Denitrification In Sediments Research Articles

Nitrogen uptake and coupled nitrification–denitrification in riverine sediments with benthic microalgae and rooted macrophytes

Riverine ecosystems receive nitrogen loads from point and diffuse sources that are transferred downstream. Such loads may undergo poorly explored retention and dissipation processes, varying along gradients of nitrogen availability due to different interactions among primary producers and microbial communities. We measured carbon uptake and nitrogen fluxes in microcosms containing riverine sediments with benthic algae and submerged macrophytes (Vallisneria spiralis L.). Coupled nitrification–denitrification rates were determined via 15N–NH4 + injection in the pore water and quantification of the produced 29N2 and 30N2. Two sites with different N–NO3 − concentration and sediment organic content were investigated. We hypothesized that: (1) N–NO3 − availability promotes water column N uptake and attenuates primary producers-bacteria competition; (2) coupled nitrification–denitrification is stimulated by radial oxygen loss from roots; (3) macrophyte meadows favour both temporary nitrogen retention and permanent removal. Sediments with V. spiralis were mostly inorganic C and N sinks and always displayed higher coupled nitrification–denitrification rates compared to sediments with microphytobenthos. Highest rates, up to 100 µmol N m−2 h−1, were measured at the more eutrophic site and in the light. This is likely due to a shift from root to leaf uptake at the N–NO3 − rich site, attenuating plant-bacteria competition, and to increased radial oxygen loss in organic-rich sediments, stimulating nitrification. High rates of N uptake and loss in lotic sediments represent natural buffers preventing N transport downstream and stress the need to preserve aquatic vegetation and its ecosystem services.

Analysis of Driving Factors on the Nitrogen Decrease in the Early Stage of the Thermal Stratification in Main Area of Zhoucun Reservoir

In order to explore the trend and driving factors of nitrogen in the early stage of the thermal stratification in Zhoucun reservoir, the water quality indicators in main reservoir area of Zhoucun reservoir were monitored regularly form February to April (2016). Meanwhile, the fresh water and surface sediments in the sampling point were collected to study the effect of water and sediment denitrification by simulation in laboratory. The results showed that: the concentration of TN decreased from (2.28±0.09) mg·L-1 to (1.08±0.09) mg·L-1, the concentration of nitrate nitrogen decreased from (1.66±0.09) mg·L-1 to (0.25±0.06) mg·L-1, whereas the concentration of ammonia nitrogen and nitrite showed little change in the early thermal stratification of Zhoucun reservoir. At the same time, chlorophyll was not significantly increased, so the algae had little effect on nitrogen; the decreasing nitrogen was mainly due to the aerobic denitrification. Gradually increased temperature,the changes of DO and pH and the composition of organic matter (low molecular weight) were beneficial to the growth of aerobic denitrifying bacteria. Furthermore, the number of bacteria increased from 1.06×105 cfu·L-1 to 8.33×106 cfu·L-1, which enhanced the denitrification of the reservoir; Meanwhile, in the simulation experiments, the TN removal rates of water and water-sediment systems reached 0.7 mg and 3.3 mg, respectively. Water and sediment denitrification ratio was generally 1:4, and the aerobic denitrification of surface sediment was an important factor in the removal of nitrogen in reservoir.

Within-lake variability and environmental controls of sediment denitrification and associated N2O production in a shallow eutrophic lake

Excess nitrogen (N) in lakes may lead to eutrophication and many attendant environmental problems such as water quality decline and loss of aquatic biodiversity. Denitrification in lake sediments can alleviate the effects of eutrophication through removal of N to the atmosphere as N2O and N2. However, N2O contributes to the greenhouse effect and global warming. In this study, we measured three denitrification parameters (i.e., potential denitrification rate, unamended denitrification rate and net N2O production rate) in surface sediments which were collected from 7 sub-lakes of the Lake Donghu, one of the most eutrophic lakes in China. The results showed that a range of water quality and sediment characteristics (e.g., total N and total phosphorus) varied significantly among sub-lakes. The unamended denitrification rate varied between 0.51 and 26.0ngNg−1h−1, while the N2O production rate ranged from less than 0 to 1.68ngNg−1h−1. However, there was no significant difference among the sub-lakes in these denitrification parameters. The unamended denitrification rate was positively related to the water NO3− concentration and sediment moisture and bulk density. The findings of the present study suggest that sediments in eutrophic lakes can remove large quantities of N through denitrification and may become a significant source of N2O if the N input is maintained or to increase.

