Nitrogen uptake rates and phytoplankton composition across contrasting North Atlantic Ocean coastal regimes north and south of Cape Hatteras.

Understanding nitrogen (N) uptake rates respect to nutrient availability and the biogeography of phytoplankton communities is crucial for untangling the complexities of marine ecosystems and the physical, biological, and chemical forces shaping them. In the summer of 2016, we conducted measurements of bulk microbial uptake rates for six 15N-labeled substrates: nitrate, nitrite, ammonium, urea, cyanate, and dissolve free amino acids across distinct marine provinces, including the continental shelf of the Mid-and South Atlantic Bights (MAB and SAB), the Slope Sea, and the Gulf Stream, marking the first instance of simultaneously measuring six different N uptake rates in this dynamic region. Total measured N uptake rates were lowest in the Gulf Stream followed by the SAB. Notably, the MAB exhibited significantly higher N uptake rates compared to the SAB, likely due to the excess levels of pre-existing phosphorus present in the MAB. Together, urea and nitrate uptake contributed approximately 50% of the total N uptake across the study region. Although cyanate uptake rates were consistently low, they accounted for up to 11% of the total measured N uptake at some Gulf Stream stations. Phytoplankton groups were identified based on specific pigment markers, revealing a dominance of diatoms in the shelf community, while Synechococcus, Prochlorococcus, and pico-eukaryotes dominated in oligotrophic Gulf Stream waters. The reported uptake rates in this study were mostly in agreement with previous studies conducted in coastal waters of the North Atlantic Ocean. This study suggests there are distinct regional patterns of N uptake in this physically dynamic region, correlating with nutrient availability and phytoplankton community composition. These findings contribute valuable insights into the intricate interplay of biological and chemical factors shaping N dynamics in disparate marine ecosystems.

The Occurrence of Karenia species in mid-Atlantic coastal waters: Data from the Delmarva Peninsula, USA.

A bloom of Karenia papilionacea that occurred along the Delaware coast in late summer of 2007 was the first Karenia bloom reported on the Delmarva Peninsula (Delaware, Maryland, and Virginia, USA). Limited spatial and temporal monitoring conducted by state agencies and citizen science groups since 2007 have documented that several Karenia species are an annual component of the coastal phytoplankton community along the Delmarva Peninsula, often present at background to low concentrations (100 to 10,000 cells L-1). Blooms of Karenia (> 105 cells L-1) occurred in 2010, 2016, 2018, and 2019 in different areas along the Delmarva Peninsula coast. In late summer and early autumn of 2017, the lower Chesapeake Bay experienced a K. papilionacea bloom, the first recorded in Bay waters. Blooms typically occurred summer into autumn but were not monospecific; rather, they were dominated by either K. mikimotoi or K. papilionacea, with K. selliformis, K. brevis-like cells, and an undescribed Karenia species also present. Cell concentrations during these mid-Atlantic Karenia spp. blooms equalled concentrations reported for other Karenia blooms. However, the negative impacts to environmental and human health often associated with Karenia red tides were not observed. The data compiled here report on the presence of multiple Karenia species in coastal waters of the Delmarva Peninsula detected through routine monitoring and opportunistic sampling conducted between 2007 and 2022, as well as findings from research cruises undertaken in 2018 and 2019. These data should be used as a baseline for future phytoplankton community analyses supporting coastal HAB monitoring programs.

Imaging Technologies Build Capacity and Accessibility in Phytoplankton Species Identification Expertise for Research and Monitoring: Lessons Learned During the COVID-19 Pandemic.

