N2 Efflux Research Articles

The influence of oyster reefs and surrounding sediments on nitrogen removal – An in-situ study along the East coast of Australia

Oyster reefs play a crucial role in the removal of nitrogen (N) from aquatic systems by facilitating nutrient regeneration and denitrification, both in their tissues and shells and surrounding sediments. However, we still have a limited understanding about the contribution of each component of the reefs (e.g. oysters vs sediments) to N processes, and whether rates are dependent on site-specific characteristics. To address these knowledge gaps, we conducted an experiment across six oyster reefs along 1080 km of the Eastern Australian coast with different sediment characteristics. By using in-situ clear and dark incubation chambers, we assessed how benthic metabolism, nutrient and dinitrogen gas (N2) fluxes varied among the following treatments: ‘oysters’, ‘sediments’, and ‘sediments + oysters’ that were used to represent components of the whole reef habitat (i.e. reef matrix vs surrounding sediments vs the interaction among them, respectively), and sites. We found that during dark conditions and at siltier sites, N2 effluxes from oysters can be up to 23 times higher than sediments, while N2 effluxes from chambers with both sediments and oysters were similar to sediment treatments, and lower than oyster treatments. These results can be explained by sediment processes including nutrient assimilation by benthic microalgae and/or lower nutrient diffusion into interstitial space. Additionally, oyster treatments showed an uptake of nitrate (NO3-) that was likely converted into N2, whereas sediment treatments showed an overall release of NO3-. In dark conditions, ammonium (NH4+) fluxes remained consistent across treatments and sites, indicating that any exports from oyster excretion (in those treatments including oysters) were either counterbalanced by or comparable to exports from sediments. This study provides evidence that the crucial contribution of oyster reefs to N removal is dependent on interactions between reef components and environmental factors.

Biotic and Abiotic Controls on Dinitrogen Production in Coastal Sediments

AbstractDinitrogen gas (N2) production removes fixed N from the marine biosphere; however, questions remain over the relative influence of microbial processes in determining N2 efflux from coastal sediments. Here, we quantify N2 production processes and controlling factors along a ∼2,500 km continental shelf sediment transect of China via combined molecular and isotopic approaches. We show that denitrification is the dominant pathway of sedimentary N2 production, which is greatly higher than anaerobic ammonium oxidation. N2 production rates by denitrification increase with nosZ gene abundance and decrease with bacterial diversity. The effects of temperature and dissolved oxygen on denitrification are mainly through their influence on biotic factors including functional genes, microbial biomass, and bacterial diversity. The rate of anaerobic ammonium oxidation increases with hzsB gene abundance. Temperature, dissolved oxygen, and dissolved organic carbon appear to indirectly affect anaerobic ammonium oxidation rate by regulating hzsB gene abundance. When extrapolated to the entire sampling region, the annual flux of sedimentary denitrification is estimated at ∼9.2 Tg N yr−1. These results improve our understanding of sediment N removal processes and the mechanistic factors that regulate denitrification fluxes.

The First N2 Channels Identified by a Novel Technique for Assessing Nitrogen Gas Efflux from Oocytes