Spatial variation in rates of benthic denitrification and environmental controls in four New Zealand estuaries

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 556:59-77 (2016) - DOI: https://doi.org/10.3354/meps11865 Spatial variation in rates of benthic denitrification and environmental controls in four New Zealand estuaries Catherine Gongol1, Candida Savage1,2,* 1Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand 2Department of Biological Sciences, University of Cape Town, Rondebosch, South Africa *Corresponding author: candida.savage@otago.ac.nz ABSTRACT: Denitrification plays a key role in global biogeochemical cycles as the main nitrogen (N) loss process that limits availability of N to primary producers. This study presents the first measurements of sediment denitrification rates for New Zealand estuaries. Spatially explicit rates of direct denitrification (Dw) and coupled nitrification-denitrification (Dn) were quantified using the isotope pairing method in 4 oligotrophic to moderately nutrient-enriched estuaries on the South Island, New Zealand (Avon-Heathcote, Waikouaiti, Tokomairiro, Tautuku). Coupled nitrification-denitrification dominated (>86%), with most sites having low dissolved inorganic nitrogen (DIN) concentrations (<30 µmol l-1). Rates of total denitrification (Dtot) and Dn were negatively correlated with percent sand and positively related to oxygen penetration depth (OPD). Surface sediment chlorophyll a concentrations and percent carbon (%C) were also identified as significant predictors of Dtot and Dn in the single-factor models. By contrast, Dw was the main nitrogen removal process at moderately nutrient-enriched sites and was positively related to DIN concentrations in the water column and negatively correlated with percent sand. OPD and %C were also identified as significant predictors of Dw. While Dw was positively related to DIN concentrations, overall rates of denitrification were not, demonstrating that predictive relationships between N loading and removal are not always possible in oligotrophic estuaries. Our findings demonstrate the importance of grain size as a driver of denitrification rates in estuaries and imply that an increase in fine-grained sediments from land-use intensification may alter rates of N removal in estuaries through complex direct and indirect processes. KEY WORDS: Denitrification · Estuaries · New Zealand · Nitrogen · Benthic · Sediment grain size · Isotope pairing method · Oxygen flux Full text in pdf format PreviousNextCite this article as: Gongol C, Savage C, (2016) Spatial variation in rates of benthic denitrification and environmental controls in four New Zealand estuaries. Mar Ecol Prog Ser 556:59-77. https://doi.org/10.3354/meps11865 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 556. Online publication date: September 08, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Living oysters and their shells as sites of nitrification and denitrification

Oysters provide a critical habitat, are a food resource for higher trophic levels and support important commercial fisheries throughout the world. Oyster reefs can improve water quality by removing phytoplankton. While sediment denitrification may be enhanced adjacent to oyster reefs, little is known about nitrification and denitrification associated with living oysters and their shells. We measured nitrification and denitrification in living oysters (Crassostrea virginica and Crassostrea gigas) and empty oyster shells. Nitrification was similar between live oysters and empty oyster shells, however, denitrification was enhanced significantly on living oysters compared to shells. This is the first demonstration of nitrification and denitrification associated with living oysters and their shells. Our data suggest that loss of historic oyster reefs has likely affected the resilience of estuaries to eutrophication. The additional benefit of oyster mediated denitrification should be considered in restoration of oyster reefs as a tool for managing eutrophication.