As primary producers, phytoplankton play an integral role in global biogeochemical cycles through their production of oxygen and fixation of carbon. They also provide significant ecosystem services, by supporting secondary production and fisheries. Phytoplankton biomass and diversity have been identified by the Global Ocean Observing System (GOOS) as Essential Ocean Variables (EOVs), properties that need to be monitored to better understand and predict the ocean system. Phytoplankton identification and enumeration relies on the skills and expertise of highly trained taxonomic analysts. The training of new taxonomic analysts is intensive and requires months to years of supervised training before an analyst is able to independently and consistently apply identification skills to a sample. During the COVID-19 pandemic, access to laboratories was greatly restricted and social distancing requirements prevented supervised training. However, access to phytoplankton imaging technologies such as the Imaging FlowCytobot (IFCB), FlowCam, and PlanktoScope, combined with open online taxonomic identification platforms such as EcoTaxa, provided a means to continue monitoring, research, and training activities remotely when in-person activities were restricted. Although such technologies can not entirely replace microscopy, they have a great potential for supporting an expansion in taxonomic training, monitoring, surveillance, and research capacity. In this paper we highlight a set of imaging and collaboration tools and describe how they were leveraged during laboratory lockdowns to advance research and monitoring goals. Anecdotally, we found that the use of imaging tools accelerated the training of new taxonomic analysts in our phytoplankton analysis laboratory. Based on these experiences, we outline how these technologies can be used to increase capacity in taxonomic training and expertise, as well as how they can be used more broadly to expand research opportunities and capacity.

Understanding controls on Margalefidinium polykrikoides blooms in the lower Chesapeake Bay.

A time-dependent model of Margalefidinium polykrikoides, a mixotrophic dinoflagellate, cell growth was implemented to assess controls on blooms in the Lafayette River, a shallow, tidal sub-tributary of the lower Chesapeake Bay. Simulated cell growth included autotrophic and heterotrophic contributions. Autotrophic cell growth with no nutrient limitation resulted in a bloom but produced chlorophyll concentrations that were 45% less than observed bloom concentrations (~80 mg Chl m-3 vs. 145 mg Chl m-3) and a bloom progression that did not match observations. Excystment (cyst germination) was important for bloom initiation, but did not influence the development of algal biomass or bloom duration. Encystment (cyst formation) resulted in small losses of biomass throughout the bloom but similarly, did not influence M. polykrikoides cell density or the duration of blooms. In contrast, the degree of heterotrophy significantly impacted cell densities achieved and bloom duration. When heterotrophy contributed a constant 30% to cell growth, and dissolved inorganic nitrogen was not limiting, simulated chlorophyll concentrations were within those observed during blooms (maximum ~140 mg Chl m-3). However, nitrogen limitation quenched the maximum chlorophyll concentration by a factor of three. Specifying heterotrophy as an increasing function of nutrient limitation, allowing it to contribute up to 50% and 70% of total growth, resulted in simulated maximum chlorophyll concentrations of 90 mg Chl m-3 and 180 mg Chl m-3, respectively. This suggested that blooms of M. polykrikoides in the Lafayette River are fortified and maintained by substantial heterotrophic nutritional inputs. The timing and progression of the simulated bloom was controlled by the temperature range, 23 °C to 28 °C, that supports M. polykrikoides growth. Temperature increases of 0.5 °C and 1.0 °C, consistent with current warming trends in the lower Chesapeake Bay due to climate change, shifted the timing of bloom initiation to be earlier and extended the duration of blooms; maximum bloom magnitude was reduced by 50% and 65%, respectively. Warming by 5 °C suppressed the summer bloom. The simulations suggested that the timing of M. polykrikoides blooms in the Lafayette River is controlled by temperature and the bloom magnitude is determined by trade-offs between the severity of nutrient limitation and the relative contribution of mixotrophy to cell growth.

Developing a 3D mechanistic model for examining factors contributing to harmful blooms of Margalefidinium polykrikoides in a temperate estuary.

Blooms of Margalefidinium (previously Cochlodinium) polykrikoides occur almost annually in summer in the lower Chesapeake Bay and its tributaries (e.g., the James and York Rivers). The Lafayette River, a sub-tributary of the lower James River, has been recognized as an initiation location for blooms in this region. The timing of bloom initiation varies interannually, ranging from late June to early August. To fully understand critical environmental factors controlling bloom initiation and interactions between physical and biological processes, a numerical module simulating M. polykrikoides blooms was developed with a focus on the bloom initiation. The module also includes life cycle and behavioral strategies such as mixotrophy, vertical migration, cyst dynamics and grazing suppression. Parameterizations for these behaviors were assigned based on published laboratory culture experiments. The module was coupled with a 3D physical-biogeochemical model for the James River that examined the contribution of each environmental factor and behavioral strategy to bloom initiation and development. Model simulation results highlight the importance of mixotrophy in maintaining high growth rates for M. polykrikoides in this region. Model results suggest that while many factors contribute to the initiation process, temperature, physical transport processes, and cyst germination are the three dominant factors controlling the interannual variability in the timing of bloom initiation.

