Nitrate Depletion Research Articles

Denitrification in a hyporheic riparian zone controlled by river regulation in the Seine river basin (France)

AbstractThe purpose of this paper is to study denitrification and the conditions for its development in a hyporheic zone. The study site is the riparian zone of a former branch of the Seine River, where the river stage is kept almost constant during the year by hydraulic regulation. Hydrological and geochemical surveys were performed by monitoring four wells, ten shorter piezometers and the river over a 15‐month period. The water fluxes originating from the chalky hillsides and the river converge in a zone parallel to the river that acts as a drainage flow path through the floodplain. The riparian zone between this flow path and the river shows an important depletion of nitrate during the summer and autumn period, which cannot be explained by a simple mixing of waters coming from the river and the chalky hillsides. It can be attributed to denitrification as it occurs when oxygen concentration is below 2 mg l−1, and goes along with a consumption of dissolved organic carbon and a decrease of redox potential. The river completely controls these hydro‐geochemical conditions. It also keeps the wetness of the riparian zone almost constant, which allowed us to isolate the high temperatures in summer and autumn as an important triggering factor for denitrification through its influence on the reaction rate and oxygen deficits. We also found a small isotopic enrichment of nitrate, suggesting that denitrification occurs after diffusion of nitrate through the sediment and riparian zone matrix, which is consistent with the hyporheic functioning of the study site. Copyright © 2008 John Wiley & Sons, Ltd.

Nitrate-related down-regulation of respiratory nitrate reductase from Bradyrhizobium sp. (Lupinus).

Previously, we showed that anaerobic induction of respiratory nitrate reductase (NR) activity in Bradyrhizobium sp. (Lupinus) USDA 3045 is strongly enhanced by nitrate or nitrite through de novo synthesis. Here, multiple NR-active soluble forms, ranging from 75 kDa to 190 kDa, were observed under anaerobic conditions. Electrophoretic activity band patterns differed depending on the level and the type of the N oxyanion added. The intensity of the membrane-bound NR activity band of 230 kDa changed with time along with consumption of 2 mM nitrate. It was associated with a parallel 5-fold increase and then 2-fold reduction in the amount of membrane-bound NR protein. In contrast, on 4 mM nitrate, the level of NR protein was much more stable, apparently due to slower nitrate depletion. Moreover, in cells anaerobically grown without nitrate addition, a 42-kDa derivative of NR degradation was immunodetected, which was not observed if nitrate was present in the medium. These findings suggest that the amount of the respiratory NR protein could be negatively regulated by endogenous proteases in relation to the level of nitrate available. It seems, therefore, that multiple native forms might be not different isoenzymes but immature complexes or derivatives of the enzyme protein turnover. This report adds to a modest list of bacterial enzymes apparently regulated by proteolysis, such as GS, MurAA, EnvA, GdhA, and MetA.

Phosphorylation of the Yeast Nitrate Transporter Ynt1 Is Essential for Delivery to the Plasma Membrane during Nitrogen Limitation

Ynt1 is the sole high affinity nitrate transporter of the yeast Hansenula polymorpha. It is highly regulated by the nitrogen source, by being down-regulated in response to glutamine by repression of the YNT1 gene and Ynt1 ubiquitinylation, endocytosis, and vacuolar degradation. On the contrary, we show that nitrogen limitation stabilizes Ynt1 levels at the plasma membrane, requiring phosphorylation of the transporter. We determined that Ser-246 in the central intracellular loop plays a key role in the phosphorylation of Ynt1 and that the nitrogen permease reactivator 1 kinase (Npr1) is necessary for Ynt1 phosphorylation. Abolition of phosphorylation led Ynt1 to the vacuole by a pep12-dependent end4-independent pathway, which is also dependent on ubiquitinylation, whereas Ynt1 protein lacking ubiquitinylation sites does not follow this pathway. We found that, under nitrogen limitation, Ynt1 phosphorylation is essential for rapid induction of nitrate assimilation genes. Our results suggest that, under nitrogen limitation, phosphorylation prevents Ynt1 delivery from the secretion route to the vacuole, which, aided by reduced ubiquitinylation, accumulates Ynt1 at the plasma membrane. This mechanism could be part of the response that allows nitrate-assimilatory organisms to cope with nitrogen depletion.

