Nitrogen-depleted Waters Research Articles

Variability of early autumn planktonic assemblages in the strait of Gibraltar: a regionalization analysis

The Strait of Gibraltar (SG) is the only connection of the Mediterranean Sea with the global circulation. The SG is an outstanding marine region to explore physical-biological coupling of pelagic communities due to its hydrodynamic complexity, including strong tidal forcing and marked spatial gradients and fronts. The authors have unravelled the role of the fortnightly tidal scale (spring and neap tides) and local processes (upwelling and tidal-topographic mixing) that shape planktonic assemblages in the Strait. To do so, an oceanographic cruise was taken in early autumn 2008 with a high-resolution grid sampling and spring/neap tidal conditions. The planktonic features were captured using different automatic and semi-automatic techniques of plankton analyses (flow cytometry, FlowCAM, LOPC and Ecotaxa) that allowed covering a wide range of sizes of the community from pico- to mesoplankton. The SG was sectorized into two clusters based on the biogeochemical and main water column properties. Cluster 1 (CL1) covered shallow productive areas around Cape Trafalgar (CT). CL1 presented higher concentrations of chlorophyll and nutrients, and phytoplankton was mostly represented by Synechococcus and coastal diatoms while zooplankton had the highest percentage of meroplankton (31%). In contrast, cluster 2 (CL2) covered open ocean waters and presented more oligotrophic features, i.e. nitrogen-depleted waters with lower chlorophyll concentrations and a picoplankton community dominated by Prochlorococcus and holoplankton predominance in mesozooplankton. Under early autumn conditions with overall nutrient-depleted and stratified waters, the CT area emerges as an ecosystem where the constant tidal mixing and nutrients supply is coupled with an active production also being favored by high residence times and finally shaping a plankton community with unique features in the area.

Accumulation of dissolved organic carbon and nitrogen in the photic zone in the nitrogen-depleted waters of the Bay of Bengal

Dissolved organic matter (DOM) accumulates in the upper ocean for several days to years and supports both autotrophic and heterotrophic activities. To examine the source of DOM and its role in plankton metabolic processes, 18 stations were sampled in the Bay of Bengal (BoB) during June 2019. The concentration of DOC in the upper ocean exhibited a significant relationship with Chlorophyll-a (Chl-a), particulate organic carbon (POC) and DOC exudation rates, suggesting possible sources through in situ biological processes. The mean DOC: DON ratio (8.3 ± 2) and isotopic composition of carbon in DOC (δ13CDOC of −21.2 ± 1‰) indicates a marine origin in the BoB during the study period. On the other hand, dissolved organic nitrogen (DON) exhibited a significant weak inverse relationship with salinity in the northern BoB indicating some contribution from the riverine source. Phytoplankton uptake rates of amino acids (glycine or aspartic acid) were 10 times lower than dissolved inorganic nitrogen (DIN). The accumulation time of semi-labile DOC, produced by phytoplankton, in the upper ocean varied between 2.5 and 7.5 months in the BoB with the longer rate in the anticyclonic (ACE; 5.6 ± 1 months) than cyclonic eddy (CE; 3.6 ± 0.5 months) regions. This study suggests that DOM is produced through in situ processes and supports phytoplankton biomass in the BoB than hitherto hypothesized as river-borne sources and refractory in nature.

Identification of oceanic hotspots for production of the neurotoxin β-N-methylamino-L-alanine: a multidisciplinary ocean-prospecting study