Effects of tidal flooding on estuarine biogeochemistry: Quantifying flood-driven nitrogen inputs in an urban, lower Chesapeake Bay sub-tributary.

Sea level rise has increased the frequency of tidal flooding even without accompanying precipitation in many coastal areas worldwide. As the tide rises, inundates the landscape, and then recedes, it can transport organic and inorganic matter between terrestrial systems and adjacent aquatic environments. However, the chemical and biological effects of tidal flooding on urban estuarine systems remain poorly constrained. Here, we provide the first extensive quantification of floodwater nutrient concentrations during a tidal flooding event and estimate the nitrogen (N) loading to the Lafayette River, an urban tidal sub-tributary of the lower Chesapeake Bay (USA). To enable the scale of synoptic sampling necessary to accomplish this, we trained citizen-scientist volunteers to collect 190 flood water samples during a perigean spring tide to measure total dissolved N (TDN), dissolved inorganic N (DIN) and phosphate concentrations, and Enterococcus abundance from the retreating ebb tide while using a phone application to measure the extent of tidal inundation. Almost 95% of Enterococcus results had concentrations that exceeded the standard established for recreational waters (104 MPN 100 mL-1). Floodwater dissolved nutrient concentrations were higher than concentrations measured in natural estuarine waters, suggesting floodwater as a source of dissolved nutrients to the estuary. However, only DIN concentrations were statistically higher in floodwater samples than in the estuary. Using a hydrodynamic model to calculate the volume of water inundating the landscape, and the differences between the median DIN concentrations in floodwaters and the estuary, we estimate that 1,145 kg of DIN entered the Lafayette River during this single, blue sky, tidal flooding event. This amount exceeds the annual N load allocation for overland flow established by federal regulations for this segment of the Chesapeake Bay by 30%. Because tidal flooding is projected to increase in the future as sea levels continue to rise, it is crucial we quantify nutrient loading from tidal flooding in order to set realistic water quality restoration targets for tidally influenced water bodies.

Potential virus-mediated nitrogen cycling in oxygen-depleted oceanic waters.

Viruses play an important role in the ecology and biogeochemistry of marine ecosystems. Beyond mortality and gene transfer, viruses can reprogram microbial metabolism during infection by expressing auxiliary metabolic genes (AMGs) involved in photosynthesis, central carbon metabolism, and nutrient cycling. While previous studies have focused on AMG diversity in the sunlit and dark ocean, less is known about the role of viruses in shaping metabolic networks along redox gradients associated with marine oxygen minimum zones (OMZs). Here, we analyzed relatively quantitative viral metagenomic datasets that profiled the oxygen gradient across Eastern Tropical South Pacific (ETSP) OMZ waters, assessing whether OMZ viruses might impact nitrogen (N) cycling via AMGs. Identified viral genomes encoded six N-cycle AMGs associated with denitrification, nitrification, assimilatory nitrate reduction, and nitrite transport. The majority of these AMGs (80%) were identified in T4-like Myoviridae phages, predicted to infect Cyanobacteria and Proteobacteria, or in unclassified archaeal viruses predicted to infect Thaumarchaeota. Four AMGs were exclusive to anoxic waters and had distributions that paralleled homologous microbial genes. Together, these findings suggest viruses modulate N-cycling processes within the ETSP OMZ and may contribute to nitrogen loss throughout the global oceans thus providing a baseline for their inclusion in the ecosystem and geochemical models.

Genome-resolved viral ecology in a marine oxygen minimum zone.