Seasonal dynamics of new production from Trichodesmium N2 fixation and nitrate uptake in the upstream Kuroshio and South China Sea basin

The seasonal dynamics of primary production in the upstream Kuroshio was studied to understand why pelagic fish are relatively populous in this oligotrophic tropical ocean region. The relative importance of new production from nitrate uptake vs. Trichodesmium N2 fixation was compared to differentiate the seasonal production dynamics in the upstream Kuroshio from that in the neighboring and interacting northern South China Sea (SCS). Unlike the SCS, which was most productive in the winter, seasonal fluctuation of primary production (0.51‐0.53 g C m−2 d−1) was not apparent in the upstream Kuroshio. The seasonality in new production in the Kuroshio, in contrast, was pronounced. Its nitrate‐uptake‐based new production was high in the winter (0.27 g C m−2 d−1) and low in the summer (0.16 g C m−2 d−1); conversely, Trichodesmium N2‐fixation‐based new production was low in the winter (2.4 µmol N m−2 d−1) and high in the summer (168.1 µmol N m−2 d−1). In the summer when nitrate‐uptake‐based new production was low due to nitrate depletion, the Kuroshio remained productive because of enhanced Trichodesmium N2 fixation from strong stratification and a deepened nitracline. The standing crop of Trichodesmium was consistently much higher in the Kuroshio (34.61 × 106 and 0.49 × 106 trichomes m−2 in the summer and winter, respectively) than in the SCS (4.87 × 106 and 0.15 × 106 trichomes m−2, respectively). In both regions, surface water temperature correlated significantly with increased Trichodesmium stocks. At a given temperature, higher Trichodesmium standing crop in the Kuroshio than the SCS was related to the deeper nitracline and ensuing decrease in nitrate availability in the Kuroshio.

Bacterial Succession within an Ephemeral Hypereutrophic Mojave Desert Playa Lake

Ephemerally wet playas are conspicuous features of arid landscapes worldwide; however, they have not been well studied as habitats for microorganisms. We tracked the geochemistry and microbial community in Silver Lake playa, California, over one flooding/desiccation cycle following the unusually wet winter of 2004-2005. Over the course of the study, total dissolved solids increased by approximately 10-fold and pH increased by nearly one unit. As the lake contracted and temperatures increased over the summer, a moderately dense planktonic population of approximately 1x10(6) cells ml(-1) of culturable heterotrophs was replaced by a dense population of more than 1x10(9) cells ml(-1), which appears to be the highest concentration of culturable planktonic heterotrophs reported in any natural aquatic ecosystem. This correlated with a dramatic depletion of nitrate as well as changes in the microbial community, as assessed by small subunit ribosomal RNA gene sequencing of bacterial isolates and uncultivated clones. Isolates from the early-phase flooded playa were primarily Actinobacteria, Firmicutes, and Bacteroidetes, yet clone libraries were dominated by Betaproteobacteria and yet uncultivated Actinobacteria. Isolates from the late-flooded phase ecosystem were predominantly Proteobacteria, particularly alkalitolerant isolates of Rhodobaca, Porphyrobacter, Hydrogenophaga, Alishwenella, and relatives of Thauera; however, clone libraries were composed almost entirely of Synechococcus (Cyanobacteria). A sample taken after the playa surface was completely desiccated contained diverse culturable Actinobacteria typically isolated from soils. In total, 205 isolates and 166 clones represented 82 and 44 species-level groups, respectively, including a wide diversity of Proteobacteria, Bacteroidetes, Actinobacteria, Firmicutes, Gemmatimonadetes, Acidobacteria, and Cyanobacteria.