Similar temperature responses suggest future climate warming will not alter partitioning between denitrification and anammox in temperate marine sediments

Removal of biologically available nitrogen (N) by the microbially mediated processes denitrification and anaerobic ammonium oxidation (anammox) affects ecosystem N availability. Although few studies have examined temperature responses of denitrification and anammox, previous work suggests that denitrification could become more important than anammox in response to climate warming. To test this hypothesis, we determined whether temperature responses of denitrification and anammox differed in shelf and estuarine sediments from coastal Rhode Island over a seasonal cycle. The influence of temperature and organic C availability was further assessed in a 12-week laboratory microcosm experiment. Temperature responses, as characterized by thermal optima (Topt ) and apparent activation energy (Ea ), were determined by measuring potential rates of denitrification and anammox at 31 discrete temperatures ranging from 3 to 59°C. With a few exceptions, Topt and Ea of denitrification and anammox did not differ in Rhode Island sediments over the seasonal cycle. In microcosm sediments, Ea was somewhatlower for anammox compared to denitrification across all treatments. However, Topt did not differ between processes, and neitherEa norTopt changed withwarming or carbon addition.Thus, the two processes behaved similarly in terms of temperature responses, and these responses were not influenced by warming. This led us to reject the hypothesis that anammox is more cold-adapted than denitrification in our study system. Overall, our study suggests that temperature responses of both processes can be accurately modeled for temperate regions in the future using a single set of parameters, which are likely not to change over the next century as a result of predicted climate warming. We further conclude that climate warming will not directly alter the partitioning of N flow through anammox and denitrification.

Denitrification occurring on suspended sediment in a large, shallow, subtropical lake (Poyang Lake, China)

Many lakes and rivers are enriched with high levels of suspended sediments (SPS). Denitrification occurring on suspended sediments (DSS) may play an important role in nitrogen removal in water columns with high SPS concentrations. Poyang Lake, with dramatic hydrologic variations, has high spatial and seasonal variation of SPS, and we hypothesized that DSS and nitrogen removal in this lake would vary similarly. DSS in Poyang Lake was determined by the traditional acetylene-inhibition method combined with a batch mode assay. Laboratory simulation experiments were also conducted to examine the factors controlling denitrification occurring on SPS. Seasonally, DSS rates at 15 sampling sites in Poyang Lake were 0.63 ± 0.24, 0.29 ± 0.17, 0.25 ± 0.18, and 0.52 ± 0.37 μmol N·L−1·d−1, respectively in spring, summer, autumn, and winter. Spatially, average DSS rates were higher in the northern lake area, which is connected to the Yangtze River, than in the upstream and central lake area. Lowest DSS rates occurred in semi-closed bay and dish lakes. Spatial and seasonal variations of DSS rates were affected by a combination of factors, in which nitrate concentrations, SPS composition, and concentrations of organic-SPS were the most important. These influencing factors were seasonally dependent, with nitrate concentrations having stronger effects on DSS during wet seasons than dry seasons. Results from a multiple stepwise regression model also demonstrated that DSS tended to occur on fine particles (e.g., clay particles, <4 μm). Evaluation of annual nitrogen loss by DSS was estimated according to the seasonal water budget and DSS rates in Poyang Lake. The total nitrogen loss by DSS was estimated to be 10800 ± 6090 t, which accounted for 2.8–9.9% of the nitrogen input, and this proportion was comparable to nitrogen removal by sediment denitrification. This result confirms that DSS was an important nitrogen sink in this large, turbid lake.

Carbon nanotubes affect the toxicity of CuO nanoparticles to denitrification in marine sediments by altering cellular internalization of nanoparticle.

