Organic Nitrogen Release Research Articles

Bacterial transcriptional response to labile exometabolites from photosynthetic picoeukaryote Micromonas commoda

Dissolved primary production released into seawater by marine phytoplankton is a major source of carbon fueling heterotrophic bacterial production in the ocean. The composition of the organic compounds released by healthy phytoplankton is poorly known and difficult to assess with existing chemical methods. Here, expression of transporter and catabolic genes by three model marine bacteria (Ruegeria pomeroyi DSS-3, Stenotrophomonas sp. SKA14, and Polaribacter dokdonensis MED152) was used as a biological sensor of metabolites released from the picoeukaryote Micromonas commoda RCC299. Bacterial expression responses indicated that the three species together recognized 38 picoeukaryote metabolites. This was consistent with the Micromonas expression of genes for starch metabolism and synthesis of peptidoglycan-like intermediates. A comparison of the hypothesized Micromonas exometabolite pool with that of the diatom Thalassiosira pseudonana CCMP1335, analyzed previously with the same biological sensor method, indicated that both phytoplankton released organic acids, nucleosides, and amino acids, but differed in polysaccharide and organic nitrogen release. Future ocean conditions are expected to favor picoeukaryotic phytoplankton over larger-celled microphytoplankton. Results from this study suggest that such a shift could alter the substrate pool available to heterotrophic bacterioplankton.

A New Composite Biomaterial Made from Sunflower Proteins, Urea, and Soluble Polymers Obtained from Industrial and Municipal Biowastes to Perform as Slow Release Fertiliser

Controlled-release fertilizers (CRF) are needed under current agriculture practice to decrease the environmental impact caused by fertilizer doses applied in excess of plants’ uptake rate. Commercial CRF are available. They are manufactured from mineral fertilizers encapsulated into a synthetic polymer matrix or coated by a polymer layer. However, substitution of fossil sourced organic polymers with biopolymers is a major environmental concern. In the present paper, we describe the manufacture by a continuous twin-screw extrusion process, and the mechanical and chemical properties of injection-molded composite pellets containing 90% sunflower protein concentrate (SPC) matrix, and 5–10% of a biopolymer (BP) obtained from municipal biowastes (MBW), and/or urea (U). The reported results show that SPC-BP-U behaves as an efficient eco-friendly CRF. BP contributes to several benefits to the performance of the composite pellets, upon increasing surface hardness, and controlling the formation of ammonia from urea hydrolysis and the release of organic nitrogen. The SPC-BP-U appears a powerful eco-friendly CRF to supply organic C and the three major N, P, and K nutrients to soil and plants. It offers worthwhile scope for being tested in the cultivation of specific plants under the real operational conditions of agriculture practices.

Feasibility of Focused‐Pulsed Treated Waste Activated Sludge as a Supplemental Electron Donor for Denitrification

We evaluated the feasibility of using waste activated sludge (WAS) from a wastewater treatment plant as an internal electron donor to fuel denitrification, by increasing its bioavailability with Focused-Pulsed (FP) technology. The focused-pulsed treatment of WAS (producing FP-WAS) increased the semi-soluble chemical oxygen demand (SSCOD) by 26 times compared with the control WAS. The maximum denitrification rate of FP-WAS (0.25 g nitrate-nitrogen [NO3- -N]/g volatile suspended solids [VSS] x d) was greater than for untreated WAS (0.05 g NO3- -N/g VSS x d) and methanol (0.15 NO3- -N/g VSS x d). Centrifuging out the larger suspended solids created FP-centrate, which had a rate (0.14 g NO3- -N/g VSS x d) comparable with that of methanol. Thus, FP treatment of WAS created SSCOD, which was an internal electron donor that was able to drive denitrification at a rate similar to or greater than methanol. One trade-off of using FP-WAS for denitrification is an increase in total Kjeldahl nitrogen (TKN) loading. While FP-WAS achieved the lowest total nitrogen and NO3- -N concentrations in the batch denitrification test, its final ammonia-nitrogen (NH3-N) concentration was the highest, as a result of the release of organic nitrogen from the FP-treated biomass; FP-centrate had less release of total soluble nitrogen. While the return of total nitrogen (TN) is small compared with the SSCOD, the effects of the added nitrogen loading need to be considered.

