Dissolved Organic Nitrogen Pool Research Articles

Isotopic compositions of organic and inorganic nitrogen reveal processing and source dynamics at septic influenced and undeveloped estuary sites

Historically, dissolved organic nitrogen (DON) has not been characterized in the nitrogen profiles of most estuaries despite its significant contribution to total nitrogen and projected increase in loading. The characterization of dissolved inorganic nitrogen (DIN) and DON processing from groundwater to surface water also remains unconstrained. This study attempts to fill in these knowledge gaps by quantifying the DON pool and potential sources in a semiarid, low inflow estuary (Baffin Bay, Texas) using stable isotope techniques. High NO3− and DON concentrations, and high δ15N-NH4+ (+55.0 ± 56.7 ‰), δ15N-NO3− (+23.9 ± 8.6 ‰) and δ15N-DON (+22.3 ± 6.5 ‰) were observed in groundwaters of a septic-influenced estuarine area, indicating coupled septic contamination and nitrification/denitrification. In contrast, groundwater of an undeveloped area provided evidence of inundation by bay water through high NH4+ concentrations and δ15N-NH4+ (+8.4 ± 3.0 ‰) resembling estuary porewater. NH4+ was the dominant nitrogen species in porewater of both areas and δ15N-NH4+ indicated production via organic nitrogen mineralization and dissimilatory nitrate reduction to ammonium. Surface water had similar nitrogen profiles (DON constituted ∼98 % of dissolved nitrogen pool) and potential source contributions, despite distinct nitrogen processing and profiles found in each water table. This was attributed to low nitrogen removal rates and prolonged mixing associated with long residence time. This study emphasizes the importance of DON in a low-inflow estuary and the isotopic approach to comprehensively examine both inorganic and organic N processing and sources serving as a guide to investigate N cycling in high DON estuaries globally.

D-Alanine Metabolism via d-Ala Aminotransferase by a Marine Gammaproteobacterium, Pseudoalteromonas sp. Strain CF6-2.

As the most abundant d-amino acid (DAA) in the ocean, d-alanine (d-Ala) is a key component of peptidoglycan in the bacterial cell wall. However, the underlying mechanisms of bacterial metabolization of d-Ala through the microbial food web remain largely unknown. In this study, the metabolism of d-Ala by marine bacterium Pseudoalteromonas sp. strain CF6-2 was investigated. Based on genomic, transcriptional, and biochemical analyses combined with gene knockout, d-Ala aminotransferase was found to be indispensable for the catabolism of d-Ala in strain CF6-2. Investigation on other marine bacteria also showed that d-Ala aminotransferase gene is a reliable indicator for their ability to utilize d-Ala. Bioinformatic investigation revealed that d-Ala aminotransferase sequences are prevalent in genomes of marine bacteria and metagenomes, especially in seawater samples, and Gammaproteobacteria represents the predominant group containing d-Ala aminotransferase. Thus, Gammaproteobacteria is likely the dominant group to utilize d-Ala via d-Ala aminotransferase to drive the recycling and mineralization of d-Ala in the ocean. IMPORTANCE As the most abundant d-amino acid in the ocean, d-Ala is a component of the marine DON (dissolved organic nitrogen) pool. However, the underlying mechanism of bacterial metabolization of d-Ala to drive the recycling and mineralization of d-Ala in the ocean is still largely unknown. The results in this study showed that d-Ala aminotransferase is specific and indispensable for d-Ala catabolism in marine bacteria and that marine bacteria containing d-Ala aminotransferase genes are predominantly Gammaproteobacteria widely distributed in global oceans. This study reveals marine d-Ala-utilizing bacteria and the mechanism of their metabolization of d-Ala. The results shed light on the mechanisms of recycling and mineralization of d-Ala driven by bacteria in the ocean, which are helpful in understanding oceanic microbial-mediated nitrogen cycle.