Oxygen minimum zones (OMZs) are critical to marine nitrogen cycling and global climate change. While OMZ microbial communities are relatively well-studied, little is known about their viruses. Here, we assess the viral community ecology of 22 deeply sequenced viral metagenomes along a gradient of oxygenated to anoxic waters (<0.02 µmol/l O2 ) in the Eastern Tropical South Pacific (ETSP) OMZ. We identified 46 127 viral populations (≥5 kb), which augments the known viruses from ETSP by 10-fold. Viral communities clustered into six groups that correspond to oceanographic features. Oxygen concentration was the predominant environmental feature driving viral community structure. Alpha and beta diversity of viral communities in the anoxic zone were lower than in surface waters, which parallels the low microbial diversity seen in other studies. ETSP viruses were largely endemic, with the majority of shared viruses (87%) also present in other OMZ samples. We detected 543 putative viral-encoded auxiliary metabolic genes (AMGs), of which some have a distribution that reflects physico-chemical characteristics across depth. Together these findings provide an ecological baseline for viral community structure, drivers and population variability in OMZs that will help future studies assess the role of viruses in these climate-critical environments.

Cyanobacteria and cyanophage contributions to carbon and nitrogen cycling in an oligotrophic oxygen-deficient zone.

Up to half of marine N losses occur in oxygen-deficient zones (ODZs). Organic matter flux from productive surface waters is considered a primary control on N2 production. Here we investigate the offshore Eastern Tropical North Pacific (ETNP) where a secondary chlorophyll a maximum resides within the ODZ. Rates of primary production and carbon export from the mixed layer and productivity in the primary chlorophyll a maximum were consistent with oligotrophic waters. However, sediment trap carbon and nitrogen fluxes increased between 105 and 150 m, indicating organic matter production within the ODZ. Metagenomic and metaproteomic characterization indicated that the secondary chlorophyll a maximum was attributable to the cyanobacterium Prochlorococcus, and numerous photosynthesis and carbon fixation proteins were detected. The presence of chemoautotrophic ammonia-oxidizing archaea and the nitrite oxidizer Nitrospina and detection of nitrate oxidoreductase was consistent with cyanobacterial oxygen production within the ODZ. Cyanobacteria and cyanophage were also present on large (>30 µm) particles and in sediment trap material. Particle cyanophage-to-host ratio exceeded 50, suggesting that viruses help convert cyanobacteria into sinking organic matter. Nitrate reduction and anammox proteins were detected, congruent with previously reported N2 production. We suggest that autochthonous organic matter production within the ODZ contributes to N2 production in the offshore ETNP.

Utilization of urea and cyanate in waters overlying and within the eastern tropical north Pacific oxygen deficient zone.

In marine oxygen deficient zones (ODZs), which contribute up to half of marine N loss, microbes use nitrogen (N) for assimilatory and dissimilatory processes. Here, we examine N utilization above and within the ODZ of the Eastern Tropical North Pacific Ocean, focusing on distribution, uptake and genes for the utilization of two simple organic N compounds, urea and cyanate. Ammonium, urea and cyanate concentrations generally peaked in the oxycline while uptake rates were highest in the surface. Within the ODZ, concentrations were lower, but urea N and C and cyanate C were taken up. All identified autotrophs had an N assimilation pathway that did not require external ammonium: ODZ Prochlorococcus possessed genes to assimilate nitrate, nitrite and urea; nitrite oxidizers (Nitrospina) possessed genes to assimilate nitrite, urea and cyanate; anammox bacteria (Scalindua) possessed genes to utilize cyanate; and ammonia-oxidizing Thaumarchaeota possessed genes to utilize urea. Urease genes were present in 20% of microbes, including SAR11, suggesting the urea utilization capacity was widespread. In the ODZ core, cyanate genes were largely (∼95%) associated with Scalindua, suggesting that, within this ODZ, cyanate N is primarily used for N loss via anammox (cyanammox), and that anammox does not require ammonium for N loss.

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.

Biological nitrogen fixation in the oxygen-minimum region of the eastern tropical North Pacific ocean.