Biogenic Nanoscale Colloids in Wastewater Effluents

The size, surface area, metal complexation capacity, organic pollutant sorption potential, reactivity with disinfectants, and elevated nitrogen content of biogenic organic nanoscale material (BONM) can potentially affect aquatic environments. BONM in effluents from 11 full-scale wastewater treatment plants (WWTPs), which use a range of biological processes, were characterized in two ways. First, BONM was measured by hydrodynamic size-exclusion chromatography coupled with an online organic carbon and UV detector. Second, BONM was isolated from the wastewater using rotary evaporation and dialysis and then characterized by elemental analysis, transmission electron microscopy, and Fourier transform infrared spectroscopy. The wastewaters contained 6-10 mg/L of dissolved organic carbon (DOC). BONM accounted for 5%-50% of the DOC in wastewater effluent organic matter, and the largest size fraction (>10 kDa) of organic carbon correlated with the organic carbon content determined after rotary evaporation and dialysis. Membrane bioreactor WWTPs had the lowest fraction of BONM (<10% of the DOC), followed by conventional activated sludge (10% to 30% of the DOC), with other processes (e.g., trickling filters, aerated lagoons) containing larger BONM percentages. BONM had a lower carbon to nitrogen ratio (6.2 ± 1.7) compared with the literature values for humic or fulvic acids, exhibited chemical bonds that were indicative of amides and polysaccharides, and contained fibril entangled networks. This work has important implications for operations efficiency of WWTPs, including controlling membrane fouling and release of organic nitrogen into sensitive environments.

Acute sensitivity of activated sludge bacteria to erythromycin

The presence of antibiotics in water resources has been disturbing news for the stakeholders who are responsible for public health and the drinking water supply. In many cases, biological wastewater treatment plants are the final opportunity in the water cycle to trap these substances. The sensitivity of activated sludge bacteria to erythromycin, a macrolide widely used in human medicine was investigated in batch toxicity tests using a concentration range of 1–300 mg L −1. Erythromycin, a protein synthesis inhibitor, has been found to significantly inhibit ammonification, nitritation and nitratation at concentrations higher than 20 mg L −1. The degree of inhibition increased with greater concentrations of the antibiotic. Exposure to erythromycin also clearly affected heterotrophs, particularly filamentous bacteria, causing floc disintegration and breakage of filaments. Cell lysis was observed with the concomitant release of organic nitrogen (intracellular proteins) and soluble COD. Although erythromycin exhibits properties of a surfactant, this characteristic alone cannot explain the damage to heterotrophs: the effects from erythromycin were greater than those of Tween 80, a commonly used surfactant. Floc disruption can lead to the release of isolated bacteria, and possibly antibiotic resistance genes, into the environment.

Chara hispida beds as a sink of nitrogen: Evidence from growth, nitrogen uptake and decomposition

Chara hispida forms dense beds (0.78–0.95 kg DW m −2) in Colgada Lake. The ability of Chara meadows to act as a nitrogen source or sink was evaluated by the following methods: (1) investigating Chara growth, (2) nitrogen incorporation and decomposition laboratory experiments and (3) relating experimental results to field conditions. Sediment oospores were germinated in large aquaria and observed growth rates were 0.001 m day −1 (shoot length) and 0.0002 g day −1 (dry weight). Nitrogen uptake rates were determined by addition of K 15NO 3 during two different periods of Chara growth and the rates were 1.21 and 3.86 μM g DW −1 h −1 when charophytes were 166 days old (not sexually mature) and 323 days old (sexually mature), respectively. After the uptake experiments, the same charophytes were allowed to decompose within two types of litter bags (3 mm-pore litter bags and entire, non-porous plastic litter bags). Decomposition rates of Ch. hispida were 0.016 and 0.009 day −1 in perforated and non-perforated bags, respectively, and fit a negative exponential model. The nitrogen release rate, calculated as the disappearance of N content from Chara tissues, was 0.012 day −1 and there were no statistically significant differences between the values from the two different bag types. The dissolved organic nitrogen concentrations in aquarium and non-perforated litter bags waters increased linearly with time due to the leaching of soluble compounds from Chara. The rate of N loss from Chara tissues, total nitrogen and dissolved organic nitrogen release rates and the decrease in initial dry weight rate were all lower than the daily rate of Chara N uptake. By extrapolating laboratory data to field situations, we determined that approximately 38% of the N taken up by charophytes in Colgada Lake during the growth period is retained. Given the high charophyte biomass in the lake, its ability to incorporate nitrogen, its low decomposition rate and its ability to over-winter, we conclude that Chara beds could be acting as nitrogen sinks in this ecosystem.