Decompression illness (DCI) is a serious problem for both divers and pilots. Divers must schedule their ascents to minimize the risk of nitrogen (N2) bubble formation as the partial pressure of N2 (PN2) decreases. Pilots can experience the same effects via decreases in cabin pressure. In order to approach management or even prevention of DCI, it would be beneficial to understand the mechanism of N2 transport across cell membranes. Specific aquaporins and rhesus (Rh) proteins can conduct CO2, NH3, NO, and probably O2 across membranes. Certain solute transporters (e.g., NBCe1) may also be able to conduct gases. Whereas several assays exist for CO2, O2 and NH3 permeability, an effective assay for N2 movements across membranes has been elusive. Here we present the first such assay, based on maintaining the neutral buoyancy of a Xenopus oocyte previously injected with a 200‐nl air bubble (79% N2, 21% O2). We apply pressure to the air above a saline solution, causing the bubble to constrict sufficiently that the oocyte falls to a depth of ~5 cm. As air molecules dissolve in and diffuses through the cytoplasm, and eventually exit the cell, the bubble tends to collapse. A feedback system (webcam, computer, digitally controlled pressure regulator) reacts by lowering the neutral‐buoyancy air pressure (PNB) to maintain the oocyte at a 5‐cm depth. The pressure inside the bubble (PBubble > PNB by fixed amount) falls proportionally with the decreasing number of air molecules. The rate at which PNB falls thus reflects gas efflux from the bubble. For control oocytes injected with H2O rather than cRNA, PNB falls slowly, reflecting a slow exit of N2 (+ O2) from the bubble (kPNB = 1.16 × 10−3 ± 0.02 s−1, n=47). We estimate that 10–50% of kPNB represents N2 dissolving in cellular water/lipids. For oocytes injected with cRNA to express NOD26—a plant AQP in cell membranes of root nodules of the N2‐fixing soybean plant, Glycine max—PNB falls 13.0% more rapidly (kPNB = 1.31 × 10−3 ± 0.04 s−1, p=0.018, n=5), presumably reflecting an increased N2 (+ O2) efflux across the oocyte membrane. kPNB for oocytes expressing human AQP5 or the plant AQP, AtPIP 2;1—AQPs known to function as CO2 channels—were not significantly different from H2O‐injected control oocytes (n=20 & 5, respectively). However, kPNB was 8.6% greater in oocytes expressing human RhAG (kPNB = 1.26 × 10−3 ± 0.03 s−1, p=0.003, n=26) vs. H2O‐injected control oocytes. 200 mM 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonate (DIDS) decreased the RhAG‐dependent component of kPNB by 89% (kPNB = 1.17 × 10−3 ± 0.03 s−1, p=0.04, n=12). DIDS had no effect on H2O‐injected controls (p=0.65, n=11). Together these experiments demonstrate a simple and robust assay for the efflux of N2 across a biological membrane, as well as the first examples of N2 channels in a biological membrane.Support or Funding InformationFunding: Office of Naval Research (N00014‐15‐1‐2060 & N00014‐16‐1‐2535)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Estimate of gas transfer velocity in the presence of emergent vegetation using argon as a tracer: Implications for whole-system denitrification measurements

Denitrification associated with emergent macrophytes is a pivotal process underlying the treatment performance of wetlands and slow-flow waterways. Laboratory scale experiments targeting N losses via denitrification in sediments colonized by emergent macrophytes require the use of mesocosms that are necessarily open to the atmosphere. Thus, the proper quantification of N2 effluxes relies on the accurate characterization of the air–water gas exchanges. In this study, we present a simple approach for direct measurements of the gas transfer velocity, in open-top mesocosms with Phragmites australis, by using argon as a tracer. Different conditions of water velocity (0, 1.5, 3, and 6 cm s−1) and temperature (8.5, 16, and 28 °C), were tested, along with, for the first time, the presence of emergent vegetation. The outcomes demonstrated that water velocity and temperature are not the only factors regulating aeration at the mesocosm scale. Indeed, the gas transfer velocity was systematically higher, in the range of 42–53%, in vegetated compared to unvegetated sediments. The increase of small-local turbulence patterns created within water parcels moving around plant stems translated into significant modifications of the reaeration process. The adopted approach may be used to improve the accuracy of denitrification measurements by N2 efflux-based methods in wetland and slow-flow waterway sediments colonized by emergent macrophytes. Moreover, the present outcomes may have multiple implications for whole-system metabolism estimations from which largely depend our understanding of biogeochemical dynamics in inland waters that have strong connections to worldwide issues, such as nitrate contamination and greenhouse gas emissions.

Nitrous Oxide and Dinitrogen: The Missing Flux in Nitrogen Budgets of Forested Catchments?