Active dissolved organic nitrogen cycling hidden in large river and environmental implications

Large rivers are important terrestrial dissolved organic matter (DOM) sources to marginal seas, and dissolved organic nitrogen (DON) plays an essential role in DOM cycling. The Yellow River ranks as the fifth largest river (in length) in the world and is well-known for its high dissolved inorganic nitrogen (DIN) concentration and relatively low DON concentration, leading to extreme measuring uncertainties in DON and nitrogen isotopic composition (δ15N), consequently leaving its DON cycling as an unresolved puzzle. To fill such a knowledge gap, we analyzed 17 samples from the middle to downstream with a combination of spectroscopy, tangential flow filtration, nitrogen isotope, and DNA sequencing. DON<1kDa dominated the DON pool and significantly correlated inversely with DIN, indicating the DON<1kDa mineralized into nitrate. This finding was further supported by the observed Rayleigh fractionation in δ15NDON<1kDa and the spatial distribution pattern of ammonia-oxidizing bacteria/archaea abundance. The redundancy analysis revealed that the geographical features and the microbial community were closely related, which joined together to drive the DON cycling. In addition, we propose a rational method to quantify the flux of mineralized DON in large rivers. This study discovered the active DON cycling hidden in high DIN large river and highlighted the importance of DON mineralization as well as its role in marginal seas carbon cycling.

Organic carbon and nitrogen dynamics during a peatland storm event: How dissolved combined amino acids reveal the spatial and temporal separation of organic molecules

The export of fixed carbon from peatlands is likely important in driving freshwater ecosystem productivity in head water streams. Here we studied the stream composition from three peatlands, and the sub-catchment containing these peatlands, in response to a natural storm event. We investigated the dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) mobilised by this event as well as the dissolved combined amino acids (DCAAs; proteins, peptides and free AAs) component of the DOC/DON. The storm pulse was characterised by a strong DOC/DON/DCAA pulse reflecting the mobilisation of materials from the peat profile. Through the storm cycle a transient increase in the C:N ratio of the DOC was observed (ranging over 20 – 50 across the four sites), likely reflecting the mobilisation of less decomposed material from higher in the peat soil profile. By comparing the DCAA and DON dynamics we show that a DCAA-rich component (fast-DCAA) is mobilised earlier in the storm cycle, likely reflecting a higher mobility in the peat soil. Similarly, comparison of the DCAA and DOC dynamics suggests that at least part of the DOC is mobilised more rapidly than fast-DCAA (fast-DOC). While we may not expect the composition of a storm pulse to be uniform through the cycle, this study shows that within the DOC and DON pools there is spatial and temporal separation of molecular groups. Proteins (in principal) represent both highly bioavailable N-sources as well as potential biological trigger molecules, so our work shows that these compounds are more dominant in the early part (rising limb) of the storm pulse. This understanding of the changing composition of DOC through a storm peak also provides greater insight into the concentration-discharge (C-Q) response of peatlands; our work shows that the activation of water paths higher in the peat profile at high flows leads to the mobilisation of different pools of DOC such that the molecular characteristics of DOC change through the storm cycle.

Abiotic and biotic controls of soil dissolved organic nitrogen along a precipitation gradient on the Tibetan plateau

Dissolved organic nitrogen (DON) has been increasingly recognized as a crucial component of the terrestrial nitrogen (N) cycle that regulates the ecosystem feedback to climate change. Yet, little information is available about the factors that control soil DON in the alpine ecosystems of the Tibetan Plateau, a region that is extremely sensitive to climate change. Here, we examined the relationship between DON and climate, plant, and soil attributes along a precipitation gradient in five alpine ecosystems (alpine wetland, alpine meadow, alpine shrub, alpine steppe, and alpine desert) across 20 sites that were up to 4000 km apart on the Tibetan Plateau. Results showed that soil DON concentration varied significantly across these alpine ecosystems, and had a positive relationship with mean annual precipitation. Belowground biomass, soil moisture, micro biomass carbon (MBC), and soil total phosphorus (TP) explained 84% of the variation in DON concentrations. The percentage of DON in soil total dissolved N (TDN) varied with vegetation type, with the lowest percentage observed in the alpine meadow (52%) and the highest in the alpine desert (77%), indicating the slow turnover of DON into inorganic N in systems with low litter input and microbial activity. Precipitation and plant biomass input determine the concentration and turnover of soil DON on the Tibetan Plateau. A warmer and wetter climate that has been predicted for the Tibetan grasslands may lead to a larger and more active DON pool.