Dissolved Organic Nitrogen Release and Bacterial Activity in the Upper Layers of the Atlantic Ocean

The variability of the percentage of extracellular dissolved organic nitrogen (DON) release (PER), along with the relationship between DON release and bacterioplankton activity, was examined during five oceanographic cruises, carried out in the upwelling region of the NW Iberian Peninsula, the SW Bay of Biscay, and a latitudinal transect in the Atlantic Ocean (50 degrees N-35 degrees S). Rates of nitrogen uptake, DON release, and bacterial production were measured at 66 stations and sampled between August 1998 and October 2000. The percentage of DON release relative to the gross uptake of ammonium (PERNH4+) ranged from 3 to 46%, whereas that relative to total nitrogen (NH4+ + NO3- + urea) gross uptake (PERtotal) varied between 21 and 82%. The highest values for both PERNH4+ and PER(total) were found in oligotrophic oceanic waters (< 0.25 mg chlorophyll a m(-3)). In oceanic environments, a positive relationship was found between nitrogen uptake and DON release rates, with a log-log linear regression slope significantly lower than 1, suggesting an inverse relationship between PER and gross nitrogen uptake rates. In contrast, in areas with higher productivity levels (> 6 mg chlorophyll a m(-3)), such as the continental shelf off the NW Iberian Peninsula, PER held constant as nitrogen uptake increased. These results suggest the dominance of different processes controlling DON release in oceanic and neritic zones. DON release rates accounted for less than 15% of the variability observed in bacterial production rates, suggesting a weak response of bacterioplankton to phytoplankton on short time scales (hours). Furthermore, nitrogen budgets showed an excess of DON release in relation to bacterial requirements.

Nitrogen uptake and dissolved organic nitrogen release in planktonic communities characterised by phytoplankton size–structure in the Central Atlantic Ocean

Ammonium, nitrate and urea uptake rates, as well as the release of dissolved organic nitrogen (DON), were measured along a latitudinal transect (49°N–33°S) in the Atlantic Ocean during the 2000 summer-to-autumn transition period and subsequently related to phytoplankton-size structure. The transect included upwelling areas, oligotrophic gyres and temperate regions and, as a result, euphotic layer integrated uptake rates of ammonium (18–355 mmol m −2 h −1 ), nitrate (5–2830 mmol m −2 h −1 ) and urea (26–1212 mmol m −2 h −1 ) covered a wide productivity spectrum. The percentage of extracellular DON release relative to total nitrogen uptake (PER) was highest when nitrate was used as the substrate (mean 42±SE 3.1%) whereas the uptake of ammonium and urea produced lower PER values (22±2.1 and 26±1.6%, respectively). Small cells (<2 μm) contributed between 20% and 80% of the total primary production; however, they accounted for 50–80% of total chlorophyll- a. In upwelling areas of the ocean, dominated by large phytoplankton, PER accounted for less than 30% of total nitrogen uptake and could be predicted from a regression equation relating the contribution of <2 μm phytoplankton to either total biomass or production. In contrast, in oligotrophic areas, small cells formed the bulk of the phytoplankton biomass, but not of primary production. In these areas, PER often exceeded 50% and was very variable, displaying a low correlation with phytoplankton size, which suggests that processes not directly related to the size of the primary producers influence DON release in the oligotrophic ocean.

Nitrogen Mobility at the Sediment-Water Interface of Lake Maracaibo, Venezuela

Nitrogen is one of the two most important elements in the metabolism of aquatic eco- systems. At low concentrations it can limit primary productivity and when present at very high concentrations it can participate in the eutrophication process of these environments. The mech- anism of nitrogen transport in sediments is almost unknown, nevertheless it is of vital importance for establishing mass balances in aquatic systems. In the study presented here, we assessed the nitrogen flux in sediments of the central part of tropical Lake Maracaibo, Venezuela, in particular with regard to dissolved oxygen concentrations. Experiments were performed under laboratory conditions in a batch system, and at varying dissolved oxygen regimes (aerobic and anaerobic). Every two days, during 3 months, overlying water samples were taken to analyze nitrite, nitrate, ammonium and Kjeldahl total nitrogen. Average release rates of total nitrogen were 0.86 in aerobic, and 1.06 mmol N m −2 d −1 in anaerobic systems corresponding to 41.7% of total N input to the lake. The behavior of nitrogen was strongly influenced by nitrate concentrations under aerobic conditions, and by organic nitrogen under anaerobic conditions during the course of the experiment. A major trend for the release of organic nitrogen during anaerobic conditions, and of nitrate, during aerobic conditions, was observed. Also fluxes of NO − ,N H + , organic N and Total N across the sediment-water interface were measured. In anaerobic conditions, which are predominant in the hypolimnetic cone of Lake Maracaibo, denitrification was estimated to be 0.02 mmol N m −2 d −1 , which corresponds to 1.89% of the total N released from sediments. This is to our knowledge the first study of nitrogen fluxes in sediments from Lake Maracaibo. The laboratory data presented here reflects conditions in the lake when major nutrients accumulation occurs.