Temporal variation of diatom benthic propagules in a monsoon-influenced tropical estuary

Temporal variations in the diatom benthic propagule (DBP) community and their role in the phytoplankton community in a monsoon-affected tropical estuary, Zuari estuary, Goa (India) are presented. The DBP from the sediments was enumerated using an extinction dilution method (most probable number method), which allows estimation of resting stages through examination of germinated vegetative cells in culture. The DBP community was dominated by planktonic species belonging to the genera Skeletonema, Fragilariopsis, Thalassiosira, and Chaetoceros. Benthic propagules (BPs) of Skeletonema costatum and Fragilariopsis sp. were dominant throughout the year. Between these two species, only S. costatum showed a linear relationship between the BP and planktonic cells, indicating that this species is particularly important in coupling of pelagic and benthic ecosystems. During the onset and restart of monsoon after an intermittent break, water column was stratified, with a low-salinity layer arising from riverine discharge and precipitation at the surface and relatively cold, saline, low-oxygen waters at the bottom. Stratification favored blooming of S. costatum and Fragilariopsis sp. in nutrient-rich surface and bottom waters, respectively. The decline in these blooms ensuing nitrate depletion and salinity change resulted in an increased abundance of BP. Chaetoceros bloom was observed during the monsoon break as well as during non-monsoon period and on both the occasions the decline in bloom was coupled with freshwater discharge. During the non-monsoon season, Thalassiosira blooms were encountered subsequent to high nitrate inputs. These findings suggest that in such shallow tropical regions, physical processes during monsoon (freshwater discharge) and non-monsoon seasons (currents, waves and tides) cause resuspension of diatom BP. Since light is not a limiting factor for germination in such regions, the blooming of resuspended BP depends on nutrient availability.

Insights into nutrient assimilation and export in naturally iron-fertilized waters of the Southern Ocean from nitrogen, carbon and oxygen isotopes

Abstract The KErguelen Ocean and Plateau compared Study (KEOPS) documented enhanced iron input and phytoplankton biomass over the deep Kerguelen plateau in comparison to surrounding high-nutrient low-chlorophyll (HNLC) waters in late summer 2005. We examined the influence of this iron on nitrogen and carbon metabolism by the microbial food-web, by comparing samples from on-plateau and off-plateau. Suspended particulate organic carbon (POC) was ∼5 times more abundant on-plateau and exhibited greater POC/PON (∼6.5 vs. ∼5.5), δ13C-POC (∼−21.5 vs. ∼−24.5‰) and δ15N-PON (∼+2 vs. ∼0‰) than off-plateau. These differences arose in part from changes in ecosystem structure as demonstrated by size-fractionation (1, 5, 20, 55, 210, and 335-μm filters in series), which revealed large isotopic variations with size (δ13C-POC ranged from −28 to −19‰ and δ15N-PON from −3 to +5‰) and greater abundances of 13C- and 15N-enriched large phytoplankton over the plateau. The 13C enrichment in POC reflected faster growth rates and greater draw-down of dissolved inorganic carbon over the plateau. Quantitative comparison to the δ15N of dissolved nitrate indicates that the δ15N-PON enrichment derived from increased assimilation of nitrate, corresponding to new production f-ratios of 0.7–0.9 on-plateau vs. 0.4–0.6 off-plateau. Results from a sparse set of free-drifting sediment trap samples suggest control of export by zooplankton grazing. The 15N and 18O enrichments in dissolved nitrate exhibited a 1:1 correlation, indicating that phytoplankton assimilation controls nitrate availability and only a relatively small amount of nitrate was regenerated by nitrification. The δ15N-NO3 values yield indistinguishable isotopic fractionation factors on and off the plateau ( 15 e NO 3 of 4–5‰). This suggests that variations in iron availability may not bias the interpretation of paleo-environmental 15N records, and leaves intact the view that higher sedimentary δ15N-PON values during the last glacial maximum indicate greater fractional nitrate depletion in the Southern Ocean.