Most forest nitrogen budgets are imbalanced, with nitrogen inputs exceeding nitrogen outputs. The denitrification products nitrous oxide (N2O) and dinitrogen (N2) represent often-unmeasured fluxes that may close the gap between explained nitrogen inputs and outputs. Gaseous N2O and N2 effluxes, dissolved N2O flux, and traditionally measured dissolved nitrogen species (i.e., nitrate, ammonium, and dissolved organic nitrogen) were estimated to account for the annual nitrogen output along hillslope gradients from two catchments in a temperate forest. Adding N2O and N2 effluxes to catchment nitrogen output not only reduced the discrepancy between nitrogen inputs and outputs (9.9 kg ha-1 yr-1 and 6.5 or 6.3 kg ha-1 yr-1, respectively), but also between nitrogen outputs from two catchments with different topographies (6.5 kg ha-1 yr-1 for the catchment with a large wetland, 6.3 kg ha-1 yr-1 for the catchment with a very small wetland). Dissolved N2O comprised a very small portion of the annual nitrogen outputs. Nitrogen inputs exceeded nitrogen outputs throughout the year except during spring runoff, and also during autumn storms in the catchment with the large wetland. Failing to account for denitrification products, especially during summer rainfall events, may lead to underestimation of annual nitrogen losses.

Sediment-Water Nitrogen Exchange along the Potomac River Estuarine Salinity Gradient

ABSTRACT Cornwell, J.C.; Owens, M.S.; Boynton, W.R., and Harris, L.A., 2016. Sediment-water nitrogen exchange along the Potomac River estuarine salinity gradient. Observations of N2 efflux in estuarine sediments across the salinity gradient of the tidal Potomac River, a eutrophic subestuary of the Chesapeake Bay, were used to evaluate environmental controls of microbial denitrification. Rates of denitrification were measured using N2:Ar ratios in core incubations and were similar to other nitrogen-enriched estuaries, with summer and spring N2-N efflux rates averaging 54 ± 47 and 153 ± 97 μmol m−2 h−1, respectively. The paradigm of higher denitrification rates at lower salinities was not supported by observations during summer and spring conditions along this estuarine salinity gradient. Low bottom water oxygen concentrations in the lower, more saline part of the estuary resulted in low rates of coupled nitrification/denitrification. The most favorable region for denitrification in the tidal Potomac River ...

Summer storms trigger soil N2O efflux episodes in forested catchments

AbstractClimate change and climate‐driven feedbacks on catchment hydrology and biogeochemistry have the potential to alter the aquatic versus atmospheric fate of nitrogen (N) in forests. This study investigated the hypothesis that during the forest growth season, topography redistributes water and water‐soluble precursors (i.e., dissolved organic carbon and nitrate) for the formation of gaseous N species. Soil nitrous oxide (N2O) and nitrogen (N2) efflux and soil physical and chemical properties were measured in a temperate forest in Central Ontario, Canada from 2005 to 2010. Hotspots and hot moments of soil N2O and N2 efflux were observed in topographic positions that accumulate precipitation, which likely triggered the formation of redox conditions and in turn intercepted the conversion of nitrate N flowing to the stream by transforming it to N2O and N2. There was a strong relationship between precipitation and N2O efflux (y = 0.44x1.22, r2 = 0.618, p < 0.001 in the inner wetland; y = 1.30x1.16 r2 = 0.72, p < 0.001 in the outer wetland) and significantly different N2:N2O ratios in different areas of the wetland (19.6 in the inner wetland and 10.1 in the outer wetland). Soil N2O + N2 efflux in response to precipitation events accounted for 16.1% of the annual N input. A consequence of the higher frequency of extreme precipitation events predicted under climate change scenarios is the shift from an aquatic to atmospheric fate for N, resulting in a significant forest N efflux. This in turn creates feedbacks for even warmer conditions due to increased effluxes of potent greenhouse gases.

