Bermuda Atlantic Time Series Station Research Articles

Deep metagenomic sequencing unveils novel SAR202 lineages and their vertical adaptation in the ocean

SAR202 bacteria in the Chloroflexota phylum are abundant and widely distributed in the ocean. Their genome coding capacities indicate their potential roles in degrading complex and recalcitrant organic compounds in the ocean. However, our understanding of their genomic diversity, vertical distribution, and depth-related metabolisms is still limited by the number of assembled SAR202 genomes. In this study, we apply deep metagenomic sequencing (180 Gb per sample) to investigate microbial communities collected from six representative depths at the Bermuda Atlantic Time Series (BATS) station. We obtain 173 SAR202 metagenome-assembled genomes (MAGs). Intriguingly, 154 new species and 104 new genera are found based on these 173 SAR202 genomes. We add 12 new subgroups to the current SAR202 lineages. The vertical distribution of 20 SAR202 subgroups shows their niche partitioning in the euphotic, mesopelagic, and bathypelagic oceans, respectively. Deep-ocean SAR202 bacteria contain more genes and exhibit more metabolic potential for degrading complex organic substrates than those from the euphotic zone. With deep metagenomic sequencing, we uncover many new lineages of SAR202 bacteria and their potential functions which greatly deepen our understanding of their diversity, vertical profile, and contribution to the ocean’s carbon cycling, especially in the deep ocean.

Change of coral carbon isotopic response to anthropogenic Suess effect since around 2000s

The stable carbon isotope composition (δ13C) in coral skeletons can be used to reconstruct the evolution of the dissolved inorganic carbon (DIC) in surface seawater, and its long-term declining trend during the past 200 years (~1800–2000) reflects the effect of anthropogenic Suess effect on carbonate chemistry in surface oceans. The global atmospheric CO2 concentration still has been increasing since 2000, and the Suess effect is intensifying. Considering the coral's ability of resilience and acclimatization to external environmental stressors, the response of coral δ13C to Suess effect may change and needs to be re-evaluated. In this study, ten long coral δ13C time series synthesized from different oceans were used to re-evaluate the response of coral carbonate chemistry to Suess effect under the changing environments. These δ13C time series showed a long-term declining trend since 1960s, but the declining rates slowed in eight time series since around 2000s. Considering that the declining rates of the DIC-δ13C in surface seawater from the Hawaii Ocean Time-series Station and Bermuda Atlantic Time-series Station has not changed since 2000 compared with those during 1960–1999, the change in the coral δ13C trends at eight of ten locations may indicate that the response of coral δ13C to the anthropogenic Suess effect has changed since around 2000s. This change may have resulted from coral acclimatization to external environmental stressors. To adapt to acidifying oceans, coral may have the ability to regulate the source of DIC in extracellular calcifying fluid and/or the utilization way of DIC, therefore the response of coral δ13C to anthropogenic Suess effect will change accordingly.

A marine carbon monoxide (CO) model with a new parameterization of microbial oxidation

Traditionally, marine carbon monoxide (CO) models assume that the microbial oxidation of CO is only dependent on the concentration of CO in the water column. However, CO oxidation rates in the ocean have been reported to vary up to two orders of magnitude both spatially and temporally. Here, we developed a new model assuming that CO microbial oxidation is dependent on bacterial carbon biomass other than CO concentration. In addition to microbial oxidation, the model also describes CO photochemical production, vertical mixing, and air-sea gas exchange. The new CO model has been embedded in the European Regional Seas Ecosystem Model (ERSEM) and coupled with the General Ocean Turbulence Model (GOTM). The CO-GOTM-ERSEM model was implemented at the Bermuda Atlantic Time Series (BATS) station to simulate CO concentrations observed in March 1993 by Kettle (1994). The proposed second-order function describing CO microbial oxidation introduces a new parameter, the bacteria biomass specific oxidation rate, which was estimated to be 5.7 ± 0.2 (μg C m−3)−1 h−1. Statistical metrics indicates that the new CO model performs better than a previously published model with a first-order decay function to describe microbial oxidation, acknowledging the dependence of microbial oxidation on bacterial abundance is realistic. A long-term (1992 - 1994) simulation carried out with CO-GOTM-ERSEM reproduced the spatial and seasonal variability of CO reported in the literature. Our model provides a realistic description of the CO dynamics and is potentially usable in different environmental contexts worldwide.