Biological nitrogen fixation (BNF) was investigated above and within the oxygen-depleted waters of the oxygen-minimum zone of the Eastern Tropical North Pacific Ocean. BNF rates were estimated using an isotope tracer method that overcame the uncertainty of the conventional bubble method by directly measuring the tracer enrichment during the incubations. Highest rates of BNF (~4 nM day-1) occurred in coastal surface waters and lowest detectable rates (~0.2 nM day-1) were found in the anoxic region of offshore stations. BNF was not detectable in most samples from oxygen-depleted waters. The composition of the N2-fixing assemblage was investigated by sequencing of nifH genes. The diazotrophic assemblage in surface waters contained mainly Proteobacterial sequences (Cluster I nifH), while both Proteobacterial sequences and sequences with high identities to those of anaerobic microbes characterized as Clusters III and IV type nifH sequences were found in the anoxic waters. Our results indicate modest input of N through BNF in oxygen-depleted zones mainly due to the activity of proteobacterial diazotrophs.

Growth and nitrogen uptake kinetics in cultured Prorocentrum donghaiense.

We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO3-), ammonium (NH4+), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5-500 µmol N L-1 for NO3- and NH4+, 0.5-50 µmol N L-1 for urea, 0.5-100 µmol N L-1 for glu and cyanate, and 0.5-200 µmol N L-1 for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (µm, 1.51±0.06 d-1 and 1.50±0.05 d-1, respectively). However, cultures grown on cyanate, NO3-, and NH4+ had lower half saturation constants (Kµ, 0.28-0.51 µmol N L-1). N uptake kinetics were measured in NO3--deplete and -replete batch cultures of P. donghaiense. In NO3--deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO3-, NH4+, urea and algal amino acids; uptake was saturated at or below 50 µmol N L-1. In NO3--replete batch cultures, NH4+, urea, and algal amino acid uptake kinetics were similar to those measured in NO3--deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth.

Emergence of Algal Blooms: The Effects of Short-Term Variability in Water Quality on Phytoplankton Abundance, Diversity, and Community Composition in a Tidal Estuary.

Algal blooms are dynamic phenomena, often attributed to environmental parameters that vary on short timescales (e.g., hours to days). Phytoplankton monitoring programs are largely designed to examine long-term trends and interannual variability. In order to better understand and evaluate the relationships between water quality variables and the genesis of algal blooms, daily samples were collected over a 34 day period in the eutrophic Lafayette River, a tidal tributary within Chesapeake Bay's estuarine complex, during spring 2006. During this period two distinct algal blooms occurred; the first was a cryptomonad bloom and this was followed by a bloom of the mixotrophic dinoflagellate, Gymnodinium instriatum. Chlorophyll a, nutrient concentrations, and physical and chemical parameters were measured daily along with phytoplankton abundance and community composition. While 65 phytoplankton species from eight major taxonomic groups were identified in samples and total micro- and nano-phytoplankton cell densities ranged from 5.8 × 106 to 7.8 × 107 cells L-1, during blooms, cryptomonads and G. instriatum were 91.6% and 99.0%, respectively, of the total phytoplankton biomass during blooms. The cryptomonad bloom developed following a period of rainfall and concomitant increases in inorganic nitrogen concentrations. Nitrate, nitrite and ammonium concentrations 0 to 5 days prior were positively lag-correlated with cryptomonad abundance. In contrast, the G. insriatum bloom developed during periods of low dissolved nitrogen concentrations and their abundance was negatively correlated with inorganic nitrogen concentrations.

Dinitrogen fixation in aphotic oxygenated marine environments.

We measured N2 fixation rates from oceanic zones that have traditionally been ignored as sources of biological N2 fixation; the aphotic, fully oxygenated, nitrate (NO(-) 3)-rich, waters of the oligotrophic Levantine Basin (LB) and the Gulf of Aqaba (GA). N2 fixation rates measured from pelagic aphotic waters to depths up to 720 m, during the mixed and stratified periods, ranged from 0.01 nmol N L(-1) d(-1) to 0.38 nmol N L(-1) d(-1). N2 fixation rates correlated significantly with bacterial productivity and heterotrophic diazotrophs were identified from aphotic as well as photic depths. Dissolved free amino acid amendments to whole water from the GA enhanced bacterial productivity by 2-3.5 fold and N2 fixation rates by ~2-fold in samples collected from aphotic depths while in amendments to water from photic depths bacterial productivity increased 2-6 fold while N2 fixation rates increased by a factor of 2 to 4 illustrating that both BP and heterotrophic N2 fixation were carbon limited. Experimental manipulations of aphotic waters from the LB demonstrated a significant positive correlation between transparent exopolymeric particle (TEP) concentrations and N2 fixation rates. This suggests that sinking organic material and high carbon (C): nitrogen (N) micro-environments (such as TEP-based aggregates or marine snow) could support high heterotrophic N2 fixation rates in oxygenated surface waters and in the aphotic zones. Indeed, our calculations show that aphotic N2 fixation accounted for 37 to 75% of the total daily integrated N2 fixation rates at both locations in the Mediterranean and Red Seas with rates equal or greater to those measured from the photic layers. Moreover, our results indicate that that while N2 fixation may be limited in the surface waters, aphotic, pelagic N2 fixation may contribute significantly to new N inputs in other oligotrophic basins, yet it is currently not included in regional or global N budgets.