Nitrous oxide cycling in the water column and sediments of the oxygen minimum zone, eastern subtropical North Pacific, Southern California, and Northern Mexico (23°N-34°N)

Identifying sources and sinks of N2O can illuminate N cycling processes in marine systems, particularly where changes in dissolved O2 can lead to changes in N cycling pathways (i.e., nitrification versus denitrification). We measured N2O and concentration and their stable isotope ratios (δ15N and δ18O) in the water column and sediments of the oxygen minimum zone in the nearshore eastern subtropical North Pacific (23°N–34°N). Atmospheric efflux of N2O ranged from 2.2 to 17.9 μmol m−2 d−1 or about 2–20 times higher than in oxygenated regions of the North Pacific. Surface waters were a source of 15N-depleted and 18O-enriched N2O to the atmosphere, indicating a bacterial, not archaeal, nitrification N2O source. Stable isotopes indicated that nitrification in both surface and intermediate waters (∼0–200 m) was the major source of N2O in this study area, with denitrification acting as a small N2O sink in strongly O2-depleted waters. Denitrification had a larger impact on observed patterns of N2O and concentrations and isotope ratios in the southern oxygen minimum zone. Sediments were generally neutral or a weak sink for N2O, with only one site (Soledad basin) showing a positive efflux of +3.5 ± 1.0 μmol N2O-N m−2 d−1. Sediment fluxes of N2O at all sites were several orders of magnitude smaller than fluxes of dinitrogen, nitrate, and ammonium measured in previous studies and did not appear to impact water column N2O concentrations. N2O was less than 0.1% of the N2 efflux from sedimentary denitrification.

Dinitrogen, oxygen, and nutrient fluxes at the sediment–water interface and bottom water physical mixing on the eastern Chukchi Sea shelf

We measured fluxes of net dissolved N2 and O2, NO3−, and PO4−3 at the sediment–water interface in whole cores in relation to bottom water temperature and salinity in late summer 2010 on the shelf of the eastern Chukchi Sea. Cold (−1.68°C) and high-salinity (33.72) bottom water, characteristic of Pacific winter water, overlaid the sediment over the northeastern region of the shelf while relatively warmer (8.33°C) and less saline (29.88) water with properties of Alaska Coastal Water covered the seabed in the southeastern region. Latitudinal variability of gas fluxes was not linked to differences in temperature or salinity. Relatively higher N2 efflux was measured for sediment located at the water mixing front between cold and warm bottom waters. Benthic O2 consumption in the southeastern region was relatively intense as were NO3− and PO4−3 effluxes from the sediments into the water column. Overall, sediments in the Chukchi Sea are sites of intense denitrification but these losses appear compensated by benthic NO3− regeneration, as NO3− efflux from sediments into the overlying water exceed those of N2. Exposure of sediment cores to incoming light did not affect benthic fluxes significantly compared to dark conditions.

Seasonal, daily and diel N<sub>2</sub> effluxes in permeable carbonate sediments

Abstract. Benthic metabolism and inorganic nitrogen and N2 flux rates (denitrification) were measured in permeable carbonate sands from Heron Island (Great Barrier Reef). Some of the N2 flux rates were among the highest measured in sediments. All benthic fluxes showed a significant difference between seasons with higher rates in summer and late summer. There was no distinct response of the benthic system to mass coral spawning. Instead, changes in benthic fluxes over 12 days in summer appear to be driven by tidal changes in water depth and associated changes in phytosynthetically active radiation reaching the sediments. Dark N2 fluxes were strongly correlated to benthic oxygen consumption across all sites and seasons (r2 = 0.63; p < 0.005; slope = 0.035). However, there were seasonal differences with a steeper slope in summer than winter, reflecting either more efficient coupling between respiration and nitrification–denitrification at higher temperatures or different sources of organic matter. Adding data from published studies on carbonate sands revealed two slopes in the dark N2 flux versus benthic oxygen consumption relationship. The lower slope (0.035) was most likely due to high carbon : nitrogen (C : N) organic matter from coral reefs, and associated assimilation of nitrogen by heterotrophic bacteria including enhanced heterotrophic N-fixation, but competition by benthic microalgae or inefficient coupling between respiration and nitrification–denitrification cannot be excluded. The steeper slope (0.089) was most likely due to respiration being driven by low C : N phytodetritus. If the different slopes were driven by the sources of organic matter, then global estimates of continental shelf denitrification are probably about right. In contrast, global estimates of continental shelf denitrification may be over-estimated if the low slope was due to inefficient coupling between respiration and nitrification–denitrification and also due to reduced N2 effluxes in the light associated with competition by benthic microalgae for nitrogen and N-fixation.

