was conducted using the experimental data of a reported H2-based MBfR. With a good level of identifiability, the SeO42− affinity constant and the SeO32− affinity constant were estimated at 9.80 ± 0.51 g Se m−3 and 1.83 ± 0.38 g Se m−3, respectively. The model was then applied to evaluate the effects of key operating conditions on the single-stage H2-based MBfR and the role of reactor configuration through comparing two-stage to single-stage MBfR systems. The results showed that (i) high SeO42− or low NO3− concentration in the influent favored the growth of selenate-reducing bacteria (SeRB) and therefore benefited the Se removal, (ii) the influent dissolved oxygen slightly inhibited the Se removal through enhancing the aerobic microbial respiration on H2, (iii) the H2 supply should be controlled at a proper level to avoid SeRB suppression and H2 wastage, (iv) thin biofilm should be avoided to ensure a protected niche for SeRB and therefore a promising Se removal, and (v) the two-stage MBfR configuration offered relatively higher efficiency in removing Se and NO3− simultaneously under the same loading condition.

Organic carbon sources and controlling processes on aquifer arsenic cycling in the Jianghan Plain, central China

Groundwater arsenic contamination is a common environmental problem that threatens the health of over 100 million people globally. Apparent seasonal fluctuations in groundwater arsenic concentrations have been reported in various locations worldwide, including the Jianghan Plain, central China. This phenomenon has been attributed to shifts in redox conditions induced by seasonal incursions of surface water. However, it is not clear what processes during the incursion lead to changes in the redox conditions and what is the source of the organic carbon driving these processes. Therefore, we conducted a long-term investigation of stable carbon isotopic compositions in surface water and groundwater, as well as long-term monitoring of hydraulic gradients and geochemical compositions at the Jianghan Plain. Results indicated that a series of biogeochemical processes occurred during surface water incursion, including aerobic microbial respiration, nitrate and sulfate reduction. Groundwater arsenic was removed by adsorption on iron oxyhydroxides produced during oxidation of ferrous iron, resulting in dramatic decreases in arsenic concentrations during surface water recharge seasons. These processes were likely driven by organic carbon vertically transported from surface water and released from the surficial aquitard above 15 m. Groundwater pumping may accelerate the vertical infiltration of oxidizing recharge water and drive exogenous organic carbon to depth. Findings of this study advance the understandings of the mechanisms that cause temporal variations in groundwater As and the importance of exogenous organic carbon that may influence the temporal behavior of arsenic in groundwater.

Characterization of iron oxide nanoparticle films at the air–water interface in Arctic tundra waters

Massive amounts of organic carbon have accumulated in Arctic permafrost and soils due to anoxic and low temperature conditions that limit aerobic microbial respiration. Alternative electron acceptors are thus required for microbes to degrade organic carbon in these soils. Iron or iron oxides have been recognized to play an important role in carbon cycle processes in Arctic soils, although the exact form and role as an electron acceptor or donor remain poorly understood. Here, Arctic biofilms collected during the summers of 2016 and 2017 from tundra surface waters on the Seward Peninsula of western Alaska were characterized with a suite of microscopic and spectroscopic methods. We hypothesized that these films contain redox-active minerals bound to biological polymers. The major components of the films were found to be iron oxide nanoparticle aggregates associated with extracellular polymeric substances. The observed mineral phases varied between films collected in different years with magnetite (Fe2+Fe23+O4) nanoparticles (<5nm) predominantly identified in the 2016 films, while for films collected in 2017 ferrihydrite-like amorphous iron oxyhydroxides were found. While the exact formation mechanism of these Artic iron oxide films remains to be explored, the presence of magnetite and other iron oxide/oxyhydroxide nanoparticles at the air–water interface may represent a previously unknown source of electron acceptors for continual anaerobic microbial respiration of organic carbon within poorly drained Arctic tundra.

