Oligotrophic Seawater Research Articles

Marine Aerosol Production via Detrainment of Bubble Plumes Generated in Natural Seawater With a Forced‐Air Venturi

AbstractDuring September–October 2016, a marine aerosol generator configured with forced‐air Venturis was deployed at two biologically productive and two oligotrophic regions of the western North Atlantic Ocean to investigate factors that modulate primary marine aerosol (PMA) production. The generator produced representative bubble size distributions with Hinze scales (0.32 to 0.95 mm radii) and void fractions (0.011 to 0.019 Lair Lsw‐1) that overlapped those of plumes produced in the surface ocean by breaking wind waves. Hinze scales and void fractions of bubble plumes varied among seawater hydrographic regions, whereas corresponding peaks and widths of bubble size distributions did not, suggesting that variability in seawater surfactants drove variability in plume dynamics. Peaks in size‐resolved number production efficiencies for model PMA (mPMA) emitted via bubble bursting in the generator were within a narrow range (0.059 to 0.069 μm geometric mean diameter) over wide ranges in subsurface bubble characteristics, suggesting that subsurface bubble size distributions were not the primary controlling factors as was suggested by previous work. Total mass production efficiencies for mPMA decreased with increasing air detrainment rates, supporting the hypothesis that surface bubble rafts attenuate mPMA mass production. Total mass and Na+ production efficiencies for mPMA from biologically productive seawater were significantly greater than those from oligotrophic seawater. Corresponding mPMA number distributions peaked at smaller sizes during daytime, suggesting that short‐lived surfactants of biological and/or photochemical origin modulated diel variability in marine aerosol production.

Microsphere packages of carotenoids: intact sea urchin eggs tracked by Raman spectroscopy tools.

Although natural exposure to ambient UV radiation in oligotrophic seawater at small depths can reach the levels responsible for cellular damage, the sea urchin Paracentrotus lividus is frequently in such sites, particularly on the southern Adriatic Sea shore. Spawning their eggs and spending their early life stage in rocky shores at depths of 0.5-2 m are the results of their successful adaptation strategies, although adults may dwell at greater depths. Surprisingly, there is a paucity of reports regarding the carotenoid content in sea urchin eggs. Beyond their important role in photoprotection against high UV exposure, cell division and early development, the content and distribution of carotenoids contribute to the successful survival of sea urchins and also determine the color of their gonads (roe), which is of commercial importance as a delicacy. Herein, for the first time, we have described the carotenoid content and distribution in intact, freshly released eggs of P. lividus species, non-destructively employing resonance Raman spectroscopy and imaging; near-infrared Raman spectroscopy revealed additional molecular carotenoid content. Echinenone and β-carotene resonance Raman signals were the most intense, and they were identified as the principal carotenoids that are preferentially accumulated in eggs rather than in gonads. Raman imaging in confocal mode revealed the uniform distribution of the carotenoid signal over the whole eggs, while the distribution of proteins appeared spotted. Egg carotenoids generally maintained their identity after 2 months of dry storage, with slight signs of C[double bond, length as m-dash]C bond oxidation. The potential utilization of P. lividus sea urchin eggs as valuable microsphere packages of native carotenoids is discussed.

Properties of Seawater Surfactants Associated with Primary Marine Aerosol Particles Produced by Bursting Bubbles at a Model Air-Sea Interface.

Surfactants account for minor fractions of total organic carbon in the ocean but can significantly influence the production of primary marine aerosol particles (PMA) at the sea surface via modulation of bubble surface tension. During September and October 2016, model PMA (mPMA) were produced from seawater by bursting bubbles at two biologically productive and two oligotrophic stations in the western North Atlantic Ocean. Total concentrations of surfactants extracted from mPMA and seawater were quantified and characterized via measurements of surface tension isotherms and critical micelle concentrations (CMCs). Surfactant CMCs in biologically productive seawater were lower than those in the oligotrophic seawater suggesting that surfactant mixtures in the two regions were chemically distinct. mPMA surfactants were enriched in all regions relative to those in the associated seawater. Surface tension isotherms indicate that mPMA surfactants were weaker than corresponding seawater surfactants. mPMA from biologically productive seawater contained higher concentrations of surfactants than those produced from oligotrophic seawater, supporting the hypothesis that seawater surfactant properties modulate mPMA surfactant concentrations. Diel variability in concentrations of seawater and mPMA surfactants in some regions is consistent with biological and/or photochemical processing. This work demonstrates direct links between surfactants in mPMA and those in the associated seawater.