Testing a Simple Field Method for Assessing Nitrate Removal in Riparian Zones1

Abstract: Being able to identify riparian sites that function better for nitrate removal from groundwater is critical to using efficiently the riparian zones for water quality management. For this purpose, managers need a method that is quick, inexpensive, and accurate enough to enable effective management decisions. This study assesses the precision and accuracy of a simple method using three ground water wells and one measurement date for determining nitrate removal characteristics of riparian buffer zones. The method is a scaled‐down version of a complex field research method that consists of a large network of wells and piezometers monitored monthly for over two years. Results using the simplified method were compared to those from the reference research method on a date‐by‐date basis on eight sites covering a wide range of hydrogeomorphic settings. The accuracy of the three‐well, 1 day measurement method was relatively good for assessing nitrate concentration depletion across riparian zones, but poor for assessing the distance necessary to achieve a 90% nitrate removal and for estimating water and nitrate fluxes compared to the reference method. The simplified three‐well method provides relatively better estimates of water and nitrate fluxes on sites where ground‐water flow is parallel to the water table through homogeneous aquifer material, but such conditions may not be geographically widespread. Despite limited overall accuracy, some parameters that are estimated using the simplified method may be useful to water resource managers. Nitrate depletion information may be used to assess the adequacy of existing buffers to achieve nitrate concentration goals for runoff. Estimates of field nitrate runoff and buffer removal fluxes may be adequate for prioritizing management toward sites where riparian buffers are likely to have greater impact on stream water quality.

The response of microzooplankton (20–200 μm) to coastal upwelling and summer stratification in the southeastern Arabian Sea

During the late summer monsoon (LSM), coastal upwelling and river runoff have increased the nutrient concentration in the inshore region of the southeastern Arabian Sea. This has resulted in elevated chlorophyll- a (av. 40±18 mg m −2), phytoplankton abundance (av. 474±116×10 6 ind. m −2) and primary production (av. 917±616 mg C m −2 d −1). Diatoms were the major component of the phytoplankton community (av. 60±8% of the total abundance) during this period, followed by dinoflagellates (18±12%). However, the inshore locations of 10°N and 13°N transects behaved differently, with an exceptional abundance of phytoflagellates (>10 10 ind. m −2). The microzooplankton (MZP) community was comprised of heterotrophic dinoflagellates (av. 60%), ciliates (av. 30%) and copepod nauplii (av. 5%); these showed marked variation in abundance between stations. The highest abundance (av. 283×10 4 ind. m −2) was at the inshore location of 10°N transect, where phytoflagellates were abundant. Contrasting to the LSM, intense surface layer stratification and depletion of nitrate (with a nitracline at 60 m depth), observed in the southeastern Arabian Sea during the spring intermonsoon (SIM) period, caused low phytoplankton abundance (av. 141±86×10 6 ind. m −2), chlorophyll- a (av. 19±11 mg m −2) and primary production (av. 146±68 mg C m −2 d −1). High temperature (>29 °C) and nitrate-depleted surface waters favoured the proliferation of Trichodesmium erythraeum at most of the locations. Total abundance of MZP during the SIM was markedly low (av. 20×10 4±13×10 4 ind. m −2), as was the species richness and diversity (0.36±0.02 and 0.57±0.15, respectively) compared to the LSM (1.17±0.31 and 2.72±0.34, respectively). During the SIM, the MZP community that occurs in the southeastern Arabian Sea is important, since smaller individuals are widespread and form the majority of phytoplankton community. The present study points also to the fact that the MZP could play an important role, even in nutrient-enriched environment, if smaller phytoplankton are abundant.