Hydrologic Versus Biogeochemical Controls of Denitrification in Tidal Freshwater Wetlands

Tidal freshwater wetlands (TFW) alter nitrogen concentrations in river water, but the role of these processes on a river’s downstream nitrogen delivery is poorly understood. We examined spatial and temporal patterns in denitrification in TFW of four rivers in North Carolina, USA and evaluated the relative importance of denitrification rate and inundation on ecosystem-scale N2 efflux. An empirical model of TFW denitrification was developed to predict N2 efflux using a digital topographic model of the TFW, a time series of water level measurements, and a range of denitrification rates. Additionally, a magnitude-frequency analysis was performed to investigate the relative importance of storm events on decadal patterns in N2 efflux. Spatially, inundation patterns exerted more influence on N2 efflux than did the range of denitrification rate used. Temporal variability in N2 efflux was greatest in the lower half of the tidal rivers (near the saline estuary) where inundation dynamics exerted more influence on N2 efflux than denitrification rate. N2 efflux was highest in the upper half of the rivers following storm runoff, and under these conditions variation in denitrification rate had a larger effect on N2 efflux than variability in inundation. The frequency-magnitude analysis predicted that most N2 efflux occurred during low flow periods when tidal dynamics, not storm events, affected TFW inundation. Tidal hydrology and riparian topography are as important as denitrification rate in calculating nitrogen loss in TFW; we present a simple empirical model that links nitrogen transport in rivers with loss due to denitrification in TFW.

Differences in benthic metabolism, nutrient fluxes, and denitrification in Caulerpa taxifolia communities compared to uninvaded bare sediment and seagrass (Zostera capricorni) habitats

Benthic metabolism, nutrient fluxes, and denitrification were measured in Caulerpa taxifolia communities that had recently invaded (between 2003 and 2008) bare sediments following the loss of Zostera capricorni (prior to 2003), and in adjacent Z. capricorni communities and bare sediments, to determine the likely changes in ecosystem function following C. taxifolia colonization. Areas colonized by C. taxifolia had higher gross primary production (GPP), net primary production (NPP), and dark benthic N2 effluxes than adjacent bare sediments, but lower dark benthic NH+4 effluxes. C. taxifolia invasion of bare sediments had a beneficial role in removing excess nitrogen from the system through enhanced denitrification. In contrast, areas occupied by C. taxifolia had similar GPP and NPP relative to Z. capricorni beds and similar benthic N2 and NH+4 effluxes. Thus, C. taxifolia appears to play a comparable role to Z. capricorni in terms of some ecosystem functions, although the degree of similarity is likely to be affected by the relative densities of the two macrophytes. However, replacement of Z. capricorni communities by C. taxifolia appears to significantly alter the quantity and quality (higher ratio) of benthic dissolved organic carbon and dissolved organic nitrogen fluxes.

Denitrification, N-fixation and nitrogen and phosphorus fluxes in different benthic habitats and their contribution to the nitrogen and phosphorus budgets of a shallow oligotrophic sub-tropical coastal system (southern Moreton Bay, Australia)