Environmental and nutrient controls of marine nitrogen fixation

Biological dinitrogen (N2) fixation by diazotrophic cyanobacteria has great biogeochemical implications in nitrogen (N) cycling in the ocean as this process represents the major source of new N input to the oceans, thereby controlling the marine primary productivity. Numerous factors can affect the extent of N2 fixation. A better understanding of the major environmental and nutrient factors governing this process is highly required. Iron (Fe) and/or phosphorus (P) are thought to be limiting factors in most oceanic regions. Special attention has been given in the present study to evaluate the effects of mineral dust deposition which is believed to stimulate N2 fixation as it increases the availability of both Fe and P. Through three laboratory bioassays (+Fe, +P, +Dust) via incubation experiments performed on Trichodesmium IMS101, we found that each addition of Fe, P or desert dust could stimulate the growth and N2 fixation of Trichodesmium IMS101. Several adaptive nutrient utilization strategies were observed, such as a Fe luxury uptake mechanism, a P-sparing effect and colony formation. In addition, during a field study using natural phytoplankton assemblages from the temperate Northeast Atlantic Ocean the critical role of dissolved Fe (DFe) was again highlighted by the remarkably enhanced N2 fixation rate observed after the addition of DFe under low temperature and P-depleted conditions. At the time of our study no Trichodesmium filaments were found in the studied region, the diazotrophic community was dominated by unicellular cyanobacteria symbiont (prymnesiophyte-UCYN-A1) and heterotrophic diazotrophs, therefore demonstrating that DFe could as well be the ultimate factor limiting N2 fixation of these smaller diazotrophs. Recently, the effects of ongoing climate change (ocean warming and acidification) on N2 fixation drew much attention, but various studies led to controversial conclusions. Semi-continuous dilution growth experiments were conducted on Trichodesmium IMS101 under present-day and future high pCO2 (400 and 800 µatm, respectively) and warming seawater (24 and 28 °C) conditions. The results indicate that higher pCO2 and therefore ocean acidification may be beneficial for Trichodesmium growth and N2 fixation. However, our observations suggest that Fe or P limitation in oligotrophic seawaters may offset the stimulation induced on Trichodesmium IMS101 resulting from ocean acidification. In contrast, ocean warming may not play an important role in Trichodesmium growth and N2 fixation with a 4 °C increase from 24 °C to 28 °C. Nevertheless, ocean warming is predicted to cause a shift in the geographical distribution of Trichodesmium species toward higher latitudes, extending its niche to subtropical ocean regions and potentially reducing its coverage in tropical ocean basins.

On the fluorometric measurement of ammonium in oligotrophic seawater: Assessment of reagent blanks and interferences

AbstractDynamically distributed at trace levels in the open ocean, ammonium is one of the most important and reactive nitrogen compounds in the marine environment. Obtaining reliable measurements of ammonium concentrations is thus a prerequisite to fully understand its role in marine biogeochemical processes, but remains challenging. Among others, quantification and identification of different sources of blanks is an outstanding issue, due partly to the fact that ammonium‐free water is very difficult to prepare and/or preserve. Building on a recently developed method using solid phase extraction combined with fluorescence detection (SPE‐Flu), we examined the kinetics of the ortho‐phthaldialdehyde‐sulfite‐ammonium reaction and introduce a new approach to quantifying reagent blanks via a “reagents addition” method. The reagent blank of the method, equivalent to 6.7 ± 1.5 nmol L−1 of ammonium under our experimental settings, accounted for up to 27% of the ammonium background in seawater samples collected from the oligotrophic ocean. We also showed that the SPE‐Flu method is highly specific, with negligible interference from three types of amines and 15 types of amino acids at nanomolar concentrations, which are typical of open ocean regimes. The determination of the reagent blank allowed for optimized data reduction, which was applied to a study in the oligotrophic South China Sea. Water column profiles showed a very well‐defined structure and smooth distribution of ammonium concentrations, consistent with the distribution of other parameters. We thus contend that our proposed approach provides a way to further optimize the quantification of ammonium concentrations in natural seawater via the SPE‐Flu method.