Concentration and turnover of dissolved free polyamines on the South Coast of Lake Erie

AbstractDissolved free polyamines (DFPAs) have been suggested as an important source of reduced nitrogen in marine systems, but their importance in the cycling of dissolved organic nitrogen (DON) in freshwater systems has yet to be assessed. This study measured the concentrations, turnover and uptake rates of DFPAs and compared them with those of dissolved free amino acids (DFAAs) in samples from 21 sites along eight coastal transects on the south shore of Lake Erie (LE), a Laurentian Great Lake. DFPA compounds were consistently detected in all samples and their concentrations (16.9–191.5 nmol L−1), turnover (1.6–3.2 d−1), and uptake rates (3.2–192.0 nmol L−1 d−1) were all significantly higher than those reported in marine environments. On the other hand, corresponding DFAA measurements (45.1–822.2 nmol L−1, 3.9–11.4 d−1, and 35–1690 nmol L−1 d−1, respectively) were within the range of reported values in marine environments. The DFPA to DFAA ratios in LE samples ranged between 0.21 and 0.65, which were much higher than what have been reported in marine environments (0.01–0.07). Within the bacterioplankton community, particle‐associated (PAB) and free‐living (FLB) proportions contributed equally to the turnover of DFPAs and DFAAs in LE. PAB and FLB also contributed equally when using requested DFPAs to fulfill their carbon (BCD; 3.1–7.9%) and nitrogen (BND; 13.5–18.5%) demands. Overall, our results suggest that DFPAs is a significant component of labile DON pool in freshwater systems, where they serve more as nitrogen than carbon sources to freshwater bacterioplankton.

Amino acid enantiomers in old and young dissolved organic matter: Implications for a microbial nitrogen pump

Abstract Dissolved organic nitrogen (DON) represents the largest reservoir of fixed N in the surface ocean and a significant portion accumulates in the deep sea, where it can persist for millennial time scales. However, like the dissolved organic carbon (DOC) pool, the origin and composition of long-lived, refractory DON remains largely unknown. In recent years, the “microbial carbon pump” hypothesis has emerged from abundant evidence showing that microbial processes are primarily responsible for refractory DOC accumulation. However, a similar mechanism for DON has rarely been investigated. In the study of DON, spectroscopic evidence has indicated a primarily amide composition, implying a dominant contribution from peptides. Therefore, if an analogous “microbial nitrogen pump” controls refractory DON accumulation, the amino acid component should bear increasing signatures of microbial origin with increasing age. Here we investigate the microbial sequestration of N via the production of refractory DON, for the first time considering together DOM Δ14C with amino acid (AA) molar abundance (Mol%) and D/L ratio (as a tracer for prokaryotic input). Measurements were made on a unique set of high and low molecular weight (HMW, LMW) DOM isolates with 14C ages and chemical compositions generally consistent with semi-labile and refractory DOM respectively. The samples were collected in the North Pacific Subtropical Gyre where deep waters contain some of the oldest DOC in the world ocean. We observe higher D/L ratios in older, LMW DOM isolates for almost all analyzed AAs. Using mass spectral data, we also quantify three D-AAs in all samples (D-valine, D-phenylalanine, and D-leucine), which have not previously been confirmed in ocean DOM. These newly identified D-AAs are concentrated in the LMW refractory DOM fraction and have oceanographically consistent depth profiles. Our results suggest that several novel D-AA subgroupings may be unique tracers for different prokaryotic source processes. D-alanine appears to have largely independent cycling from the other D-AAs with a connection to the production of HMW DON, which we hypothesize is linked to water column peptidoglycan. In contrast, D-leucine, D-valine, and D-phenylalanine appear to be most strongly related to the production of LMW DON. Trends in both the HMW and LMW fractions suggest a linkage to sinking particles and local microbial transformations, implying that LMW DON has a direct biological source rather than originating from successive microbial reprocessing of HMW DON. Taken together, our observations are consistent with the dominant production of refractory LMW DON by prokaryotic organisms and suggests that different AA sub-groupings that can be used to track different processes within the DON pool.

The bioavailability of different dissolved organic nitrogen compounds for the freshwater algae Raphidocelis subcapitata

Understanding which factors affect the algal bioavailability of dissolved organic nitrogen (DON) compounds in natural surface waters is important for our understanding of nutrient biogeochemistry and water quality management. We used nitrogen uptake kinetics and algal cell yield to characterize the algal bioavailability of 22 dissolved DON compounds that are commonly found in natural surface waters and wastewater treatment plant effluents, including urea, amino acids, amino sugars, nucleotides, pyrimidines, oraganonitriles, polyacrylamide, EDTA, caffeine, phenolic compounds and humic acids. Twelve of these compounds were highly bioavailable, including urea, dissolved free amino acids, bovine serum albumin, DNA, RNA, ATP, AMP, acetonitrile and caffeine. Four compounds had intermediate bioavailability including two humic acids (Elliott Soil and Pahokee Peat), glycylglycine, RNA and uracil. The remaining six compounds were classified as recalcitrant, i.e., EDTA, 2,3-Dinitrophenol, aminobenzoic acid, polyacrylamide and Aldrich humic acid. For many of the compounds tested, the algal cell yield was only 60–80% of expected relative to DON uptake. These results help explain why some DON compounds are more likely to persist in natural systems, and why the DON pool is often recalcitrant in surface waters.