Denitrification, N-fixation, and dissolved inorganic and organic fluxes of nitrogen (N) and phosphorus (P) were measured in each of the major benthic habitat types of a shallow oligotrophic sub-tropical coastal system, and N and P budgets were constructed to quantify the importance of each habitat to N and P cycling in the whole ecosystem. The productivity/respiration (p/r) ratio (trophic status) of the habitats was an important control on the rates, direction (uptake, efflux) and composition (dissolved inorganic N (DIN), dissolved organic N (DON), N2) of N fluxes across the sediment–water interface, with an efflux below p/r = 1.5 and an uptake above p/r = 1.5. The Zostera Seagrass Community was the most important habitat for N loss via net N2 effluxes (denitrification; 48%). Denitrification rates in seagrass were higher than those previously measured in temperate regions, most likely due to greater availability of NH4 + for coupled nitrification–denitrification. Yabby Shoals (sub-tidal shoals inhabited by burrowing shrimp, Trypaea australiensis) accounted for the second largest loss of N via denitrification, the largest recycling of DIN and dissolved inorganic P (DIP; statistically significant only during the dark in summer) across the sediment–water interface and the second largest uptake of DON (statistically significant only in summer). This study highlighted that shallow subtropical coastal systems have a complex mosaic of benthic habitats and that some less ‘iconic’ habitats (i.e. non-seagrass) also make an important functional contribution that controls the flow of N and P through the whole ecosystem.

Denitrification in shallow, sublittoral Gulf of Mexico permeable sediments

We examined nitrogen cycling over a 1‐yr period in shallow, sandy sediments at two contrasting sites near a barrier island in the northeastern Gulf of Mexico, and we provide here the direct determination of dinitrogen gas (N2) production at ambient nitrate concentrations in permeable marine sediments. Nitrogen‐stable isotope tracer techniques were used to quantify N2‐production rates and pathways in sediment cores and slurries. To simulate pore‐water advection, the dominant transport process in the upper layer of the permeable sand beds, intact sediment cores were perfused with aerated seawater. This perfusion increased denitrification rates up to 2‐fold in Apalachicola Bay sands and up to 17‐fold in Gulf of Mexico sublittoral sands, respectively, relative to static cores. Seasonal N2‐production rates were highest in spring and fall. Denitrified nitrate originated almost entirely from benthic nitrification at the exposed Gulf site, whereas water‐column nitrate dominated sedimentary denitrification at the sheltered Bay site. Sediment incubations in stirred chambers were used to determine net fluxes of oxygen (O2), N2, nitrate, and ammonium across the sediment‐water interface during varied degrees of continuous pore‐water exchange. Rates of N2 efflux correlated with rates of pore‐water flow increasing from 0.12 mmol N m‐2 d‐1 under diffusion‐limited transport conditions up to 0.87 mmol N m‐2 d‐1 with pore‐water advection. Mineralized nitrogen was completely converted to N2 gas in Gulf of Mexico sediments. Our results demonstrate that advective pore‐water circulation will accelerate benthic N2 production by coupled nitrification‐ denitrification and that substantial nitrogen loss occurs from coastal permeable sediments.

Identification of phytodetritus‐degrading microbial communities in sublittoral Gulf of Mexico sands

We identified microbial taxa that catalyze phytodetritus degradation and denitrification in permeable coastal sediments of the northeast Gulf of Mexico. Stable isotope probing experiments were used to track the assimilation of isotopically labeled substrate into bacterial deoxyribonucleic acid (DNA) and directly link the taxonomic identification of benthic microorganisms with particulate organic matter degradation and denitrification activity. Phytodetritus deposition events were simulated in the laboratory by the addition of 13C‐enriched, heat‐killed Spirulina cells to intact sediment core incubations. Immediate increases in O2 consumption (3‐fold), N2 efflux (16‐fold), and dissolved inorganic nitrogen efflux were observed after phytodetritus addition relative to unamended treatments, suggesting that the benthic microbial community was poised to immediately begin oxidizing deposited organic matter. Analyses of 16S ribosomal ribonucleic acid gene sequences amplified from 13C‐enriched DNA fractions demonstrated that members of the Gammaproteobacteria (Vibrionales and Alteromonadales), Deltaproteobacteria, Actinobacteria, Verrucomicrobia, and Planctomycetes metabolized the phytodetritus amendment. Terminal restriction length polymorphism analyses showed increases in the relative abundance of Gammaproteobacteria, Planctomycetes, and Bacteroidetes with phytodetritus addition. Alphaproteobacteria were identified as metabolically active denitrifiers by phylogenetic analysis of nitrous oxide reductase gene sequences from 13C‐enriched DNA fractions. This study provides the first identification of microorganisms responsible for organic matter degradation in marine sediments by DNA sequence analysis. Microbial assemblages recognized for high‐molecular‐weight organic matter oxidation in the marine water column were important in catalyzing these processes in permeable sediments.