Backgroundβ-N-Methylamino-L-alanine (BMAA) is an inducer of neurodegenerative disorders. Chronic ingestion of BMAA through the food chain has been suggested to cause amyotrophic lateral sclerosis and other related pathologies, including Parkinson's. BMAA is produced by cyanobacteria in various environments, including the ocean, and BMAA production increases under nitrogen-depleted conditions in pure cultures of non-nitrogen-fixing cyanobacteria. In the ocean, a large proportion of primary production of biomassis done by cyanobacteria, mostly in regions connected to highly productive eastern boundary upwelling systems. Those regions are often characterised by strong oxygen depletion, and enhanced nitrogen loss, but provide an essential fraction of fish production for local and global nutritional needs. Thus, these upwelling areas connected to oxygen-depleted intermediate waters are of great interest in terms of BMAA transfer to humans through the food chain. The predicted expansion of those oxygen-depleted and nitrogen-depleted waters might thus have a severe effect on BMAA production and consequently on human health. MethodsIn this multidisciplinary ocean-prospecting study, we used a coupled approach of satellite monitoring, chlorophyll, nitrogen, and oxygen sensing via autonomous ocean profilers (gliders) and ship-board measurements, and metagenomics mining to identify possible hotspots of BMAA production in two upwelling systems near Mauritania and Peru. The obtained environmental datasets were complemented with studies of cyanobacterial pure cultures and enrichments to quantify potential rates of BMAA production. From this, we could identify the amount of BMAA produced under different levels of oxygen and nitrogen depletion. We used a multivariate statistical model to estimate the BMAA production in the two upwelling systems under different nitrogen availabilities, and the amount of BMAA found in anchovies, which form a major part of the fish catch in those areas. FindingsSince December, 2009, we have repeatedly (at least twice per year) identified blooms of Prochlorococcus species capable of BMAA production along several hundred kilometres of the coasts of Peru and Mauritania by satellite analysis and direct pigment measurements. We measured high abundances of cyanobacteria in the range of 108 cells per mL seawater, accounting for up to 80% of picoplankton in the photic zone with the genetic capability for BMAA production. Our culture experiments with Prochlorococcus species present in both investigated regions showed a BMAA production of around 30 μg BMAA per g cells. BMAA production doubled under nitrogen-depleted conditions, with nitrogen concentrations of 3 μM as present in the oxygen-depleted waters near Peru leading to production of up to 70 μg BMAA per g cell. Considering the cell abundance of Prochlorococcus in those nitrogen depleted waters, our multivariate model predicts an accumulation of BMAA in anchovies of 3–27 μg BMAA per g, which might, depending on diet, accumulate to up to 1320 μg BMAA per g in human beings living at the coast. InterpretationFollow-up analysis will target a potential increase in BMAA production and its transport through the food chains in both upwelling systems in the context of climate change and expansion of nitrogen depleted waters along productive upwelling regions. FundingThe EU H2020 programme, grant number 704272 (NITROX), and the Danish Institute of Advanced Study.

Epiphytic cyanobacteria of the seagrass Cymodocea rotundata: diversity, diel nifH expression and nitrogenase activity

Seagrasses are photoautotrophic, ecologically important components of many globally widespread coastal ecosystems, in which combined nitrogen may limit their production. We examined the biodiversity and diazotrophic capacity of microbial epiphytes associated with the phyllosphere of the seagrass Cymodocea rotundata of the Western Indian Ocean. Light microscopy, 16S rRNA and nifH gene analysis revealed the dominance of cyanobacteria in the epiphytic microbial community. Most phylotypes were related to free-living uncultured benthic cyanobacteria, while some to cyanobacterial endosymbionts of marine diatoms. Novel and potentially diazotrophic species, some of known pantropical distribution, were also discovered. Significant diel nitrogenase activities (acetylene reduction assay) were recorded (up to 358 ± 232 nmol C2H4 g(-1) of seagrass FW h(-1)). The nifH gene expression patterns showed that heterocystous phylotypes may be the dominant diazotrophs during the day and non-heterocystous at night. These data show that C. rotundata is colonized by diverse diazotrophic cyanobacteria species and suggest that these may be beneficial partners of seagrasses in nitrogen-depleted waters.