Chromatographic determination of nanomolar cyanate concentrations in estuarine and sea waters by precolumn fluorescence derivatization.

Recent studies suggest that cyanate (OCN(-)) is a potentially important source of reduced nitrogen (N) available to support the growth of aquatic microbes and, thus, may play a role in aquatic N cycling. However, aquatic OCN(-) distributions have not been previously described because of the lack of a suitable assay for measuring OCN(-) concentrations in natural waters. Previous methods were designed to quantify OCN(-) in aqueous samples with much higher reduced N concentrations (micromolar levels) than those likely to be found in natural waters (nanomolar levels). We have developed a method to quantify OCN(-) in dilute, saline environments. In the method described here, OCN(-) in aqueous solution reacts with 2-aminobenzoic acid to produce a highly fluorescent derivative, 2,4-quinazolinedione, which is then quantified using high performance liquid chromatography. Derivatization conditions were optimized to simultaneously minimize the reagent blank and maximize 2,4-quinazolinedione formation (>90% reaction yield) in estuarine and seawater matrices. A limit of detection (LOD) of 0.4 nM was achieved with only minor matrix effects. We applied this method to measure OCN(-) concentrations in estuarine and seawater samples from the Chesapeake Bay and coastal waters from the mid-Atlantic region. OCN(-) concentrations ranged from 0.9 to 41 nM. We determined that OCN(-) concentrations were stable in 0.2 µm filtered seawater samples stored at -80 °C for up to nine months.

Marine phytoplankton temperature versus growth responses from polar to tropical waters--outcome of a scientific community-wide study.

"It takes a village to finish (marine) science these days" Paraphrased from Curtis Huttenhower (the Human Microbiome project) The rapidity and complexity of climate change and its potential effects on ocean biota are challenging how ocean scientists conduct research. One way in which we can begin to better tackle these challenges is to conduct community-wide scientific studies. This study provides physiological datasets fundamental to understanding functional responses of phytoplankton growth rates to temperature. While physiological experiments are not new, our experiments were conducted in many laboratories using agreed upon protocols and 25 strains of eukaryotic and prokaryotic phytoplankton isolated across a wide range of marine environments from polar to tropical, and from nearshore waters to the open ocean. This community-wide approach provides both comprehensive and internally consistent datasets produced over considerably shorter time scales than conventional individual and often uncoordinated lab efforts. Such datasets can be used to parameterise global ocean model projections of environmental change and to provide initial insights into the magnitude of regional biogeographic change in ocean biota in the coming decades. Here, we compare our datasets with a compilation of literature data on phytoplankton growth responses to temperature. A comparison with prior published data suggests that the optimal temperatures of individual species and, to a lesser degree, thermal niches were similar across studies. However, a comparison of the maximum growth rate across studies revealed significant departures between this and previously collected datasets, which may be due to differences in the cultured isolates, temporal changes in the clonal isolates in cultures, and/or differences in culture conditions. Such methodological differences mean that using particular trait measurements from the prior literature might introduce unknown errors and bias into modelling projections. Using our community-wide approach we can reduce such protocol-driven variability in culture studies, and can begin to address more complex issues such as the effect of multiple environmental drivers on ocean biota.

Diversity, distribution, and expression of diazotroph nifH genes in oxygen-deficient waters of the Arabian Sea.