Changes in species evenness of litter have no effect on decomposition processes

Most studies on the importance of litter diversity to decomposition processes have examined effects of species richness, and have not considered the other important aspect of biodiversity, namely evenness (relative abundance of species). Using microcosms, we investigated how changes in species evenness and identity of litters (taken from a regenerating Betula pendula forest) affected decomposition processes. We showed that single litter species varied in their rate of decomposition (bilberry 44.4%, willow 49.9%, rowan 53.7% and birch 54.8% weight loss after 234 days). However, when in mixtures, changes in the evenness of these litters and the identity of the dominant species had no detectable effect on measures of decomposition such as weight loss (42.6–56.6% after 234 days), microbial respiration and dissolved organic carbon and inorganic nitrogen release. Dissolved organic nitrogen release was found to be significantly greater from high evenness litter mixtures, 0.236 mg l −1, than the single species treatments, 0.145 mg l −1, but only during the initial stages of the experiment. These results, therefore, provide no support for the idea that changes in evenness of litter inputs and the identity of dominant litters will affect decomposition processes and soil nutrient availability.

Release of dissolved organic nitrogen from Scenedesmus quadricauda (Chlorophyta) and Microcystis novacekii (Cyanobacteria)

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 27:275-284 (2002) - doi:10.3354/ame027275 Release of dissolved organic nitrogen from Scenedesmus quadricauda (Chlorophyta) and Microcystis novacekii (Cyanobacteria) Fumitaka Nagao1, Tatsuo Miyazaki2,* 1Laboratory of Marine Environmental Microbiology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan 2Marine Biosystems Research Center, Chiba University, Amatsu-Kominato, Awa-gun, Chiba-ken 299-5502, Japan *Corresponding author. E-mail: myzkt@beige.ocn.ne.jp ABSTRACT: We measured the time-course of dissolved organic nitrogen (DON) release and inorganic nitrogen assimilation in the freshwater phytoplankton Scenedesmus quadricauda (Turp.) Brébission and Microcystis novacekii (Kom.) Comp. by the 15N tracer method. The algae were cultivated under nitrogen-limited or -replete conditions. We estimated the total DON release, release of recently assimilated nitrogen (RANR) and net nitrogen assimilation (NNA), using ammonium chloride or sodium nitrate as the nitrogen source. In the 15N tracer incubation of S. quadricauda prior to 1 h and in M. novacekii prior to 3-6 h, minimal release of total DON was observed under the nitrogen-limited conditions. This result suggests that nitrogen-limited cells efficiently use incorporated nitrogen. Appreciable release of total DON was observed in the 15N incubation of nitrogen-replete S. quadricauda and M. novacekii. RANR in both nitrogen-limited and -replete algae was smaller than, and lagged behind, release of total DON, indicating that newly assimilated nitrogen was less easily released. NNA by nitrogen-limited S. quadricauda increased from the onset of the tracer incubation, showing that ambient nitrogen was needed for nitrogen-limited cells from the beginning of the incubation. Nitrogen-replete S. quadricauda showed no NNA during the first 1 h of the tracer incubation, suggesting that the alga did not require external nitrogen immediately after being transferred from nitrogen-replete conditions. This study shows that nitrogen release and assimilation are markedly affected by the nutritional states of the algae. KEY WORDS: Ammonium · Nitrogen assimilation · Organic nitrogen release · Microcystis novacekii · Nitrate · Scenedesmus quadricauda Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 27, No. 3. Online publication date: April 16, 2002 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2002 Inter-Research.