Exploring dissolved organic carbon cycling at the stream–groundwater interface across a third-order, lowland stream network

The stream–groundwater interface (SGI) is thought to be an important location within stream networks for dissolved organic carbon (DOC) processing (e.g., degradation, removal), since it is considered a hotspot for microbial activity and biogeochemical reactions. This research is one of the first attempts to collect and assess DOC conditions at the SGI across a stream network—an entire third-order, lowland watershed in Michigan, USA. We present an initial exploration of this unique data set and highlight some of the challenges when working at these scales. Overall, our results show that SGI DOC conditions are complex at the network scale and do not conform to predictions based upon previous point- and small-scale studies. We found no strong pattern of DOC removal within the SGI at the network scale even after using chloride and temperature as natural tracers to differentiate between hydrological processes and biogeochemical reactions influencing DOC cycling. Instead, trends in DOC quantity and molecular qualities suggest that potential biotic reactions, including aerobic microbial respiration, had an influence on DOC concentrations at only some of the sites, while physical mixing of ground and stream surface waters appears to explain the majority of the observed changes in DOC concentrations at other sites. In addition, results show that neither SGI sites indicating DOC removal nor DOC molecular quality shifts measured correlated with stream order. It did reveal that DOC variability across surface water, groundwater, and the SGI locations was consistently greatest in the shallow sediments of the SGI, demonstrating that the SGI is a systematically distinct location for DOC conditions in the watershed. Our empirical stream network-scale SGI data are some of the first that are compatible with recently developed process-based, network-scale, SGI models. Our results indicate that these process-based models may not accurately represent SGI exchange of lowland, groundwater discharge-dominated streams like the one in this study. Lastly, this study shows that new methods are needed to achieve the goal of making and linking SGI observations to network-scale biogeochemical processes and theory. To help develop these methods we provide a discussion of the approach we used (i.e., “lessons-learned”) that might become the basis for systematic data analysis of SGI porewaters in future, network-wide biogeochemical studies of the SGI.

Temperature Dependence of Apparent Respiratory Quotients and Oxygen Penetration Depth in Contrasting Lake Sediments

AbstractLake sediments constitute an important compartment in the carbon cycle of lakes, by burying carbon over geological timescales and by production and emission of greenhouse gases. The degradation of organic carbon (OC) in lake sediments is linked to both temperature and oxygen (O2), but the interactive nature of this regulation has not been studied in lake sediments in a quantitative way. We present the first systematic investigation of the effects of temperature on the apparent respiratory quotient (RQ, i.e., the molar ratio between carbon dioxide (CO2) production and O2 consumption) in two contrasting lake sediments. Laboratory incubations of sediment cores of a humic lake and an eutrophic lake across a 1–21°C temperature gradient over 157 days revealed that both CO2 production and O2 consumption were positively, exponentially, and similarly dependent on temperature. The apparent RQ differed significantly between the lake sediments (0.63 ± 0.26 and 0.99 ± 0.28 in the humic and the eutrophic lake, respectively; mean ± SD) and was significantly and positively related to temperature. The O2 penetration depth into the sediment varied by a factor of 2 over the 1–21°C temperature range and was significantly, negatively, and similarly related to temperature in both lake sediments. Accordingly, increasing temperature may influence the overall extent of OC degradation in lake sediments by limiting O2 supply to aerobic microbial respiration to the topmost sediment layer, resulting in a concomitant shift to less effective anaerobic degradation pathways. This suggests that temperature may represent a key controlling factor of the OC burial efficiency in lake sediments.

Issues for storing plant-based alternative fuels in marine environments

Two coastal seawaters (Key West, FL, USA and the Persian Gulf, Bahrain, representing oligotrophic and eutrophic environments, respectively) were used to evaluate potential biodegradation and corrosion problems during exposure to alternative and conventional fuels. Uncoated carbon steel was exposed at the fuel/seawater interface and polarization resistance was monitored. Under typical marine storage conditions, dioxygen in natural seawater exposed to fuel and carbon steel was reduced to <0.1parts-per-million within 2d due to consumption by corrosion reactions and aerobic microbial respiration. Sulfides, produced by anaerobic sulfate-reducing bacteria, and chlorides were co-located in corrosion products. Transient dioxygen influenced both metabolic degradation pathways and resulting metabolites. Catechols, indicative of aerobic biodegradation, persisted after 90d exposures. Detection of catechols suggested that initial exposure to dioxygen resulted in the formation of aerobic metabolites that exacerbated subsequent corrosion processes.