Nitrogen ebullition in a Colorado plains river

Rates of ebullition and composition of bubbles were measured along a nutrient-enriched segment of the South Platte River below Denver, Colorado. Ebullition was widespread in the South Platte up to 81 km downstream from Denver. Ebullitive fluxes of 0.44 and 0.29 g N m−2 d−1 were recorded at two sites downstream of Denver and represented 6–16% of the diffusive N2 efflux from this region. These data indicate that not accounting for ebullitive N2 losses can, at some locations, lead to a considerable underestimation of dentrification rates using the open-channel (gas exchange) method. Gas bubbles often were >98% N2; methane dominated in a few organic-rich areas. Rates of ebullition related significantly to variations in temperature and dissolved organic carbon. Ebullition was not observed in four tributaries of the South Platte River, despite their moderate to high concentrations of nitrate and dissolved organic carbon. The data demonstrate that ebullition can contribute significantly to N2 effluxes in running waters exhibiting high rates of denitrification.

The Metallo-β-lactamase GOB Is a Mono-Zn(II) Enzyme with a Novel Active Site

Metallo-beta-lactamases (MbetaLs) are zinc-dependent enzymes able to hydrolyze and inactivate most beta-lactam antibiotics. The large diversity of active site structures and metal content among MbetaLs from different sources has limited the design of a pan-MbetaL inhibitor. Here we report the biochemical and biophysical characterization of a novel MbetaL, GOB-18, from a clinical isolate of a Gram-negative opportunistic pathogen, Elizabethkingia meningoseptica. Different spectroscopic techniques, three-dimensional modeling, and mutagenesis experiments, reveal that the Zn(II) ion is bound to Asp120, His121, His263, and a solvent molecule, i.e. in the canonical Zn2 site of dinuclear MbetaLs. Contrasting all other related MbetaLs, GOB-18 is fully active against a broad range of beta-lactam substrates using a single Zn(II) ion in this site. These data further enlarge the structural diversity of MbetaLs.

Effects of Hydrological Flow Regime on Sediment-water Interface and Water Column Nitrogen Dynamics in a Great Lakes Coastal Wetland (Old Woman Creek, Lake Erie)

Sediment-water interface nitrogen (N) transformations and water column ammonium cycling rates were measured along a stream to lake gradient at three sites within Old Woman Creek (OWC) and one near-shore Lake Erie site during two hydrological regimes: one with open flow to the lake after a rain event (July 2003), and another with a sand barrier blocking flow (July 2004). Net N2 effluxes in OWC at all times and at the near-shore Lake Erie site in July 2003 suggest that sediments are a N sink via denitrification. Observed dissimilatory nitrate reduction to ammonium (DNRA) may counteract some of this N removal, particularly when the creek mouth is closed. Upstream, a closed creek mouth led to higher sediment oxygen demand, net N 2 flux, potential DNRA, and potential denitrification rates. The lake site exhibited lower rates of these processes with the creek mouth closed except denitrification potential, which was unchanged. Denitrification in OWC appeared to drive N limitation in the lower wetland when the sand barrier was blocking flow to the lake. Higher potential versus in situ denitrification estimates imply that water column NO 3 − limits and drives denitrification in OWC. Water column to sediment regeneration ratios suggest that sediment recycling may drive primary production in the OWC interior when the creek mouth is closed and new N inputs from runoff are absent, but more data are needed to confirm these apparent trends. Overall, hydrological regime in OWC appeared to have a greater impact on sediment N processes than on water column cycling.