Carbon Fixation, Ammonium Uptake and Regeneration in an Equatorial Lake: Biological Versus Physical Control

Ammonium uptake and regeneration were measured in the euphotic zone of Petit Saut Lake, French Guyana, to examine nitrogen cycling in this recently flooded equatorial forest environment. Am-monium regeneration rates were extremely high (mostly in the range 1–6 μmol N l–1 h–1), and are due to the very high grazing rates of the microzooplankton, which consumed between 56 and 95% of the phytoplankton production in any given incubation. These regeneration rates were about an order of magnitude higher than the net ammonium uptake rates. This imbalance is probably due to dissolved organic nitrogen release during grazing. At the bottom of the euphotic zone (4–5 m), photosynthetic bacteria are responsible for ammonium uptake. Diffusion-driven ammonium fluxes are an order of magnitude lower than biologically driven fluxes. Therefore, ammonium fluxes are dominated by biology rather than by physics in this lake.

Use of 15 N to measure dissolved organic nitrogen release by marine phytoplankton (reply to comment by Bronk and Ward)

Use of ¹⁵ N to measure dissolved organic nitrogen release by marine phytoplankton (reply to comment by Bronk and Ward)

Magnitude of dissolved organic nitrogen release relative to gross nitrogen uptake in marine systems

Magnitude of dissolved organic nitrogen release relative to gross nitrogen uptake in marine systems

Nitrogen regeneration and dissolved organic nitrogen release during spring in a NW Mediterranean coastal zone (Gulf of Lions):implications for the estimation of new production

Nitrogen regeneration fluxes of ammonium (NH 4 + ) and nitrate (NO 3 - ) as well as losses of dissolved organic nitrogen (DON) by phytoplankton were investigated over a 2 mo period (spring 1997) in a NW coastal Mediterranean area (Gulf of Lions) using 15 N-tracer techniques. Profiles of dissolved inorganic nitrogen (DIN) concentrations were almost uniform with values of 600, 150 and 35 nM for NO 3 - , NO 2 - and NH 4 + , respectively, except at the end of the study period when the upper layer became nitrogen-depleted (<50 nM down to 40 m). Chlorophyll (chl) distributions showed a surface maximum (to 0.85 mg m -3 ) and a deep maximum (to 1.25 mg m -3 ) at 40 m. Plankton DIN utilization (net uptake) was most of the time highest at the surface, with rates reaching 62 and 40 nM d -1 for NH 4 + and NO 3 - , respectively. However, a deepening (to 60 m) of maximum NO 3 - uptake rates with a corresponding deepening of the nitracline sometimes occurred during the experiment. Therefore, f-ratio profiles depicted maximum surface values (∼0.40) at the beginning of the experiment and a deep maximum at the end. NH 4 + regeneration rates were 1 order of magnitude higher (up to 220 nM d -1 ) than nitrification and DIN loss (as DON) rates, and could largely sustain more than 100% of the plankton NH 4 + demand. Underestimation of NH 4 + uptake rates due to 15 N isotope dilution had only a small effect on the ∫-ratio calculation (overestimation <5%). Nitrification occurred from the surface (10 to 20 nM d -1 ) down to the base of the euphotic layer (30 nM d -1 ), and corresponded to 90% and »100% of the plankton NO 3 - demand at the surface and in the nitracline, respectively. Consequently, a great part of NO 3 - uptake did not correspond to new production and should be considered as regenerated production, particularly in the NO 3 - depleted surface layer. Profiles of DIN loss (as DON) well paralleled those of DIN net uptake with values highest at surface reaching 35 and 14 nM d -1 for NH 4 + and NO 3 - , respectively. DIN loss rates represented on average ∼23% of gross DIN uptake (gross DIN uptake = DIN losses + DIN net uptake) whatever the substrate was, indicating that (1) DIN loss (as DON) did not depend on the nitrogen source, and (2) DIN uptake was mostly due to phytoplankton and not to bacterioplankton, although the study area tended to be globally nitrogen-depleted and based on regeneration. Failure to account for DIN losses had no significant effect on the computation of ∫-ratios.

Rates of NH4+ uptake, intracellular transformation and dissolved organic nitrogen release in two clones of marine Synechococcus spp

Evidence suggests that phytoplankton can release significant amounts of dissolved organic carbon (DOC), particularly when they are nitrogen (N) deficient. To investigate this phenomenon from the perspective of dissolved organic N (DON), 15N tracer techniques were used to trace the flow of N through two clones of Synechococcus, WH7803 and WH8018, which were grown in batch culture under N-sufficient and N-deficient conditions. At various stages during the experiment, a subsample of the culture was removed and the response to a pulse of 15N-labeled NH4 + was measured with respect to rates of NH4 + uptake, intracellular NH4 + transformation, and release of total and low- molecular-weight (LMW; <10 000 Da) DON. In contrast to what is commonly observed for DOC, high rates of DON release were not observed during N starvation in either clone. The highest rates of DON release, both absolute and as a percentage of gross N uptake, occurred during N-sufficient growth, and release rates decreased by a factor of 4-7 when NH4 + was depleted in the medium.