Nitrogen nutrition in assemblages dominated by Pseudo-nitzschia spp., Alexandrium catenella and Dinophysis acuminata off the west coast of South Africa

A study was carried out in the southern Benguela upwelling system in 2006 and 2007 to establish how the nutrient regime determines community succession and the selection of harmful algal bloom species. In March 2006, Pseudo-nitzschia spp. reached concentrations of 13 × 10 6 cells l -1 , representing 80% of the total estimated phytoplankton biomass, while chlorophyll a (chl a) reached 57 µg l -1 . High rates of NO3 - uptake (ρNO3 -, maximum 0.56 µmol N l -1 h -1 ) led to NO3 - depletion (<0.1 µM). However, cell numbers remained high for several days, sustained by regenerated nitro- gen, with ƒ-ratios dropping from 0.79 to 0.12. In March 2007, a bloom of Alexandrium catenella (4.5 × 10 5 cells l -1 , 77% of the phytoplankton biomass) occurred, with surface chl a concentrations of 26 µg l -1 . NO3 - concentrations were high (10 to 17 µM), sustaining high ρNO3 - (maximum 0.61 µmol N l -1 h -1 ) and ƒ-ratios up to 0.87. In April, Dinophysis acuminata reached 3.1 × 10 4 cells l -1 (91% phytoplank- ton biomass), with NO3 - concentrations <0.5 µM and ƒ-ratios <0.1, indicative of regenerated produc- tion. Nutrient uptake kinetics showed that Pseudo-nitzschia spp. displayed the highest maximum specific uptake rates (15.0 × 10 -3 and 18.0 × 10 -3 h -1 for NO3 - and NH4 + , respectively). D. acuminata displayed the highest affinity for NH4 + , as shown by its higher α (slope of the nutrient uptake vs. con- centration curve) of 20.7 × 10 -3 , compared to 13.4 × 10 -3 and 5.9 × 10 -3 h -1 (µmol N l -1 ) -1 for Pseudo- nitzschia spp. and A. catenella, respectively. D. acuminata was an affinity strategist, successful in nitrogen-depleted waters, whereas both Pseudo-nitzschia spp. and A. catenella were velocity strate- gists, better adapted to utilising high NO3 - concentrations during upwelling pulses.

Distribution of inorganic and organic nutrients in the South Pacific Ocean − evidence for long-term accumulation of organic matter in nitrogen-depleted waters

Abstract. During the BIOSOPE cruise the RV Atalante was dedicated to study the biogeochemical properties in the South Pacific between the Marquesas Islands (141° W–8° S) and the Chilean upwelling (73° W–34° S). Over the 8000 km covered by the cruise, several different trophic situations were encountered, in particular strong oligotrophic conditions in the South Pacific Gyre (SPG, between 123° W and 101° W). In this isolated region, nitrate was undetectable between the surface and 160–180 m and only trace quantities (&lt;20 nmoles l−1) of regenerated nitrogen (nitrite and ammonium) were detected, even in the subsurface maximum. Integrated nitrate over the photic layer, which reached 165 m, was close to zero. Despite this severe nitrogen-depletion, phosphate was always present in significant concentrations (≈0.1 μmoles l−1), while silicic acid was maintained at low but classical oceanic levels (≈1 μmoles l−1). In contrast, the Marquesas region (MAR) to the west and Chilean upwelling (UPW) to the east were characterized by high nutrient concentrations, one hundred to one thousand fold higher than in the SPG. The distribution of surface chlorophyll reflected the nitrate gradient, the lowest concentrations (0.023 nmoles l−1) being measured at the centre of the SPG, where integrated value throughout the photic layer was very low (≈ 10 mg m−2). However, due to the relatively high concentrations of chlorophyll-a encountered in the DCM (0.2 μg l−1), chlorophyll-a concentrations throughout the photic layer were less variable than nitrate concentrations (by a factor 2 to 5). In contrast to chlorophyll-a, integrated particulate organic matter (POM) remained more or less constant along the study area (500 mmoles m−2, 60 mmoles m−2 and 3.5 mmoles m−2 for particulate organic carbon, particulate organic nitrogen and particulate organic phosphorus, respectively), with the exception of the upwelling, where values were two fold higher. The residence time of particulate carbon in the surface water was only 4–5 days in the upwelling, but up to 30 days in the SPG, where light isotopic δ15N signal noted in the suspended POM suggests that N2-fixation provides a dominant supply of nitrogen to phytoplankton. The most striking feature was the large accumulation of dissolved organic matter (DOM) in the SPG compared to the surrounding waters, in particular dissolved organic carbon (DOC) where concentrations were at levels rarely measured in oceanic waters (&gt;100 μmoles l−1). Due to this large pool of DOM in the SPG photic layer, integrated values followed a converse geographical pattern to that of inorganic nutrients with a large accumulation in the centre of the SPG. Whereas suspended particulate matter in the mixed layer had a C/N ratio largely conforming to the Redfield stochiometry (C/N≈6.6), marked deviations were observed in this excess DOM (C/N≈16 to 23). The marked geographical trend suggests that a net in situ source exists, mainly due to biological processes. Thus, in spite of strong nitrate-depletion leading to low chlorophyll biomass, the closed ecosystem of the SPG can accumulate large amounts of C-rich dissolved organic matter. The implications of this finding are examined, the conclusion being that, due to weak lateral advection, the biologically produced dissolved organic carbon can be accumulated and stored in the photic layer for very long periods. In spite of the lack of seasonal vertical mixing, a significant part of new production (up to 34%), which was mainly supported by dinitrogen fixation, can be exported to deep waters by turbulent diffusion in terms of DOC. The diffusive rate estimated in the SPG (134 μmolesC m−2 d−1), was quite equivalent to the particles flux measured by sediments traps.