Denitrification is an important pathway for nitrate transformation in marine sediments, and this process has been observed to be negatively affected by engineered nanomaterials. However, previous studies only focused on the potential effect of a certain type of nanomaterial on microbial denitrification. Here we show that the toxicity of CuO nanoparticles (NPs) to denitrification in marine sediments is highly affected by the presence of carbon nanotubes (CNTs). It was found that the removal efficiency of total NOX−-N (NO3−-N and NO2−-N) in the presence of CuO NPs was only 62.3%, but it increased to 81.1% when CNTs appeared in this circumstance. Our data revealed that CuO NPs were more easily attached to CNTs rather than cell surface because of the lower energy barrier (3.5 versus 36.2 kT). Further studies confirmed that the presence of CNTs caused the formation of large, incompact, non-uniform dispersed, and more negatively charged CuO-CNTs heteroaggregates, and thus reduced the nanoparticle internalization by cells, leading to less toxicity to metabolism of carbon source, generation of reduction equivalent, and activities of nitrate reductase and nitrite reductase. These results indicate that assessing nanomaterial-induced risks in real circumstances needs to consider the “mixed” effects of nanomaterials.

Influence of long-term inundation and nutrient addition on denitrification in sandy wetland sediments from Poyang Lake, a large shallow subtropical lake in China

Wetlands at aquatic/terrestrial transition zones may play an important role in nitrogen removal due to the denitrification during inundation events. In this study, air-dried sandy sediment cores from Poyang Lake’s wetlands were continuously inundated in the laboratory to investigate the rate, efficiency, and pattern of denitrification at the sediment-water interface (SWI). Denitrification rates responded to inundation in three stages: an adaptive period, a rapid growth period, and a stable period. The average denitrification rates during these stages were 52.62 ± 11.33 μmol N2 m−2 h−1, 115.74 ± 58.40 μmol N2 m−2 h−1 and 187.74 ± 16.44 μmol N2 m−2 h−1, respectively. Multiple regression analysis showed that inundation duration was the most important factor controlling denitrification rates at the SWI rather than nitrate concentrations, which were also positively correlated with denitrification rates. Changes in water content, oxidation-reduction potential (Eh), and pH in the surface sediments, which were consequences of inundation, significantly influenced the denitrification rate (p < 0.05). The N2O/(N2O + N2) flux ratio in denitrification production varied from 1.61% during the adaptive period to 0.03% in the stable period, suggesting that continuous inundation of sediments in aquatic-terrestrial transition zones of Poyang Lake could decrease the proportion of greenhouse gases (N2O). Since inputs of bioavailable nitrogen and phosphorus to Poyang Lake have increased in recent decades, the influence of elevated nutrients on denitrification under continuous inundation were also evaluated by simulation experiments. The results showed that increased nitrate or phosphate loading stimulate denitrification rates in sandy sediments during the inundation process. The addition of nitrate plus phosphate showed a stronger influence on denitrification than single nutrient additions. This study facilitates understanding of the influence of continuous inundation and elevated nutrients on denitrification in sandy sediments in aquatic/terrestrial transition zones.

Effects of macrophyte-associated nitrogen cycling bacteria on ANAMMOX and denitrification in river sediments in the Taihu Lake region of China

The coexistence of anaerobic ammonia oxidation (ANAMMOX) and denitrification can improve N removal in river sediments. This study evaluated the effects of a macrophyte (Elodea nuttallii) and immobilized nitrogen cycling bacteria (INCB) on ANAMMOX and denitrification in an inflow river in the Taihu Lake region in China. The results demonstrated that the potential ANAMMOX and denitrification rates varied among the four treatments, with the highest mean rates (0.413±0.029μmolN2cm−3h−1 and 0.656±0.038μmolN2cm−3h−1) occurring in the treatments with Elodea nuttallii and INCB added. The abundances of ANAMMOX and denitrification bacteria in the sediments with Elodea nuttallii and INCB added were also the highest among the four different treatment groups. In addition, the primary ANAMMOX bacterial genus (C. Kuenenia) and the main denitrification bacterial genera (Achromobacter sp. and Thauera) detected in the sediments benefited from the addition of Elodea nuttallii and INCB. Our results confirmed that the Elodea nuttallii-INCB assemblage could significantly increase the benthic N removal by enhancing the ANAMMOX and denitrification processes. Moreover, the total N2O emission was highest in the sediment with Elodea nuttallii and INCB added, which may be a potential environmental risk to the atmosphere.