Lateral Hyporheic Zone Chemistry in an Artificially Constructed Gravel Bar and a Re‐Meandered Stream Channel, Southern Ontario, Canada1

Abstract: The effect of stream restoration on hyporheic functions has been neglected, although channel rehabilitation projects have a potential to alter stream‐ground‐water interactions. The present study examined the effect of an artificially constructed gravel bar and re‐meandered stream channel on lateral hyporheic exchange flow and chemistry in two lowland N‐rich streams in southern Ontario, Canada. Nitrate concentrations were relatively high, ranging from 0.5 to 1.3 mg N/l in both streams during spring through fall months. However, nitrate concentrations showed a steep decline as stream water entered the gravel bar and the meander bends. Differences between observed and predicted nitrate concentrations based on conservative ion concentration patterns indicated that 40‐100 and 68‐98% of the nitrate entering the hyporheic zone was removed in the gravel bar and meanders, respectively. Rapid depletion of dissolved oxygen concentrations along lateral hyporheic flow paths and denitrifying potentials assayed by the acetylene block technique in hyporheic sediments suggests that denitrification was an important mechanism of nitrate depletion. Despite the high rate of nitrate removal, the flux of stream water laterally entering the constructed gravel bar and meander bends was very small, and hyporheic nitrate removal was <0.015% of the daily stream load during base‐flow periods in summer and fall. The effects of restoration projects on hyporheic zone dynamics are often limited in lowland streams by low channel gradients and fine floodplain sediments with low interstitial flows that restrict the magnitude of the stream‐hyporheic connection.

In situ reductive dechlorination of chlorinated ethenes in high nitrate groundwater

In situ bioremediation using carbohydrate was evaluated as an in situ treatment alternative for trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE) in groundwater containing high nitrate concentrations. Upon addition of carbohydrate to groundwater, sequential reduction of electron acceptors was observed, where nitrate was reduced early in the pilot test, followed by sulfate and TCE. Reduction of cDCE to vinyl chloride and ethene occurred in conjunction with increased iron and manganese, and increased methane concentrations, approximately 7 months into the evaluation period, following depletion of nitrate and sulfate. TCE, cDCE, and vinyl chloride concentrations decreased from approximately 500 to >10 μg/L within 21 months of operation.

Examining the effects of nutrients on the composition and size of phytoplankton using unaltered deep-sea waters

Effects of nutrients on the size and composition of phytoplankton and the resulting changes in ecosystem structure were investigated using incubations of unaltered deep-sea waters from 400 m and 700 m depths of Suruga Bay Japan. This new approach allowed us to observe the succession of phytoplankton communities from conditions of high to low nitrogen availability and explore the effects of N source and availability on the composition and size of diatoms under low grazing pressure. Diatoms dominated the phytoplankton biomass throughout the duration of experiments in both incubations. However, among diatoms, the genus Thalassiosira dominated the abundance and biomass when nutrients were replete, and the genus Chaetoceros dominated the abundance and biomass when nitrate became depleted. Our results show that diatoms can dominate other phytoplankton groups not only under nutrient replete conditions, but also (particularly for Chaetoceros) under low concentrations of recycled nitrogen if silica and phosphate are not deficient. Diatoms < 15 μm dominated the biomass in the 700 m incubation whereas those > 15 μm dominated the biomass in the 400 m incubations during the formation of blooms under nitrate-replete conditions. After nitrate depletion, diatoms > 15 μm dominated the biomass in both incubations. Our results showed that availability of N determined the composition of diatoms if Si and P are not deficient. However, for size the results showed that N source and availability did not determine the size of phytoplankton. Our results also showed that the molar ratio of Si:N uptake was < 1 for diatoms under nutrient replete conditions. Further the Si:N drawdown ratio decreased with the increasing abundance of smaller diatoms (< 15 μm).

Carbon and nitrogen metabolism during Dendrobium huoshanense protocorm-like body development in suspension culture