Influence of benthic macrofauna on microbial production of methylmercury in sediments on the New England continental shelf

Microbial methylation processes in sediment are an important source of toxic monomethylmercury (MMHg) to aquatic ecosystems. Although bioturbation activities (feeding, digging of galleries, excavations, bioirrigation) by benthic fauna are known to affect many biogeochemical processes, their influence on benthic MMHg production is poorly understood. We investigated the effect of benthic fauna on the microbial production of MMHg in sediments on the continental shelf of the northwest Atlantic Ocean in September 2009. Replicate cores of sieved (control) and unaltered sediment containing native macrofauna were incubated to examine the influence of benthic macrofauna on net MMHg production, potential gross rates of Hg methylation, sediment reworking, dissolved oxygen and organic carbon concentrations, and microbial metabolic activities. The presence of macrofauna stimulated aerobic microbial respiration and net MMHg production, but had no observed effect on short-term gross rates of Hg methylation. This suggests that bioturbation may promote net MMHg production by inhibiting demethylating microorganisms, although overall community metabolism was increased. Results from this work emphasize the need to enhance our knowledge and understanding of the interactions among benthic fauna, microorganisms, and geochemistry in affecting MMHg production.

Measuring aerobic respiration in stream ecosystems using the resazurin‐resorufin system

The use of smart tracers to study hydrologic systems is becoming more widespread. Smart tracers are compounds that irreversibly react in the presence of a process or condition under investigation. Resazurin (Raz) is a smart tracer that undergoes an irreversible reduction to resorufin (Rru) in the presence of cellular metabolic activity. We quantified the relationship between the transformation of Raz and aerobic bacterial respiration in pure culture experiments using two obligate aerobes and two facultative anaerobes, and in colonized surface and shallow (&lt;10 cm) hyporheic sediments using reach‐scale experiments. We found that the transformation of Raz to Rru was nearly perfectly (minr2 = 0.986), positively correlated with aerobic microbial respiration in all experiments. These results suggest that Raz can be used as a surrogate to measure respiration in situ and in vivoat different spatial scales, thus providing an alternative to investigate mechanistic controls of solute transport and stream metabolism on nutrient processing. Lastly, a comparison of respiration and mass‐transfer rates in streams suggests that field‐scale respiration is controlled by the slower of respiration and mass transfer, highlighting the need to understand both biogeochemistry and physics in stream ecosystems.

Behavior of stable isotopes of dissolved oxygen, dissolved inorganic carbon and nitrate in groundwater at a former wood treatment facility containing hydrocarbon contamination

Abstract The speciation and mobility of a variety of chemical species in groundwater are strongly influenced by redox and pH conditions. Dissolved O2 (DO) and dissolved inorganic C (DIC) concentrations are significant controls of these conditions, respectively. It is not always clear what the major processes are that influence changes in the concentration of DO and DIC across a groundwater flowpath. The combined use of the stable isotope compositions of DO (δ18O–DO) and DIC (δ13C–DIC) has the potential to help investigators discriminate between sources and sinks of DO and DIC in groundwater systems. A total of 31 monitoring wells were sampled to investigate changes in DO, DIC, δ18O–DO and δ13C–DIC in shallow groundwater at the Montana Pole and Treating Plant (MPTP), Butte, Montana, USA. The MPTP site contains significant quantities of petroleum hydrocarbons in soil and groundwater from historical operations as a wood treatment-preservative facility. Dissolved O2 concentrations decreased downgradient across the site (340–6.6 μmol L−1) while DIC concentration increased (3.6–12.8 mmol L−1). A general enrichment in δ18O–DO (16.8–31.6‰) and depletion in δ13C–DIC (−12.7‰ to −18.7‰) along the flowpath is consistent with aerobic microbial respiration. However, the correlation between δ18O–DO and δ13C–DIC was poor which may be caused in part by the remediation treatments being used to mitigate soil and groundwater contamination. The presence of an extensive suboxic zone near the center of the field area suggests that some DIC of unknown isotopic composition could have been added to the groundwater by anaerobic degradation of hydrocarbons. This idea is consistent with a limited amount of NO 3 - isotope data which suggest that denitrification is occurring at the MPTP site.