Nitrogen sources and cycling revealed by dual isotopes of nitrate in a complex urbanized environment

Elevated nutrient inputs have led to increased eutrophication in coastal marine ecosystems worldwide. An understanding of the relative contribution of different nutrient sources is imperative for effective water quality management. Stable isotope values of nitrate (δ15NNO3-, δ18ONO3-) can complement conventional water quality monitoring programs to help differentiate natural sources of NO3− from anthropogenic inputs and estimate the processes involved in N cycling within an ecosystem. We measured nutrient concentrations, δ15NNO3-, and δ18ONO3- in 76 locations along a salinity gradient from the lower end of the Pearl River Estuary, one of China's largest rivers discharging into the South China Sea, towards the open ocean. NO3− concentrations decreased with increasing salinity, indicative of conservative mixing of eutrophic freshwater and oligotrophic seawater. However, our data did not follow conservative mixing patterns. At salinities <20 psu, samples exhibited decreasing NO3−concentrations with almost unchanged NO3− isotope values, indicating simple dilution. At salinities >20 psu, NO3− concentrations decreased, while dual NO3− isotopes increased, suggesting mixing and/or other transformation processes. Our analysis yielded mean estimates for isotope enrichment factors (15ε = −2.02‰ and 18ε = −3.37‰), Δ(15,18) = -5.5‰ and δ15NNO3- - δ15NNO2- = 12.3‰. After consideration of potential alternative sources (sewage, atmospheric deposition and groundwater) we concluded that there are three plausible interpretations for deviations from conservative mixing behaviour (1) NO3− uptake by assimilation (2) in situ NO3− production (from fixation-derived nitrogen and nitrification of sewage-derived effluents) and (3) input of groundwater nitrate carrying a denitrification signal. Through this study, we propose a simple workflow that incorporates a synthesis of numerous isotope-based studies to constrain sources and behaviour of NO3− in urbanized marine environments.

Transcriptional Response of Staphylococcus aureus to Sunlight in Oxic and Anoxic Conditions.

The transcriptional response of Staphylococcus aureus strain Newman to sunlight exposure was investigated under both oxic and anoxic conditions using RNA sequencing to gain insight into potential mechanisms of inactivation. S. aureus is a pathogenic bacterium detected at recreational beaches which can cause gastrointestinal illness and skin infections, and is of increasing public health concern. To investigate the S. aureus photostress response in oligotrophic seawater, S. aureus cultures were suspended in seawater and exposed to full spectrum simulated sunlight. Experiments were performed under oxic or anoxic conditions to gain insight into the effects of oxygen-mediated and non-oxygen-mediated inactivation mechanisms. Transcript abundance was measured after 6 h of sunlight exposure using RNA sequencing and was compared to transcript abundance in paired dark control experiments. Culturable S. aureus decayed following biphasic inactivation kinetics with initial decay rate constants of 0.1 and 0.03 m2 kJ−1 in oxic and anoxic conditions, respectively. RNA sequencing revealed that 71 genes had different transcript abundance in the oxic sunlit experiments compared to dark controls, and 18 genes had different transcript abundance in the anoxic sunlit experiments compared to dark controls. The majority of genes showed reduced transcript abundance in the sunlit experiments under both conditions. Three genes (ebpS, NWMN_0867, and NWMN_1608) were found to have the same transcriptional response to sunlight between both oxic and anoxic conditions. In the oxic condition, transcripts associated with porphyrin metabolism, nitrate metabolism, and membrane transport functions were increased in abundance during sunlight exposure. Results suggest that S. aureus responds differently to oxygen-dependent and oxygen-independent photostress, and that endogenous photosensitizers play an important role during oxygen-dependent indirect photoinactivation.

100 Days of marine Synechococcus-Ruegeria pomeroyi interaction: A detailed analysis of the exoproteome.

SummaryMarine phototroph and heterotroph interactions are vital in maintaining the nutrient balance in the oceans as essential nutrients need to be rapidly cycled before sinking to aphotic layers. The aim of this study was to highlight the molecular mechanisms that drive these interactions. For this, we generated a detailed exoproteomic time‐course analysis of a 100‐day co‐culture between the model marine picocyanobacterium Synechococcus sp. WH7803 and the Roseobacter strain Ruegeria pomeroyi DSS‐3, both in nutrient‐enriched and natural oligotrophic seawater. The proteomic data showed a transition between the initial growth phase and stable‐state phase that, in the case of the heterotroph, was caused by a switch in motility attributed to organic matter availability. The phototroph adapted to seawater oligotrophy by reducing its selective leakiness, increasing the acquisition of essential nutrients and secreting conserved proteins of unknown function. We also report a surprisingly high abundance of extracellular superoxide dismutase produced by Synechococcus and a dynamic secretion of potential hydrolytic enzyme candidates used by the heterotroph to cleave organic groups and hydrolase polymeric organic matter produced by the cyanobacterium. The time course dataset we present here will become a reference for understanding the molecular processes underpinning marine phototroph‐heterotroph interactions.