Controls of Sediment Nitrogen Dynamics in Tropical Coastal Lagoons.

Sediment denitrification rates seem to be lower in tropical environments than in temperate environments. Using the isotope pairing technique, we measured actual denitrification rates in the sediment of tropical coastal lagoons. To explain the low denitrification rates observed at all study sites (<5 μmol N2 m-2 h-1), we also evaluated potential oxygen (O2) consumption, potential nitrification, potential denitrification, potential anammox, and estimated dissimilatory nitrate (NO3-) reduction to ammonium (NH4+; DNRA) in the sediment. 15NO3- and 15NH4+ conversion was measured in oxic and anoxic slurries from the sediment surface. Sediment potential O2 consumption was used as a proxy for overall mineralization activity. Actual denitrification rates and different potential nitrogen (N) oxidation and reduction processes were significantly correlated with potential O2 consumption. The contribution of potential nitrification to total O2 consumption decreased from contributing 9% at sites with the lowest sediment mineralization rates to less than 0.1% at sites with the highest rates. NO3- reduction switched completely from potential denitrification to estimated DNRA. Ammonium oxidation and nitrite (NO2-) reduction by potential anammox contributed up to 3% in sediments with the lowest sediment mineralization rates. The majority of these patterns could be explained by variations in the microbial environments from stable and largely oxic conditions at low sediment mineralization sites to more variable conditions and the prevalences of anaerobic microorganisms at high sediment mineralization sites. Furthermore, the presence of algal and microbial mats on the sediment had a significant effect on all studied processes. We propose a theoretical model based on low and high sediment mineralization rates to explain the growth, activity, and distribution of microorganisms carrying out denitrification and DNRA in sediments that can explain the dominance or coexistence of DNRA and denitrification processes. The results presented here show that the potential activity of anaerobic nitrate-reducing organisms is not dependent on the availability of environmental NO3-.

Methane dependent denitrification- from ecosystem to laboratory-scale enrichment for engineering applications

Managing nitrogen and carbon cycles in engineered and natural ecosystems is an environmental challenge. In this manuscript, we report a process which connects these two cycles with immense ecological and engineering significance. Sediments, collected from Jordan River in Salt Lake City, Utah were used as seed to start a laboratory-scale denitrification coupled to anaerobic methane oxidation (n-DAMO) reactor fed with methane (CH4) and nitrite (NO2−). Methane (CH4)-dependent denitrification in sediments of a nutrient-impaired river was found to be in the range of 40 nmol kg−1 d−1 to 70 nmol kg−1 d−1. Post 19 months of operation of the lab scale reactor, the n-DAMO reactor achieved nitrite removal rate of 2.88 mmol L−1 d−1. Enrichment of n-DAMO prokaryotes was evident from the increase in 16S rRNA gene copy number of bacteria belonging to the NC10 phylum in the reactor, corroborating with increase in the oxidation rates of CH4 coupled with NO2−-N removal from 21 μM to 190 μM of CH4 d−1. Based on stable isotope experiments by other researchers, nitric oxide dismutase (nod) functional gene was hypothesized to be responsible for splitting nitric oxide to nitrogen and oxygen and this internally generated oxygen is utilized by n-DAMO prokaryotes to oxidize methane gas. Primers targeting the unique nitric oxide dismutase (nod) gene were developed and tested on the enrichment culture for the first time. This revealed that n-DAMO organisms are closely related yet distinct from, the M. oxyfera which had been enriched in earlier studies. The results emphasize tremendous future promise to use these novel organisms for wastewater treatment purposes, especially to take advantage of the dissolved methane present in anaerobic digester effluents.