A Major Step in Opening the Black Box of High-Molecular-Weight Dissolved Organic Nitrogen by Isotopic Labeling of Synechococcus and Multibond Two-Dimensional NMR.

Dissolved organic nitrogen (DON) comprises the largest pool of fixed N in the surface ocean, yet its composition has remained poorly constrained. Knowledge of the chemical composition of this nitrogen pool is crucial for understanding its biogeochemical function and reactivity in the environment. Previous work has suggested that high-molecular-weight (high-MW) DON exists only in two closely related forms, the secondary amides of peptides and of N-acetylated hexose sugars. Here, we demonstrate that the chemical structures of high-MW DON may be much more diverse than previously thought. We couple isotopic labeling of cyanobacterially derived dissolved organic matter with advanced two-dimensional NMR spectroscopy to open the "black box" of uncharacterized high-MW DON. Using multibond NMR correlations, we have identified novel N-methyl-containing amines and amides, primary amides, and novel N-acetylated sugars, which together account for nearly 50% of cyanobacterially derived high-MW DON. This study reveals unprecedented compositional details of the previously uncharacterized DON pool and outlines the means to further advance our understanding of this biogeochemically and globally important reservoir of organic nitrogen.

Atmospheric fluxes of soluble organic C, N, and P to the Mediterranean Sea: Potential biogeochemical implications in the surface layer

Linking atmospheric deposition to marine carbon and nutrient cycle is hampered by the lack of data on atmospheric fluxes of organic matter. To fill this gap, this study reports the first quantification of atmospheric fluxes of soluble organic carbon (SOC), nitrogen (SON) and phosphate (SOP) to the NW Mediterranean Sea. Simultaneous measurements of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphate (DOP) in the surface mixed layer (SML) allowed estimating the potential contribution of atmospheric fluxes to marine DOC, DON and DOP inventories. We found an annual atmospheric flux of 59mmolCm−2year−1 for SOC, 16.4mmolNm−2year−1 for SON and 23.6µmolPm−2year−1 for SOP, with proportions of SON and SOP to total soluble nitrogen and phosphate of 40% and 25%, respectively. Assuming these annual fluxes valid for the entire western Mediterranean basin, atmospheric fluxes would be higher than DOC, DON and DOP fluxes from the Rhône River by a factor of 6, 17 and 2, for C, N and P, respectively. Inventories of DOC, DON and DOP in the surface mixed layer displayed similar trends over the study period with maximum values at the end of the stratification period. DOP contributed by 85±11% to total dissolved phosphate (TDP) pool and exhibited a labile fraction (LDOP) of 27±19%. The contribution of atmospheric deposition to the DOC, DON and DOP pools in the SML, estimated for the stratification period, was low for C (3%) and P (4.5%) and moderate for N (12%). The labile fraction of atmospheric SOP (LSOP) was quantified throughout the sampling period and showed a high variability ranging from 0 to 97%. Atmospheric fluxes of LSOP contributed by 7% to marine LDOP pool and could sustain up to 8% of the heterotrophic prokaryotic phosphate demand in the SML of the NW Mediterranean Sea during the stratification period. The results obtained in this study stress the need to include atmospheric fluxes of organic matter in marine biogeochemical models to achieve a more complete picture of carbon and nutrient cycle in the Mediterranean Sea.

Degradation dynamics and bioavailability of land-based dissolved organic nitrogen in the Bohai Sea: Linking experiment with modeling

Dissolved organic nitrogen (DON) is the major nitrogen form in the Bohai Sea. Land-based DON is released into the nitrogen pool and degraded by planktonic microbiota in coastal ocean. In this study, we evaluated the degradation of land-based DON, particularly its dynamics and bioavailability, in coastal water by linking experiment and modeling. Results showed that the degradation rate constant of DON from sewage treatment plant was significantly faster than those of other land-based sources (P<0.05). DON was classified into three categories based on dynamics and bioavailability. The supply of dissolved inorganic nitrogen (DIN) pool from the DON pool of Liao River, Hai River, and Yellow River was explored using a 3D hydrodynamic multi-DON biogeochemical model in the Bohai Sea. In the model, large amounts of DIN were supplied from DON of Liao River than the other rivers because of prolonged flushing time in Liaodong Bay.