Factors That Modulate Properties of Primary Marine Aerosol Generated From Ambient Seawater on Ships at Sea

AbstractModel primary marine aerosol (mPMA) was produced by bubbling clean air through flowing natural seawater in a high‐capacity generator deployed on ships in the eastern North Pacific and western North Atlantic Oceans. Physicochemical properties of seawater and mPMA were quantified to characterize factors that modulated production. Differences in surfactant organic matter (OM) and associated properties including surface tension sustained plumes with smaller bubble sizes, slower rise velocities, larger void fractions, and older surface ages in biologically productive relative to oligotrophic seawater. Production efficiencies for mPMA number (PEnum) and mass (PEmass) per unit air detrained from biologically productive seawater during daytime were greater and mass median diameters smaller than those in the same seawater at night and in oligotrophic seawater during day and night. PEmass decreased with increasing air detrainment rate suggesting that surface bubble rafts suppressed emission of jet droplets and associated mPMA mass. Relative to bubbles emitted at 60 cm depth, PEnum for bubbles emitted from 100 cm depth was approximately 2 times greater. mPMA OM enrichment factors (EFs) and mass fractions based on a coarse frit, fine frits, and a seawater jet exhibited similar size‐dependent variability over a wide range in chlorophyll a concentrations. Results indicate that the physical production of PMA number and mass from the ocean surface varies systematically as interrelated functions of seawater type and, in biologically productive waters, time of day; bubble injection rate, depth, size, and surface age; and physical characteristics of the air‐water interface whereas size‐resolved OM EFs and mass fractions are relatively invariant.

Determination of picomolar dissolved free amino acids along a South Atlantic transect using reversed-phase high-performance liquid chromatography

Abstract Dissolved free amino acids (DFAA) in seawater are a form of nitrogen (N) available for marine microbes. In oligotrophic environments where N-containing nutrients are the limiting factor for microbial growth, N nutrition from DFAA could be crucial, but as yet it is poorly resolved. Measurements of individual DFAA are challenging as concentrations are typically in the low nmol L − 1 range. Here we report modifications to methodology using o-phthaldialdehyde (OPA) derivatization and reversed phase high performance liquid chromatography (HPLC) that provide a 30-fold improvement in sensitivity enabling the detection of 15 amino acids in seawater with a limit of detection as low as 10 pmol L − 1 with accuracy and precision of better than 10%. This analytical methodology is now suitable for the challenging quantitation of DFAA in oligotrophic seawaters. The method was successfully applied to a suite of seawater samples collected on a cruise crossing the South Atlantic Ocean, where concentrations of DFAAs were generally low (sub nmol L − 1 ), revealing basin-scale features in the oceanographic distributions of DFAA. This unique dataset implies that DFAAs are an important component of the N cycle in both near-coastal and open oceans. Further calculations suggest that the proportions of organic N originating from DFAA sources were significant, contributing between 0.2 and 200% that of NH 4 + and up to 77% that of total inorganic nitrogen in the upper 400 m in some regions of the transect.

Determination of glycolic acid in natural seawater by liquid chromatography coupled with triple quadrupole mass spectrometry

AbstractGlycolic acid, which is produced during photorespiration, is one of the major components of dissolved organic matter released by phytoplankton, comprising up to 40% of their daily primary productivity. However, the sensitivity of existing methods does not allow for accurate measurement of glycolic acid levels in oligotrophic seawater. In this study, we present a straightforward, robust, and sensitive liquid chromatography‐mass spectrometry (LC‐MS) method to quantify glycolic acid in natural seawater following liquid‐liquid extraction, requiring only a small sample size (25 mL seawater). The method has a highly sensitive detection limit of 9 nM, at least fivefold lower than existing methods. This approach was successfully applied to the analysis of seawater samples collected from both eutrophic and oligotrophic marine environments. The results showed that glycolic acid concentrations in natural seawater are positively correlated with chlorophyll a concentrations, indicating that phytoplankton contribute significantly to glycolic acid production in marine environments. As glycolic acid is an important carbon and energy source for marine bacteria, the method developed within this study allows further investigation into the rates of its release and uptake by phytoplankton and heterotrophic bacteria, respectively, as well as the role it plays in phytoplankton–bacteria interactions.