Carbon limitation of sediment bacterial production and denitrification in high nitrate low carbon systems

The concentrations and ratios of carbon and nitrogen may affect bacterial denitrification through changes in enzyme or bacterial production, but short-term laboratory and field assays may not account for changes over longer periods. Drainage from agricultural landscapes can have different ratios of C:N depending on the water source, i.e., surface runoff has elevated C:N relative to agriculture tile drainage water. We hypothesized that differences in C:N ratios limit bacterial production and reduce denitrification rates. Sediment cores collected from surface or tile water fed wetlands were incubated in low C:N (1:1) and high C:N (4:1) ratios keeping nitrogen concentrations at 10 mg L−1 NO3-N. We used replicated laboratory incubations that were destructively sampled on days 0, 1, 3, 5, 10, and 20 to represent pulsed water inputs to wetlands to test for potential increases in denitrification over time with elevated C:N (4:1) ratios. Bacterial production and denitrification were estimated using [3H] leucine incorporation and the acetylene block method. Bacterial production and denitrification increased (p < 0.0001) in response to elevated C:N (4:1). Carbon was likely preferentially allocated for bacterial production at low C:N as explained by the lack of change in denitrification at the 1:1 C:N ratio, but facilitated a significant increase in bacterial production throughout the 20 day incubation. The results suggest that if labile carbon were to be increased in subsurface tile drainage, prolonged seasonal water inputs would likely increase nitrate removal as shown by the increased denitrification throughout the 20 day study period. New agricultural conservation practices such as the implementation of cover crops have the indirect benefit of restoring soil organic matter and may improve allochthonous inputs of labile carbon.

The role of C:N:P stoichiometry in affecting denitrification in sediments from agricultural surface and tile-water wetlands.

Nutrient stoichiometry within a wetland is affected by the surrounding land use, and may play a significant role in the removal of nitrate (NO3–N). Tile-drained, agricultural watersheds experience high seasonal inputs of NO3–N, but low phosphorus (PO4–P) and dissolved organic carbon (DOC) loads relative to surface water dominated systems. This difference may present stoichiometric conditions that limit denitrification within receiving waterways. We investigated how C:N:P ratios affected denitrification rates of sediments from tile-drained mitigation wetlands incubated for: 0, 5, 10, and 20 days. We then tested whether denitrification rates of sediments from surface-water and tile-drained wetlands responded differently to C:N ratios of 2:1 versus 4:1. Ratios of C:N:P (P < 0.05) and incubation length (P < 0.05) had a significant effect on denitrification in tile-drained wetland sediments. Carbon limitation of denitrification became evident at elevated NO3–N concentrations (20 mg L−1). Denitrification measured from tile water and surface water wetland sediments increased significantly (P < 0.05) at the 2:1 and 4:1 C:N treatments. The results from both experiments suggest wetland sediments provide a limiting pool of labile DOC to maintain prolonged NO3–N removal. Also, DOC limitation became more evident at elevated NO3–N concentrations (20 mg L−1). Irrespective of NO3–N concentrations, P did not limit denitrification rates. In addition to wetting period, residence time, and maintenance of anaerobic conditions, the availability of labile DOC is playing an important limiting role in sediment denitrification within mitigation wetlands.

The effects of temperature and resource availability on denitrification and relative N2O production in boreal lake sediments

Anthropogenic environmental stressors (like atmospheric deposition, land use change, and climate warming) are predicted to increase inorganic nitrogen and organic carbon loading to northern boreal lakes, with potential consequences for denitrification in lakes. However, our ability to predict effects of these changes is currently limited as northern boreal lakes have been largely neglected in denitrification studies. The aim of this study was therefore to assess how maximum potential denitrification and N2O production rates, and the relationship between the two (relative N2O production), is controlled by availability of nitrate (NO3−), carbon (C), phosphorus (P), and temperature. Experiments were performed using the acetylene inhibition technique on sediments from a small, nutrient poor boreal lake in northern Sweden in 2014. Maximum potential denitrification and N2O production rates at 4°C were reached already at NO3− additions of 106–120μg NO3−–N/L, and remained unchanged with higher NO3 amendments. Higher incubation temperatures increased maximum potential denitrification and N2O production rates, and Q10 was somewhat higher for N2O production (1.77) than for denitrification (1.69). The relative N2O production ranged between 13% and 64%, and was not related to NO3− concentration, but the ratio increased when incubations were amended with C and P (from a median of 16% to 27%). Combined, our results suggests that unproductive northern boreal lakes currently have low potential for denitrification but are susceptible to small changes in NO3 loading especially if these are accompanied by enhanced C and P availability, likely promoting higher N2O production relative to N2.