Effects of forest types on soil dissolved organic carbon and nitrogen in surface and deep la-yers in subtropical region, China.

Forest types have significant effects on the availability and dynamics of soil dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). By now the impacts of forest types on soil DOC and DON were mainly focused on surface soil (0-10 cm). Based on the comparisons between natural forest, Phyllostachys pubescens, Castanopsis kawakamii and Cunninghamia lanceolata plantations, we investigated the effects of forest types on soil DOC and DON pools in top (0-10 cm) and deep soils (40-60 cm). Cold water, hot water and KCl solutions were used to extract soil DOC and DON from surface and deep soils. Results showed that the effects of forest types on soil DOC, DOC/TOC, DON and soil microbial biomass carbon were only significant in the surface soil. The concentrations of DOC and DON varied with extract methods and hot water extracted the largest amounts of DOC and DON, and cold water the least. Correlations among hot water, KCl and cold water extracted DOC and DON were significant, suggesting that the organic C and N released by these three solutions might be at least partly from similar pools. The concentrations of DOC and DON and DOC/TOC in surface soil under natural forest and P. pubescens were greater than under C. kawakamii and C. lanceolata. It indicated that the concentrations of DOC and DON were greater under the natural forest and P. pubescens than under the C. kawakamii and C. Lanceolata, and more beneficial to improve soil fertility.

Direct response of dissolved organic nitrogen to nitrate availability in headwater streams

Despite decades of research documenting the quantitative significance of dissolved organic nitrogen (DON) across ecosystems, the drivers controlling its production and consumption are not well understood. As an organic nutrient DON may serve as either an energy or nitrogen source. One hypothesized control on DON concentration in streams is nitrate (NO3 −) availability. Synoptic surveys of DON and NO3 −, however, have yielded inconsistent spatial and temporal patterns. Using a nutrient pulse method we experimentally manipulated stream NO3 − and measured the response of both the manipulated solute and ambient concentrations of DON in three New Hampshire headwater streams. This direct experimental addition of NO3 − often altered ambient DON concentrations in situ, with both increases and decreases observed. The overall relationship between NO3 − and DON suggests that DON is primarily used as a nutrient source in these streams, as evidenced by net DON accumulation with added NO3 −. However, strong underlying seasonal patterns in the response to NO3 − addition are also discernable, indicating that the role of DON can switch between serving as a nutrient source to an energy source (as evidenced by net DON reduction with added NO3 −). We also observed differences in the NO3 −—DON relationship (net DON accumulation vs. net DON reduction) in two streams less than five miles apart when experiments were conducted within the same month. Based on these results, we expect the role of DON within ecosystems to vary among watersheds and throughout the growing season, alternating between serving as a nutrient and energy source depending on environmental conditions. With the incorporation of a new field-based method we demonstrate that the ambient DON pool can be manipulated in situ. This approach has the potential for furthering our understanding of DON across ecosystems.

Dissolved Organic Nitrogen Inputs from Wastewater Treatment Plant Effluents Increase Responses of Planktonic Metabolic Rates to Warming

Increased anthropogenic pressures on coastal marine ecosystems in the last century are threatening their biodiversity and functioning. Global warming and increases in nutrient loadings are two major stressors affecting these systems. Global warming is expected to increase both atmospheric and water temperatures and increase precipitation and terrestrial runoff, further increasing organic matter and nutrient inputs to coastal areas. Dissolved organic nitrogen (DON) concentrations frequently exceed those of dissolved inorganic nitrogen in aquatic systems. Many components of the DON pool have been shown to supply nitrogen nutrition to phytoplankton and bacteria. Predictions of how global warming and eutrophication will affect metabolic rates and dissolved oxygen dynamics in the future are needed to elucidate their impacts on biodiversity and ecosystem functioning. Here, we experimentally determine the effects of simultaneous DON additions and warming on planktonic community metabolism in the Baltic Sea, the largest coastal area suffering from eutrophication-driven hypoxia. Both bacterioplankton community composition and metabolic rates changed in relation to temperature. DON additions from wastewater treatment plant effluents significantly increased the activation energies for community respiration and gross primary production. Activation energies for community respiration were higher than those for gross primary production. Results support the prediction that warming of the Baltic Sea will enhance planktonic respiration rates faster than it will for planktonic primary production. Higher increases in respiration rates than in production may lead to the depletion of the oxygen pool, further aggravating hypoxia in the Baltic Sea.