The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is responsible for up to half of the global mesopelagic fixed nitrogen (N) loss from the ocean via denitrification and anammox. Dinitrogen (N(2)) fixation is usually attributed to cyanobacteria in the surface ocean. Model prediction and physiological inhibition of N(2) fixation by oxygen, however, suggest that N(2) fixation should be enhanced near the oxygen-deficient zone (ODZ) of the Arabian Sea. N(2) fixation and cyanobacterial nifH genes (the gene encoding dinitrogenase reductase) have been reported in surface waters overlying the Arabian Sea ODZ. Here, water samples from depths above and within the Arabian Sea ODZ were examined to explore the distribution, diversity, and expression of nifH genes. In surface waters, nifH DNA and cDNA sequences related to Trichodesmium, a diazotroph known to occur and fix N(2) in the Arabian Sea, were detected. Proteobacterial nifH phylotypes (DNA but not cDNA) were also detected in surface waters. Proteobacterial nifH DNA and cDNA sequences, as well as nifH DNA and cDNA sequences related to strictly anaerobic N -fixers, were obtained from oxygen-deficient depths. This first report of nifH gene expression in subsurface low-oxygen waters suggests that there is potential for active N(2) fixation by several phylogenetically and potentially metabolically diverse microorganisms in pelagic OMZs.

Reactivity and chemical characterization of effluent organic nitrogen from wastewater treatment plants determined by Fourier transform ion cyclotron resonance mass spectrometry.

In advanced wastewater treatment plants that achieve high levels of nitrogen (N) removal, up to one-third of the N in effluent is organic, herein referred to as effluent organic N (EON). While we know that inorganic N is highly labile, it is unclear what fraction of EON is bioavailable. In this study, we demonstrate the utility of a method that can be used to examine the reactivity of EON in natural receiving waters to better understand both the ecosystem response and the potential bioavailability of EON. The technique is suitable for analyzing polar organic matter in natural waters; electrospray ionization coupled with Fourier transform mass spectrometry. Bioassays were performed on samples collected at the end of the biological process from two wastewater treatment plants achieving advanced N removal. The samples were concentrated, and then added to natural water samples collected from the oligohaline James River, a major tributary of the Chesapeake Bay. Our results demonstrate that while the lignin-like fraction of the effluent dissolved organic matter (some of which contains N) was conserved, a large portion of aliphatic and aromatic compounds containing N was removed (79-100%) during incubations, while other compounds were produced. Furthermore, the two effluents exhibited differences in the degree of degradation and type of degradation, which can be related both to the various processes employed in the two WWTPs and the dramatic differences in the type of influent they received. These findings suggest that EON is highly reactive in the natural environment and that simple assays examining net consumption or production of bulk dissolved organic N pools are inadequate for assessing the bioavailability of EON.

Is dinitrogen fixation significant in the Levantine Basin, East Mediterranean Sea?

We report N(2) fixation rates measured from two stations monitored monthly off the Mediterranean coast of Israel during 2006 and 2007, and along a transect from Israel to Crete in September 2008. Analyses of time-series data revealed expression of nifH genes from diazotrophs in nifH clusters I and II, including cyanobacterial bloom-formers Trichodesmium and diatom-Richelia intracellularis associations. However, nifH gene abundance and rates of N(2) fixation were very low in all size fractions measured (> 0.7 µm). Volumetric (15) N uptake ranged from below detection (∼ 36% of > 300 samples) to a high of 0.3 nmol N l(-1) d(-1) and did not vary distinctly with depth or season. Areal N(2) fixation averaged ∼ 1 to 4 µmol N m(-2) d(-1) and contributed only ∼ 1% and 2% of new production and ∼ 0.25% and 0.5% of primary production for the mixed (winter) and stratified (spring-fall) periods respectively. N(2) fixation rates along the 2008 east-west transect were also extremely low (0-0.04 nmol N l(-1) d(-1), integrated average 2.6 µmol N m(-2) d(-1) ) with 37% of samples below detection and no discernable difference between stations. We demonstrate that diazotrophy and N(2) fixation contribute only a minor amount of new N to the P impoverished eastern Mediterranean Sea.