Cell cycle distributions of prochlorophytes in the north western Mediterranean Sea

Natural populations of oceanic prochlorophytes were sampled from the northwestern Mediterranean Sea in winter, stained with the DNA-specific fluorochrome DAPI, and analysed by flow cytometry. DNA histograms exhibited two peaks (G 1 and G 2 cells, containing one and two genome copies, respectively), separated by a trough of DNA-synthesizing cells (S cells). This suggested a cell cycle with a discrete S phase similar to that observed in eucaryotes or slow-growing procaryotes. Nitrogen and light were the two key environmental factors controlling the in situ cell cycle distributions of this procaryote. When nitrate levels were below 0.4 μM or light below 0.1% of the surface intensity, most of the cells were found in G 1, suggesting that they were not actively cycling. Cells arrested in G 1 could be induced to cycle into S + G 2 by incubating them with added nitrogen. Response was a function of initial nitrate concentration and decreased with depth, indicating that it was modulated by available light. These findings strongly suggest that prochlorophytes, which are one of the key components of the picoplankton community, may grow slowly in nitrogen-depleted waters, but still have the potential to respond quickly to nitrogen pulses.

Nitrogen uptake kinetics in three year-classes of Laminaria groenlandica (Laminariales: Phaeophyta)

Nitrate and ammonium uptake rates were measured for three year-classes of the perennial macrophyte Laminaria groenlandica Rosenvinge, collected from nitrogen-depleted waters in Barkley Sound, British Columbia, Canada, in summer 1981. A time course of uptake rate revealed that ammonium uptake was high during the first hour and then decreased for all three year-classes; the opposite pattern was exhibited for the time course of nitrate uptake rate. Nitrate uptake rate increased linearly with nitrate concentration up to the highest level tested (60 μM). The nitrate uptake rate of first-year plants was three times higher than second- and third-year plants; ammonium uptake rates showed similar patterns to those for nitrate. The interaction between nitrate and ammonium was examined for first-year plants. Nitrate and ammonium were taken up simultaneously and uptake rates were identical and equal to uptake rates when only nitrate or ammonium was present in the medium. Therefore, first-year plants are able to take up twice as much inorganic nitrogen per unit time when both nitrate and ammonium are present. First-year plants showed significant diel periodicity in ammonium uptake rates, whereas second- and third-year plants showed no periodicity in nitrate or ammonium uptake rates.