Underway analysis of nanomolar dissolved reactive phosphorus in oligotrophic seawater with automated on-line solid phase extraction and spectrophotometric system

The automated in-field determination of trace phosphate (and other nutrients) is highly valuable for studying nutrient dynamics and cycling in oligotrophic oceans. Here, we report an automated portable analyzer for week-long underway analysis of nanomolar dissolved reactive phosphorus (DRP) in seawater. The method is based on classic phosphomolybdenum blue (PMB) chemistry combined with on-line solid phase extraction (SPE) and flow analysis. Under optimized conditions, the formed PMB from sample is automatically concentrated on a hydrophilic lipophilic balanced (HLB) copolymer SPE. The PMB compound can be eluted with NaOH solution and measured in a flow-through detection system. All the components of the analyzer are computer controlled using laboratory-programmed software based on LabVIEW. The system exhibited advantages of high sensitivity (detection limit of 1.0 nmol L−1) and reproducibility (relative standard deviation of 5.4%, n = 180), insignificant carry-over effect and no interferences from salinity, silicate, arsenate and other P-containing compounds (concentrations at environmental level). The analytical time was 4–7 min/sample, depending on the DRP concentration. The accuracy of the method was validated through the analysis of reference materials and comparison with two other published methods (slope of 0.986 ± 0.027, intercept of 0.39 ± 0.64 nmol L−1, R2 of 0.9608, range of 0–80 nmol L−1, n = 57). The system has been successfully applied for a two-week continuous underway determination of DRP in surface seawater during a cruise in the South China Sea. Based on the laboratory and field evaluations, it is concluded that this system is suitable for accurate and high resolution underway DRP measurements in oligotrophic areas.

Excess phosphate loading shifts bacterioplankton community composition in oligotrophic coastal water microcosms over time

Phosphorus (P), primarily as phosphate, is considered as a key trophic factor of eutrophication in coastal environments. Information on the response of bacterial community to excess phosphate loading in oligotrophic coastal water could provide insights into the micro-ecological effects on the expanding trend of nutrient-enrichment in coastal water. In this study, temporal dynamic of bacterioplankton community composition was investigated using 16S Illumina MiSeq sequencing technique in oligotrophic seawater microcosms manipulated with no perturbation (control) and a certain level of phosphate by two loading modes: 1) one-off loading at the beginning of incubation; and 2) periodic loadings every two days over a 16-day duration. The results showed that overall bacterial alpha-diversity temporally changed and achieved the lowest value in phosphate loading (P-loading) microcosms at day 8; phosphate-addition increased the Shannon index and Pielou's evenness compared with those of the control at day 16. Bacterioplankton community turnover across all the treatments showed a similar temporal pattern. P-loading significantly affected bacterioplankton community composition with negative correlations of Methylophilaceae and Flavobacteriaceae and positive correlation of Saprospiraceae. Periodic loading treatments demonstrated a greater effect on community variation than one-off loading treatments did. Moreover, excess P-loading accelerated the temporal succession of the bacterioplankton community composition. These results provide insights into the pattern of bacterioplankton community phylogenetically responding to excess P-loading in the manipulated oligotrophic coastal system.

New bio-hybrid materials for bioremoval of crude oil spills from marine waters

We report, for the first time, on the successful use of the new type of bio-hybrid materials (BHM) on the basis of polymeric nonwoven material made from methyl methacrylate-acrylonitrile (MA) which is an efficient oil-absorbent. The biomass and biomass-derived cell-structured support materials of duckweed plants (Wolffia arrhiza, Lemna minuscula) were incorporated into polymeric fibers and association of oil-degrading bacteria was immobilized. The bacteria immobilized on BHM with duckweeds structures (BHM + AD) degraded 93–97% of n-alkanes in artificial sea water after 25 days of incubation and removed the adsorbed oil from the polymer fibers almost completely whereas the oil-degrading efficiency of the bacteria immobilized on the additive-free MA was ca. twice lower. It is suggested that, in oligotrophic sea water, the plant-derived structures might furnish biogenic elements for the immobilized microorganisms increasing their oil degrading activity. The possibilities of the use of the new type of BHM for bioremoval of crude oil spills from marine waters are discussed.