The aim of this work was to study carbon and nitrogen metabolism during suspension culture of protocorm-like bodies (PLBs) from Dendrobium huoshanense. No significant lag phase of PLB growth was found, and a maximum biomass of 288.6 g l−1 was obtained at day 30 of culture. Sucrose concentration was halved as PLB growth proceeded, while no change in glucose and fructose levels in the medium was found in the first 3 days, followed by a gradual increase until day 9 of culture. Conversely, sucrose in PLBs accumulated dramatically in the first 6 days of culture, followed by a rapid decrease. At the same time, glucose and fructose content of PLBs increased, then declined after 9 days of culture. Soluble acidic invertase (soluble acidic IT) and alkaline invertase (alkaline IT) were activated after inoculation, and reached the highest value on day 6 and day 18, respectively whereas cell wall-bound invertase (cell wall-bound IT) seemed to be repressed throughout culture. The maximum value of sucrose synthase (SuSy) activity was observed on day 18, while sucrose phosphate synthase (SPS) stayed low and constant from inoculation to the end of culture. Ammonium concentration in the medium decreased rapidly, and was hardly detectable after 12 days, when the rapid utilization of nitrate began. Conversely, ammonium in PLBs showed a sharp increase in the first 3 days of culture, followed a rapid decrease until day 12, corresponding to nitrate depletion. Peaks in glutamine synthase (GS) and glutamate synthase (GOGAT) activity were observed on day 12 and 15, respectively. Nitrate reductase (NR) was repressed in the early culture stage, and activated from day 9 to 15 of culture. These results suggest that soluble acidic IT, alkaline IT, SuSy, GS, GOGAT and NR control carbon and nitrogen metabolism at different PLB growth stages.

Temporal evolution of organic carbon and nitrogen forms in volcanic soils under broom scrub affected by a wildfire

The evolution of total N, total oxidizable C, water-soluble NH(4)(+)-N, exchangeable NH(4)(+)-N and soluble NO(3)(-)-N was studied in Canarian volcanic soils under mountainous legume scrub affected by a wildfire by June 2003. Three systematic soil samplings in the burned area and in neighbouring non-burned sampling points were carried out 3, 7 and 12 months after the fire event. The results showed an important mobilization of N (as total N and soluble and exchangeable NH(4)(+)-N) in the soil within the burned area at short term, with a simultaneous depletion of nitrates. Later on, the water-soluble NH(4)(+)-N levels remained nearly constant along the study period in the burned area, whereas the exchangeable NH(4)(+) decreased progressively. Nitrates were found to increase inside and outside the burned area, but the increase rate was much higher for the burned samples. Total N fluctuated along the year, although its levels were generally higher in the burned area. However, such a response pattern of N to fire in this environment was insufficient to prompt the recovery of the plant cover.

DIN Retention‐Transport Through Four Hydrologically Connected Zones in a Headwater Catchment of the Upper Mississippi River1

Abstract: Dissolved inorganic nitrogen (DIN) retention‐transport through a headwater catchment was synthesized from studies encompassing four distinct hydrologic zones of the Shingobee River Headwaters near the origin of the Mississippi River. The hydrologic zones included: (1) hillslope ground water (ridge to bankside riparian); (2) alluvial riparian ground water; (3) ground water discharged through subchannel sediments (hyporheic zone); and (4) channel surface water. During subsurface hillslope transport through Zone 1, DIN, primarily nitrate, decreased from ∼3 mg‐N/l to &lt;0.1 mg‐N/l. Ambient seasonal nitrate:chloride ratios in hillslope flow paths indicated both dilution and biotic processing caused nitrate loss. Biologically available organic carbon controlled biotic nitrate retention during hillslope transport. In the alluvial riparian zone (Zone 2) biologically available organic carbon controlled nitrate depletion although processing of both ambient and amended nitrate was faster during the summer than winter. In the hyporheic zone (Zone 3) and stream surface water (Zone 4) DIN retention was primarily controlled by temperature. Perfusion core studies using hyporheic sediment indicated sufficient organic carbon in bed sediments to retain ground water DIN via coupled nitrification‐denitrification. Numerical simulations of seasonal hyporheic sediment nitrification‐denitrification rates from perfusion cores adequately predicted surface water ammonium but not nitrate when compared to 5 years of monthly field data (1989‐93). Mass balance studies in stream surface water indicated proportionally higher summer than winter N retention. Watershed DIN retention was effective during summer under the current land use of intermittently grazed pasture. However, more intensive land use such as row crop agriculture would decrease nitrate retention efficiency and increase loads to surface water. Understanding DIN retention capacity throughout the system, including special channel features such as sloughs, wetlands and floodplains that provide surface water‐ground water connectivity, will be required to develop effective nitrate management strategies.