Temporal dynamics and depth variations of dissolved free amino acids and polyamines in coastal seawater determined by high-performance liquid chromatography

Short-chained aliphatic polyamines (PAs) are a class of labile dissolved organic nitrogen (DON) that has biogeochemical similarities to dissolved free amino acids (DFAAs). Here we investigated the relative contributions of DFAAs and PAs to the total DON pool and their diurnal dynamics at different depths at the Gray's Reef National Marine Sanctuary (GRNMS) in the spring and fall of 2011. A high-performance liquid chromatography (HPLC) method that uses pre-column fluorometric derivatization with o-phthaldialdehyde, ethanethiol, and 9-fluorenylmethyl chloroformate was optimized to measure 20 DFAAs and 5 PAs in seawater simultaneously. The concentrations of DFAAs and PAs varied over 5-fold during individual diurnal cycles and between seasons; and concentrations of the former (tens to hundreds nM) were typically at least one order of magnitude higher than the latter (a few nM). An exception was noted in fall surface water samples when the total PAs reached 159nM and the ratio of PAs to DFAAs was about 2:3. Compositions of individual DFAAs and PAs also exhibited temporal dynamics, with glycine and spermidine consistently the most abundant compound in each pool, respectively. DFAA concentrations appeared to track chlorophyll a, whereas, total PA concentrations were strongly correlated with bacterial cell abundance. Our results indicate that, at least occasionally, PAs may serve as an important DON pool at the GRNMS. This view is in accordance with recent molecular data but contrasts to measurements made in some other marine environments.

Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 69:69-80 (2013) - DOI: https://doi.org/10.3354/ame01621 Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium Mar Benavides1,*, Nona S. R. Agawin2, Javier Arístegui1, Jan Peene3, Lucas J. Stal3,4 1Instituto de Oceanografía y Cambio Global, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain 2Departament de Biologia, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain 3Netherlands Institute of Sea Research (NIOZ), Department of Marine Microbiology, 4400 AC Yerseke, The Netherlands 4Department of Aquatic Microbiology, IBED, University of Amsterdam, 1090 GE Amsterdam, The Netherlands *Email: mar.benavides101@doctorandos.ulpgc.es ABSTRACT: Dinitrogen (N2) fixation rates may be underestimated when recently fixed N2 is released as dissolved organic nitrogen (DON). DON release (DONr) is substantial in the filamentous cyanobacterium Trichodesmium but has never been reported in unicellular diazotrophic cyanobacteria. We used axenic cultures of the marine unicellular diazotroph Cyanothece sp. Miami BG 043511 to measure dissolved organic matter release under N2-fixing conditions. DONr was measured as the transfer of 15N2 from the culture medium to the extracellular DON pool. On average, the DON released represented ~1% of the total N2 fixed. The average release of dissolved organic carbon, as determined by 14C, represented ~2% of the total carbon fixed. These results suggest that cultured populations of unicellular diazotrophs do not release much dissolved organic matter, but it cannot be excluded that DONr is important in the field when grazers and bacteria are present, or when the organism is exposed to environmental stresses such as turbulence, excess light, temperature changes, or nutrient limitation. KEY WORDS: N2 fixation · DON release · Unicellular cyanobacteria · Cyanothece Full text in pdf format PreviousNextCite this article as: Benavides M, Agawin NSR, Arístegui J, Peene J, Stal LJ (2013) Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium. Aquat Microb Ecol 69:69-80. https://doi.org/10.3354/ame01621 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 69, No. 1. Online publication date: April 18, 2013 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2013 Inter-Research.