Rapid and highly sensitive analysis of chlorophylls and carotenoids from marine phytoplankton using ultra-high performance liquid chromatography (UHPLC) with the first derivative spectrum chromatogram (FDSC) technique

We developed a rapid and highly sensitive analytical method for chlorophylls and carotenoids derived from marine phytoplankton using ultra-high performance liquid chromatography (UHPLC). High-performance liquid chromatography (HPLC) has been widely used in phytoplankton pigment analysis since the 1980s for estimating the abundance, composition, and photosynthetic physiology of natural algal assemblages or laboratory cultures. However, the run-time of the HPLC analyses is generally ca. 30min or more, which is time-consuming for analysts. Our UHPLC technique enabled us to complete the separations of chlorophylls and carotenoids from marine phytoplankton within 7min with similar resolution as conventional HPLC methods. The analytical method was tested on authentic pigment standards, marine phytoplankton cultures, and field samples that were collected from the central tropical and subarctic Pacific plus the neritic Bering Sea. Critical pigment pairs that generally co-eluted as a single peak were successively resolved by obtaining the first derivative spectrum chromatograms (FDSCs) with a photodiode array (PDA) detector based on differences in pigment absorption spectra, e.g., chlorophyll (Chl) c2 and Mg 2,4 divinyl (DV) pheoporphyrin a5 monomethyl ester (MgDVP), as well as DVChl b and Chl b. Because the maximum injection volume of UHPLC is generally lower than that of HPLC to minimize the unwanted broadening of chromatographic peaks, the detection sensitivity needed to be increased, especially for oligotrophic seawater samples with low pigment concentration. To overcome this sensitivity issue, a PDA detector equipped with an 85mm path length capillary cell was used with a fluorescence detector. As a result, the limit of quantitation (LOQ) as determined by absorbance was of the order of 0.1ng for chlorophylls and carotenoids. Furthermore, a bead-beating technique using N,N-dimethylformamide (DMF) and zirconia beads was utilized to minimize the volume of the organic solvent used for pigment extraction. Our UHPLC method can replace the conventional HPLC techniques, and allows us to yield high-throughput data of the chlorophylls and carotenoids derived from marine phytoplankton.

Partitioning of nitrogen sources to algal endosymbionts of corals with long-term 15N-labelling and a mixing model

Corals are critical in supporting high productivity and biodiversity in oligotrophic seawaters by recycling nutrients. Here, we constructed a model of nitrogen cycling in coral endosymbionts, which provides a mechanism for the efficient nutrient metabolism of corals. First, we conducted a long-term 15N-labelling experiment on corals using flow-through aquaria, where 15N-labelled nitrate was continuously supplied to the corals Porites cylindrica and Montipora digitata for two months. After the labelling experiment, 15N isotope ratios were measured in the algal endosymbionts and their animal host and the nitrogen (N) dynamics through the endosymbiont cells were modelled. The model calculations showed that the algal endosymbionts in P. cylindrica and M. digitata derived an average of 80% and 50% of their N from the animal host, respectively. The finding indicates that the large N pool in the coral animal tissue plays an important role in supporting endosymbiotic function, photosynthesis, and consequent coral reef growth. The species-specific difference in available internal N was attributed to N biomass of the animal host per unit coral surface area. Algal N uptake from seawater was enhanced by 20% with the addition of phosphate in seawater. Thus, active endosymbiotic algal photosynthesis in oligotrophic seawater may be related to their dependence on the host-derived phosphorus as well as N.

Isotopic evidence for the turnover of biological reactive nitrogen in the Pearl River Estuary, south China

AbstractNitrate (NO3−) concentrations and the isotopic composition (δ15N and δ18O) of water samples from the Pearl River Estuary (PRE), south China, were measured to constrain N sources and processing in springtime. Nitrate concentrations decreased at higher salinities, and dual isotopic values were correlated with 1/[NO3−], both of which suggest mixing of eutrophic freshwater and oligotrophic seawater. However, δ15N and δ18O values did not closely follow the expected mixing lines. At low salinities (0–3.0), some samples exhibited high δ15N and low δ18O values compared with those of the riverine end‐member, indicating that sewage is a significant source of nitrate (up to 19.0% calculated from δ15N). At salinities of &gt;3.0, Δδ15N and Δδ18O values are linearly correlated with lnƒ (the fraction of NO3− remaining in the system), which is attributable to isotopic Rayleigh fractionation during phytoplankton uptake of nitrate. However, the linear relationship between Δδ15N and Δδ18O is different between the west and east PRE, with a slope of 1.18 in the west and 1.59 in the east. This difference most likely resulted from varying degrees of nitrification due to different water velocities and residence times in the two areas. Our data therefore indicate that dual nitrate isotopic signatures are a valuable way to constrain the sources and behavior of nitrate in river‐dominated estuaries.