Modeling studies investigating the causes of preferential depletion of silicic acid relative to nitrate during SERIES, a mesoscale iron enrichment in the NE subarctic Pacific

Abstract Numerical modeling experiments were conducted to examine the reasons for observed changes in the silicic acid ([Si(OH)4]) to nitrate ( [ NO 3 - ] ) drawdown ratio after the onset of algal iron stress during SERIES. During phytoplankton blooms and immediately after them, cells encounter a range of iron stress (between iron-replete and iron-deplete) and therefore show a range of growth rates. For these reasons, the potential influence of phytoplankton growth rate, under conditions of algal iron stress, on silicic acid and nitrate depletion were investigated in numerical experiments by altering the timing of a shift in the [Si(OH)4]: [ NO 3 - ] uptake ratio. These simulations suggested that the continued growth of iron-stressed phytoplankton at sub-maximum rates, with an elevated [Si(OH)4]: [ NO 3 - ] uptake ratio, induced depletion of silicic acid in the surface water and resulted in simultaneous limitation of growth by both iron and silicic-acid supply. Therefore, bottom-up control played an important role in terminating the phytoplankton bloom in SERIES. In the model simulations, the enhancement of diatom silicification due to increased rates of biomass-normalized silicic-acid uptake, led to increases in the export flux of opal after the onset of algal iron-stress and, consequently, it stimulated the silica pump. The regulation of both the [Si(OH)4]: [ NO 3 - ] uptake ratio and the growth rate of phytoplankton by iron supply are important factors that determine the relative consumption of silicic acid and nitrate upon iron stress, although the potential influence of a floristic shift in the diatom assemblage cannot be ruled out. These findings offer insights into the impact of iron fertilization, both artificial and natural, on the biogeochemical cycling of nutrients in high-nitrate, low-chlorophyll waters.

Models of oxic respiration, denitrification and sulfate reduction in zones of coastal upwelling

Coastal upwelling zones support some of the highest rates of primary production in the oceans. The settling and subsequent decomposition of this organic matter promotes oxygen depletion. In the Eastern tropical North and South Pacific and the Arabian Sea, large tracts of anoxic water develop, where intensive N 2 production through denitrification and anammox accounts for about 1/3 of the total loss of fixed nitrogen in the marine realm. It is curious that despite extensive denitrification in these waters, complete nitrate removal and the onset of sulfate reduction is extremely rare. A simple box model is constructed here to reproduce the dynamics of carbon, oxygen and nutrient cycling in coastal upwelling zones. The model is constructed with five boxes, where water is exchanged between the boxes by vertical and horizontal mixing and advection. These primary physical drivers control the dynamics of the system. The model demonstrates that in the absence of nitrogen fixation, the anoxic waters in a coastal upwelling system will not become nitrate free. This is because nitrate is the limiting nutrient controlling primary production, and if nitrate concentration becomes too low, primary production rate drops and this reduces rates of nitrate removal through N 2 production. With nitrogen fixation, however, complete nitrate depletion can occur and sulfate reduction will ensue. This situation is extremely rare in coastal upwelling zones, probably because nitrogen-fixing bacteria do not prosper in the high nutrient, turbid waters as typically in these areas. Finally, it is predicted here that the chemistry of the upwelling system will develop in a similar matter regardless whether N 2 production is dominated by anaerobic ammonium oxidation (anammox) or canonical heterotrophic denitrification.