Habitat partitioning of marine benthic denitrifier communities in response to oxygen availability.

Denitrification is of global significance for the marine nitrogen budget and the main process for nitrogen loss in coastal sediments. This facultative anaerobic respiratory pathway is modular in nature and the final step, the reduction of nitrous oxide (N2 O), is performed by microorganisms with a complete denitrification pathway as well as those only capable of N2 O reduction. Fluctuating oxygen availability is a significant driver of denitrification in sediments, but the effects on the overall N2 O-reducing community that ultimately controls the emission of N2 O from marine sediments is not well known. To investigate the effects of different oxygen regimes on N2 O reducing communities, coastal marine surface sediment was incubated in microcosms under oxic, anoxic or oscillating oxygen conditions in the overlying water for 137 days. Quantification of the genetic potential for denitrification, anammox and respiratory ammonification indicated that denitrification supported nitrogen removal in these sediments. Furthermore, denitrifiers with a complete pathway were identified as the dominant community involved in N2 O reduction, rather than organisms that are only N2 O reducers. Specific lineages within each group were associated with different oxygen regimes suggesting that oxygen availability in the overlying water is associated with habitat partitioning of N2 O reducers in coastal marine surface sediments.

Nutrient variability and its influence on nitrogen processes in a highly turbid tropical estuary (Bangpakong, Gulf of Thailand)

Estuarine ecosystems in SE Asia have been poorly studied when compared to other tropical environments. Important gaps exist particularly in the understanding of their biogeochemical function and contribution to global change. In this work we looked into N-turnover in the water column and sediments of the Bangpakong estuary (13°N). A seasonal sampling program was performed along the salinity gradient covering different stretches of the estuary (68km). Key physical and chemical characteristics were also monitored in order to unravel possible environmental controls. Results showed the occurrence of active denitrification in sediments (5.7–50.9nmol N-N2/(cm3·hr)), and water column (3.5–1044pmol N-N2/(cm3·hr)). No seasonal or spatial variability was detected for denitrification potential in sediment samples. However, in the water column, the denitrification activity peaked during the transition season in the downstream sites coinciding with high turbidity levels. Therefore, in that period of the year, the water column compartment may be an important contributor to nitrate reduction within the estuary. The rather low nitrification rates detected were not always measurable, probably due to the reduced oxygen content and high siltation. This study is one of the few dealing simultaneously with sediments and water column processes in a highly turbid tropical estuary. Therefore, it emerges as a valuable contribution for the understanding of the dynamics of the nitrogen cycle in tropical environments by exploring the role of estuarine N microbial activity in reducing the effects of increased nitrogen loads.

Effect of Fluctuating Salinity on Potential Denitrification in Coastal Wetland Soil and Sediments