Substrate use of Pseudovibrio sp. growing in ultra-oligotrophic seawater.

Marine planktonic bacteria often live in habitats with extremely low concentrations of dissolved organic matter (DOM). To study the use of trace amounts of DOM by the facultatively oligotrophic Pseudovibrio sp. FO-BEG1, we investigated the composition of artificial and natural seawater before and after growth. We determined the concentrations of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), free and hydrolysable amino acids, and the molecular composition of DOM by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). The DOC concentration of the artificial seawater we used for cultivation was 4.4 μmol C L-1, which was eight times lower compared to the natural oligotrophic seawater we used for parallel experiments (36 μmol C L -1). During the three-week duration of the experiment, cell numbers increased from 40 cells mL-1 to 2x104 cells mL -1 in artificial and to 3x105 cells mL -1 in natural seawater. No nitrogen fixation and minor CO2 fixation (< 1% of cellular carbon) was observed. Our data show that in both media, amino acids were not the main substrate for growth. Instead, FT-ICR-MS analysis revealed usage of a variety of different dissolved organic molecules, belonging to a wide range of chemical compound groups, also containing nitrogen. The present study shows that marine heterotrophic bacteria are able to proliferate with even lower DOC concentrations than available in natural ultra-oligotrophic seawater, using unexpected organic compounds to fuel their energy, carbon and nitrogen requirements.

Investigation of the Seasonal Variation in the Phytoplankton in the Surface Waters of the Gulf of Edremit

The aim of this study was to determine the phytoplankton species existing in the Gulf of Edremit and some environmental conditions affecting them. Samples were collected in July and October 2003 and in January and April 2004 from 3 stations through horizontal tows using a plankton net. After the analysis of plankton samples, 123 species that belong to 5 classes were identified. It was determined that one species (Phalacroma cuneus Schutt, 1895) was a new record for Turkish waters. The species composition mostly consisted of dinoflagellates (53.7%) which were followed by diatoms (43.9%). Most of the species which were identified in this study were similar to those phytoplanktonic algal species which were reported from the Eastern Mediterranean Sea in previous studies. These are neritic, oceanic, temperate and subtropical climate species. It was detected that 10 out of 19 potentially harmful algal species identified in this study were toxic species; however, they did not show an extreme increase. Dinophyceae had the highest cell number in July (1100 cell/L, St.1) and Bacillariophyceae in April (2280 cell/L, St.2). The environmental variables of the seawater such as temperature (13.0-26.5 oC), salinity (35.9-38.5‰) and dissolved oxygen (4.43-8.46 mg/L) were measured in every sampling period. Consequently, the phytoplankton species and their abundances in the Gulf of Edremit were firstly determined with this study, and phytoplankton composition representing oligotrophic sea water characteristic in the Aegean Sea was obtained.

Application of Pyrosequencing Method for Investigating the Diversity of &lt;i&gt;Synechococcus&lt;/i&gt; Subcluster 5.1 in Open Ocean

Synechococcus are distributed throughout the world’s oceans and are composed of diverse genetic lineages. However, as they are much less abundant than Prochlorococcus in oligotrophic open oceans, their in-depth genetic diversity cannot be investigated using commonly used primers targeting both Prochlorococcus and Synechococcus. Thus, in this study, we designed a primer specific to the 16S–23S rRNA internal transcribed spacer (ITS) of the Synechococcus subcluster 5.1. Using the primer, we could selectively amplify Synechococcus sequences in oligotrophic seawater samples. Further, we showed that a barcoded amplicon pyrosequencing method could be applicable to investigate Synechococcus diversity using sequences retrieved in GenBank and obtained from environmental samples. Allowing sequence analyses of a large number of samples, this high-throughput method would be useful to study global biodiversity and biogeographic patterns of Synechococcus in marine environments.