Analysis of nitrite reductase (nirK and nirS) genes and cultivation reveal depauperate community of denitrifying bacteria in the Black Sea suboxic zone

Chemical profiles of the Black Sea suboxic zone show a distribution of nitrogen species which is traditionally associated with denitrification, i.e. a secondary nitrite maximum associated with nitrate depletion and a N(2) gas peak. To better understand the distribution and diversity of the denitrifier community in the Black Sea suboxic zone, we combined a cultivation approach with cloning and sequencing of PCR-amplified nitrite reductase (nirS and nirK) genes. The Black Sea suboxic zone appears to harbour a homogeneous community of denitrifiers. For nirK, over 94% of the sequences fell into only three distinct phylogenetic clusters, and for nirS, a single closely related sequence type accounted for 91% of the sequences retrieved. Both nirS and nirK genes showed a dramatic shift in community composition at the bottom of the suboxic zone, but overall, nirK-based community composition showed much greater variation across depths compared with the highly uniform distribution of nirS sequences throughout the suboxic zone. The dominant nirK and nirS sequences differed at the amino acid level by at least 17% and 8%, respectively, from their nearest database matches. Denitrifying isolates recovered from the suboxic zone shared 97% 16S rRNA gene sequence similarity with Marinobacter maritimus. Analysis of the recently discovered nirS gene from the anammox bacterium Candidatus'Kuenenia stuttgartiensis' revealed that mismatches with commonly used primers may have prevented the previous detection of this divergent sequence.

The effects of land use on stream nitrate dynamics

Summary The effects of land use and land use change on stream nitrate are poorly understood. While case studies have been presented, most process work has been done in areas with one land use (minimally disturbed or agricultural) and areas with substantial atmospheric deposition. In this paper we present results from three neighboring headwater catchments in western Oregon with similar (low) atmospheric deposition, size, and geology but with different, spatially consistent land use expressions: forest, agriculture, and residential. The climate in western Oregon has a distinct pattern of a three-month rainless period in the summer, a wetting up with many storms in the fall and winter, and a decrease of storms in the spring. We investigate how human activity alters the export of nitrate, whether the input of nitrate changes throughout the year which may affect storm response (i.e., depletion of soil water nitrate, addition of fertilizer, etc.), and how the changing contribution of source waters throughout the year affects streamflow concentrations. Our results showed marked differences in export rates between the three catchments. The forested catchment showed minimal export for three monitored storms (fall, winter, spring) through the seasonal wetting up of the catchments, and the residential catchment showed high export for all three storms. While the agricultural catchment displayed elevated export in the fall (similar to the residential catchment), exports decreased progressively throughout the rainy period (following late summer manure and green bean application). Overall, our results of storm event nitrate concentrations suggest that varying nitrate inputs have a large affect on nitrate dynamics. While within-storm nitrate concentration response patterns in the residential catchment were the same as the patterns in the reference forested catchment (a ‘‘concentration’’ pattern throughout the year), a ‘‘dilution’’ pattern was observed in the fall and winter and a ‘‘concentration’’ pattern was observed in the spring in the agricultural catchment.

Cochlodinium polykrikoides(Dinophyceae)에서 분비되는 유기물질의 특성

Organic substances are released from phytoplankton cells during all phases of growth. The type and amounts of organic substance excreted and the effects of nutrient limitation are often highly species-specific. Dinoflagellate, Cochlodinium polykrikoides grown in batch culture produced an exopolysaccharide. Exopolysaccharide and intracellular polysaccharide concentrations increased as C. polykrikoides cultures progressed from exponential phase, through stationary phase, to declining phase. In the exponential phase, the concentration of exopolysaccharide was relatively low, but in the stationary phase, it showed a rapid increase which seemed to coincide with the depletion of nitrate from the medium. Of the 20 amino acids analyzed, proline dominated in the organic matter of all cultures ranging from 48.2 to 79.9 nmol L–1, and constituting the 20-90% of total amino acids, and followed by histamine varying from 0.7 to 47.5 nmol L–1. Leucine and cysteine were also abundant in the stationary phase. The release rates of exopolysaccharide and intracellualr polysaccharide were higher the end of stationary phase than in the exponential phase. Exopolysaccharide concentration per cell was more than two times higher during the end of stationary phase than that in exponential phase. C. polykrikoides produced extracellular polysaccharide at a rate of 47.04 pg cell–1 day–1.