Management of marine fish farming in the sub-tropical environment: a modelling approach

Two deterministic models were applied to simulate hydrographic and water quality conditions within a sub-tropical marine fish culture site, where trash fish is used as feed. A two-dimensional, two-layer hydrodynamic model of tidal flows and salt transport calculated the water level, velocity and salinity in each grid cell of 50 m square in each layer within the culture area approximately every 30 s during a tidal cycle. Results from this flow model provided hydraulic data for input into a three-dimensional tidal water quality model, which was run to simulate water quality due to specified pollutant loadings from the marine fish culture operations. Simulated output from the models agreed reasonably well with observed field data, except for dissolved oxygen and nitrate levels in surface waters. This could be due to a conservative estimation of re-aeration rates in the model system. In applying the models to sub-tropical waters of Hong Kong, the simulation predicted the extent of pollution and area affected under varying fish stock and pollutant loadings. It was shown that impacts of fish culture activities on water quality at the test culture site were localized owing to strong advection by tidal residual flows. The higher simulated level of organic nitrogen in the bottom waters over the year at the test fish culture site was due to increased release of organic nitrogen through decay of wasted feed from the sea bed and from trash fish feed. Models of marine fish culture and environmental effects have applications in determining acceptable stocking levels, and the effects of changing feed and culture species, and for locating suitable culture sites in coastal areas.

The effects of irradiance on nitrate uptake and dissolved organic nitrogen release by phytoplankton in the Ross Sea

The effects of irradiance on nitrate uptake and subsequent dissolved organic nitrogen (DON) release were investigated using laboratory cultures of Phaeocystis sp. and phytoplankton assemblages dominated by Phaeocystis antarctica in the Ross Sea during cruises in November–December, 1994 and December 1995–January, 1996. 15N-labeled nitrate was used to determine nitrate uptake rates, and an ion retardation column (IRC) method was used to determine 15N-DON release rates (both the total as well as the low molecular weight (LMW) fraction). Nitrate uptake was strongly dependent on irradiance. The maximum specific nitrate uptake rate achieved by Phaeocystis cultures in short (<8 h) incubations was 0.0069 h -1, and maximum specific nitrate uptake rates by phytoplankton assemblages dominated by Phaeocystis antarctica in the Ross Sea were 0.0089 and 0.0033 h -1, respectively, for the two cruises. Photoinhibition of nitrate uptake was observed during both cruises, but was greatest during late December and January, although the mean daily irradiances for the two cruises were similar (49.9 vs. 49.5 mol m -2 d -1). From 8 to 19% of the nitrate assimilated by Phaeocystis cultures and natural phytoplankton assemblages was released as dissolved organic nitrogen. Total DON release rates increased with irradiance in the laboratory and the Ross Sea; however, LMW DON release rates were extremely low and appeared independent of irradiance in the Ross Sea. DON release was also highly correlated with nitrate uptake and phytoplankton growth rate. Release of dissolved organic nitrogen from Phaeocystis-dominated assemblages in general does not appear to play a substantial role in the nitrogen flux in the Ross Sea.

Nitrogen Uptake, Dissolved Organic Nitrogen Release, and New Production

In oceanic, coastal, and estuarine environments, an average of 25 to 41 percent of the dissolved inorganic nitrogen (NH(4) (+) and NO(3) (-)) taken up by phytoplankton is released as dissolved organic nitrogen (DON). Release rates for DON in oceanic systems range from 4 to 26 nanogram-atoms of nitrogen per liter per hour. Failure to account for the production of DON during nitrogen-15 uptake experiments results in an underestimate of gross nitrogen uptake rates and thus an underestimate of new and regenerated production. In these studies, traditional nitrogen-15 techniques were found to underestimate new and regenerated production by up to 74 and 50 percent, respectively. Total DON turnover times, estimated from DON release resulting from both NH(4) (+) and NO(3) (-) uptake, were 10 +/- 1, 18 +/- 14, and 4 days for oceanic, coastal, and estuarine sites, respectively.

Contrasting patterns of dissolved organic nitrogen release by two size fractions of estuarine plankton during a period of rapid NH/ consumption and N02-production

Contrasting patterns of dissolved organic nitrogen release by two size fractions of estuarine plankton during a period of rapid NH/ consumption and N02-production