The coastal wetlands of southern Louisiana provide an ideal environment for removing nitrate (NO₃⁻) from the Mississippi River before discharge into the Gulf of Mexico where it can contribute to hypoxia. However, denitrification, the primary mechanism of excess N removal, may be sensitive to the fluctuating salinities that characterize coastal wetlands. Salinity can shift from salt to fresh during river inflows, or from fresh to salt during storm surge events. This study investigated the impact of shifting salinity on potential denitrification rates in marsh soils and bayou sediments from the fresh and salt marshes of Brenton Sound estuary. Potential denitrification rates were quantified using bottle incubations for both short-term (2 d), and longer-term (11 d) studies. Fresh marsh sites had higher denitrification enzyme activity (DEA), but all soils and sediments achieved high denitrification when exposed to ideal conditions. Pulses of freshwater (0 ppt) in salt marsh soils decreased potential denitrification rates by 98% for an 11-d study, indicating the rapid opening of a freshwater diversion could significantly reduce that ability of coastal wetlands to provide significant nutrient-removal before discharge to the coastal ocean. Pulses of intermediate salinity (15 ppt) water stimulated denitrification rates in the fresh marsh soil by 75%, while full salinity seawater (35 ppt) suppressed potential denitrification by 73% for 11 d. This research demonstrates the sensitivity of the denitrifying microbial consortia to rapid shifts in salinity and this information can be used to guide water managers on maximizing NO₃⁻ removal in Mississippi River surface water diversions.

Factors controlling denitrification of mudflat sediments in Ariake Bay, Japan

To investigate the seasonal variation of denitrification rate (DR) and clarify the controlling factors of denitrification in the mudflat sediments of Ariake Bay, we conducted field surveys biweekly each month from April 2006 to January 2008. NH4(+)-N porewater concentration increased from summer to autumn due to the organic material mineralization under higher sediment temperatures. The seasonal pattern of NH4 (+)-N flux between sediments and overlying water interface indicated that the mudflat sediments were a source of NH4(+)-N in summer. NO3(-)+NO2(-)-N porewater concentrations were low, ranging from 0.53 to 11.46 μM, and mudflat sediments were sinks of NO3(-)+NO2(-)-N throughout the year. The mean number of denitrifiers tended to increase in July-September (2188-75,057 MPN g(-1)) and to decrease in March-May (500-3740 MPN g(-1)). DR tended to increase in summer, ranging from 76.03 to 990.21 μmol m(-2) day(-1), and to decrease in winter, ranging from 25.01 to 206.07 μmol m(-2) day(-1). There was no significant correlation between DR and denitrifier number. Environmental factors influencing DR during the investigation period were determined by multiple regression analysis with the stepwise method. The results indicated that NO3(-)+NO2(-)-N flux was an important factor in denitrification of mudflat sediments in Ariake Bay. Denitrification was depended on nitrate diffusing from overlying water into sediments under reduced sediment conditions during summer-mid-autumn. On the other hand, in late autumn-winter at Eh>+200 mV and sediment temperature >10 °C, nitrate produced by sediment nitrification was thought to be denitrified subsequently; that is, the coupled nitrification-denitrification may have taken place in the upper layer of mudflat sediments.

Nitrate Storage and Dissimilatory Nitrate Reduction by Eukaryotic Microbes.

The microbial nitrogen cycle is one of the most complex and environmentally important element cycles on Earth and has long been thought to be mediated exclusively by prokaryotic microbes. Rather recently, it was discovered that certain eukaryotic microbes are able to store nitrate intracellularly and use it for dissimilatory nitrate reduction in the absence of oxygen. The paradigm shift that this entailed is ecologically significant because the eukaryotes in question comprise global players like diatoms, foraminifers, and fungi. This review article provides an unprecedented overview of nitrate storage and dissimilatory nitrate reduction by diverse marine eukaryotes placed into an eco-physiological context. The advantage of intracellular nitrate storage for anaerobic energy conservation in oxygen-depleted habitats is explained and the life style enabled by this metabolic trait is described. A first compilation of intracellular nitrate inventories in various marine sediments is presented, indicating that intracellular nitrate pools vastly exceed porewater nitrate pools. The relative contribution by foraminifers to total sedimentary denitrification is estimated for different marine settings, suggesting that eukaryotes may rival prokaryotes in terms of dissimilatory nitrate reduction. Finally, this review article sketches some evolutionary perspectives of eukaryotic nitrate metabolism and identifies open questions that need to be addressed in future investigations.