Benthic metabolism and nitrogen cycling in a subtropical east Australian estuary (Brunswick): Temporal variability and controlling factors

We examined temporal variability in benthic metabolism and nitrogen (N) cycling in the subtropical Brunswick Estuary, Australia from December 2000 to December 2002. Benthic metabolism was tightly coupled to the production of labile carbon (C) in the water column (phytodetritus) and temperature, both of which increased in summer, resulting in increased rates of benthic metabolism and a shift to sulfate reduction. C : N ratios of remineralized material showed a consistently low return of N relative to ‘‘Redfield’’-type material over the 2-yr study period (up to 84 : 1 and averaging 31 : 1). The highest remineralized C : N ratios occurred at a sediment CO2 productivity/ respiration of ;0.4, which corresponds to maximum respiration and maximum productivity, and also when watercolumn dissolved inorganic nitrogen concentrations were lowest, reflecting potential N limitation. The ‘‘missing N’’ was most likely assimilated by heterotrophic bacteria and autotrophic benthic microalgae (BMA). Extracellular organic carbon extruded by the BMA and subsequently decomposed may also account for some of the high remineralized C : N ratios. Net N2 effluxes were controlled by a complex interaction between the supply of NO from 2 3 the water column and nitrification, the supply and decomposition of labile C, benthic productivity, and macrofauna abundance. A N mass balance for the sediment over the 2-yr study period showed that a significant proportion of the mineralized nitrogen may have been removed from the microbial loop and passed up to the metazoan levels of the food chain. About 22% of the remineralized N was permanently removed via denitrification. This active competition for limited N resources between heterotrophs, autotrophs, and chemoautotrophs appears to be a mechanism by which N-limited oligotrophic subtropical estuaries tightly recycle and conserve N. The metabolism of benthic communities is a central component of the nutrient cycling and overall productivity of shallow coastal ecosystems. Much of the carbon decomposition in shallow coastal ecosystems occurs in bottom sediments through heterotrophic-mediated processes (Hammond et al. 1985), and the associated release of nutrients can provide a large proportion of the nitrogen and phosphorus required by pelagic communities (Boynton and Kemp 1985; Cowan and Boynton 1996). Bottom sediments can also be a sink for nutrients through the burial of carbon (C), nitrogen (N), and phosphorus; the loss of carbon through CO2 efflux; and the loss of N through denitrification. Denitrification (the reduction of NO through NO to nitrous oxide [N2O] and 22 32

Effect of natural populations of burrowing thalassinidean shrimp on sediment irrigation, benthic metabolism, nutrient fluxes and denitrification

The aim of this study was to determine the effect of burrowing by natural populations of Trypaea australiensis on benthic metabolism, nutrient flux, denitrification and irrigation rates using in situ benthic chambers and exclusion nets to minimise disturbance and associated artefacts. The burrows of T. australiensis enhanced total sediment porewater exchange rates 5-fold compared to uninhabited sediments. Calculated single-burrow flow rates were <0.456 l h -1 , and active burrow irri- gation appeared to be linked to tidal movement; as such, ex situ studies (i.e. mesocosm or laboratory incubations) may underestimate burrow irrigation rates. The presence of T. australiensis increased sediment oxygen demand by 81% as compared to unoccupied sediments. Approximately 15% of this additional consumption was used by T. australiensis for respiration, with the remaining 85% con- sumed within the burrows by oxidation reactions and microbial respiration. The proportion of smaller sediment particles, porosity and organic matter content was increased in the presence of T. australi- ensis, and sediment surface chl a concentrations were ca. 50% lower than uninhabited sediments. Net denitrification rates were 4 times greater in the presence of active T. australiensis burrows and at den- sities of 22 burrows m -2 were responsible for approximately 76% of total denitrification in the study site sediments. NH4 + efflux was higher in the inhabited sediments and NO3 - and NO2 - effluxes were lower, suggesting that increased NH4 + mineralisation may have enhanced denitrification via the close coupling of nitrification-denitrification in the inhabited sediments. As such, T. australiensis appears to contribute significantly to net sediment N2 efflux in lower subtropical estuarine environments.