Molecular‐level characterization of reactive and refractory dissolved natural organic nitrogen compounds by atmospheric pressure photoionization coupled to Fourier transform ion cyclotron resonance mass spectrometry

Dissolved organic nitrogen (DON) represents a significant fraction of the total dissolved nitrogen pool in most surface waters and serves as an important nitrogen source for phytoplankton and bacteria. As with other natural organic matter mixtures, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) is the only technique currently able to provide molecular composition information on DON. Although electrospray ionization (ESI) is the most commonly used ionization method, it is not very efficient at ionizing most DON components. Positive- and negative-mode atmospheric pressure photoionization (APPI) coupled with ultrahigh resolution FTICRMS at 9.4 T were compared for determining the composition of DON before and after bioassays. Toluene was added as the APPI dopant to the solid-phase DON extracts, producing a final sample that was 90% methanol and 10% toluene by volume. Positive-mode (+) APPI proved significantly more efficient at ionizing DON; 62% of the formulas that could be assigned in the positive-ion spectrum contained at least one nitrogen atom vs. 31% in the negative-ion spectrum. FTICR mass spectral data indicated that most of the refractory DON compounds (i.e. nonreactive during the 5 days of the incubation) had molecular compositions representative of lignin-like molecules, while lipid-like and protein-like molecules comprised most of the small reactive component of the DON pool. From these data we conclude that (+) APPI FTICRMS is a promising technique for describing the molecular composition of DON mixtures. The technique is particularly valuable in assessing the bioavailability of individual components of DON when combined with bioassays.

Dissolved organic nitrogen in the global surface ocean: Distribution and fate

The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean's oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface‐accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone.

Changes in compound specific δ15N amino acid signatures and d/l ratios in marine dissolved organic matter induced by heterotrophic bacterial reworking

Compound-specific δ15N analysis of individual amino acids (δ15N-AA) represents a potentially important new tool which may reveal the molecular-level basis for δ15N signature of dissolved organic nitrogen (DON) in the ocean, as well indicate DON sources and specific mechanisms of alteration. Past work has indicated that δ15N-AA may be effective at indicating the effects of microbial heterotrophy, however the influence of bacterial degradation on δ15N-AA patterns has never been directly investigated. Here we measured molecular-level changes in δ15N-AA patterns in freshly produced algal high molecular weight (HMW) DON due to heterotrophic bacterial reworking, together with linked changes in enantiomeric (D vs. L) AA ratios and also the AA molar percentage-based degradation index (DI). Our results show a strong increase in degradation with microbial consumption of dissolved organic carbon (DOC), consistent with previous studies. The δ15N-AA data show systematically higher δ15N values for most individual AA after DOC bacterial reworking, resulting in average increases of 3–6‰ in δ15N of total proteinaceous material. The average deviation in the δ15N values of all AA (ΣV parameter) also increased with degradation, indicating an increase in δ15N-AA pattern complexity, most likely due to selected microbial resynthesis of specific AA. These results show that δ15N-AA patterns have the ability to directly track the effects of microbial resynthesis in DON. They indicate that δ15N-AA represents a highly specific tracer that provides independent, and yet strongly complimentary, information vs. existing AA-based degradation indicators. Together, our data suggests that heterotrophic microbial degradation in the ocean would be expected to increase δ15N values of the oceanic DON pool vs. autotrophic sources. This conclusion is consistent with recent results on δ15N signatures of total and HMWDON pool in the open sea, however it also strongly implicates bacterial sources as the likely mechanism for δ15N-DON changes. Reevaluating existing DON isotopic data in light of these results may improve our understanding of the influence and mechanism of bacterial reworking on DON long-term preservation in the marine water column.

Amino acid, peptide and protein mineralization dynamics in a taiga forest soil

The availability of inorganic N has been shown to be one of the major factors limiting primary productivity in high latitude ecosystems. The factors regulating the rate of transformation of organic N to nitrate and ammonium, however, remain poorly understood. The aim of this study was to investigate the nature of the soluble N pool in forest soils and to determine the relative rate of inorganic N production from high and low molecular weight (MW) dissolved organic nitrogen (DON) compounds in black spruce forest soils. DON was found to be the dominant N form in soil solution, however, most of this DON was of high MW of which >75% remained unidentified. Free amino acids constituted less than 5% of the total DON pool. The concentration of NO3− and NH4+ was low in all soils but significantly greater than the concentration of free amino acids. Incubations of low MW DON with soil indicated a rapid processing of amino acids, di- and tri-peptides to NH4+ followed by a slower transformation of the NH4+ pool to NO3−. The rate of protein transformation to NH4+ was slower than for amino acids and peptides suggesting that the block in N mineralization in taiga forest soils is the transformation of high MW DON to low MW DON and not low MW DON to NH4+ or NH4+ to NO3−. Calculated turnover rates of amino acid-derived C and N immobilized in the soil microbial biomass were similar with a half-life of approximately 30 d indicating congruent C and N mineralization.