Global database of ratios of particulate organic carbon to thorium-234 in the ocean: improving estimates of the biological carbon pump

Abstract. The ocean's biological carbon pump (BCP) plays a major role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer as settling particles (e.g., marine snow). Since the seminal works on the BCP, global estimates of the global strength of the BCP have improved but large uncertainties remain (from 5 to 20 Gt C yr−1 exported below the euphotic zone or mixed-layer depth). The 234Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage of allowing a downward flux to be determined by integrating the deficit of 234Th in the upper water column and coupling it to the POC∕234Th ratio in sinking particles. However, the factors controlling the regional, temporal, and depth variations of POC∕234Th ratios are poorly understood. We present a database of 9318 measurements of the POC∕234Th ratio in the ocean, from the surface down to >5500 m, sampled on three size fractions (∼>0.7 µm, ∼1–50 µm, ∼>50 µm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived in the data repository PANGAEA® under https://doi.org/10.1594/PANGAEA.911424 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2018, and the data have been obtained from published papers and open datasets available online. Unpublished data have also been included. Multiple measurements can be found in most of the open ocean provinces. However, there is an uneven distribution of the data, with some areas highly sampled (e.g., China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific, and the south Indian oceans. Some coastal areas, although in a much smaller number, are also included in this global compilation. Globally, based on different depth horizons and climate zones, the median POC∕234Th ratios have a wide range, from 0.6 to 18 µmol dpm−1.

Determination of the copper isotope composition of seawater revisited: A case study from the Mediterranean Sea

A new technique for the determination of dissolved copper isotope composition (δ65Cu) of seawater was applied to examine copper sources and internal cycling in the Mediterranean Sea along the GEOTRACES GA04N transect. A succession of chelating resin with nitrilotriacetic acid functional groups and strong base anion exchange resin, together with optimization of the multi-collector inductively coupled plasma mass spectrometry set-up allowed to isolate copper from seawater matrix and to measure the 65Cu/63Cu ratios in seawater with an external precision of 0.06‰ (2 s.d.). This method was first applied for inter-comparison measurements to surface and deep waters sampled at the Bermuda Atlantic Time-Series station (BATS) in the North Atlantic Ocean during the GEOTRACES Intercalibration Cruise 1. Disparities in δ65Cu reported here and in the literature over these samples showed a need to investigate whether a new inter-comparison sample should be decided upon, or whether the use of UV-oxidation can also affect the measurement of δ65Cu. We also measured total dissolved Cu concentration ([dCu]T) and δ65Cu for 12 stations in the Mediterranean Sea. The δ65Cu distribution showed significant variations in the euphotic zone, at the Chl a maximum and at bottom depths. Copper isotope ratios ranged from +0.21‰ to +0.76‰ (±0.06‰, 2 s.d.), yielding an average of +0.51‰ (±0.20‰, 2 s.d.; n = 96) for the Mediterranean Sea. A strong zonation between natural aerosol deposition to the South and anthropogenic aerosol deposition to the North was seen in the sample set (Dulaquais et al. 2017; Gerringa et al. 2017; Rolison et al. 2015). Natural dust deposits seemed to draw δ65Cu toward lower values and deeper in the euphotic zone whereas the impact of anthropogenic aerosols seemed restricted to an increase in [dCu]T. At the Chl a maximum, δ65Cu showed significant increase which we attributed to scavenging on and/or uptake by phytoplankton. The isotope signature of Cu sources to the Mediterranean Sea was also investigated and we inferred a significant source of isotopically heavy Cu in the Gibraltar area, potentially originating from a release of Cu sulfide leached in the mining area of Southern Spain (i.e. Iberian Pyrite Belt) in rivers and transported to the Mediterranean Sea in surface by seasonal water mass circulation. Conversely, marine sediments appear to be a source of isotopically-light Cu to deep waters. This study provides new hints on the use of copper isotopes to study sources and sinks of Cu in seawater. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Clade and strain specific contributions of Synechococcus and Prochlorococcus to carbon export in the Sargasso Sea

AbstractThe marine picocyanobacteria Prochlorococcus and Synechococcus dominate primary production in oligotrophic oceans, but their role in the particulate organic carbon (POC) flux into the deeper ocean remains unresolved. Here, we present for the first‐time data on the absolute clade and strain‐specific contribution to POC flux of two of the most abundant clades of Synechococcus (II and III) and strains of Prochlorococcus (high‐light strain MIT 9312 and low‐light strain MIT 9313) in the Sargasso Sea, by targeting the 23S‐16S rDNA internally transcribed spacer region using quantitative Polymerase Chain Reaction. We collected seawater samples and sinking particles using shallow sediment traps during winter and summer 2012 around the Bermuda Atlantic Time Series station. Both clades of Synechococcus had a higher contribution to the total POC flux during winter compared to the summer, with both clades contributing up to 2.7% to the total winter POC flux. Prochlorococcus was more abundant during the summer, but both strains of Prochlorococcus contributed less than 0.2% to the POC flux in both seasons. We found these differences in the contribution to flux to be mainly due to the smaller size of the Prochlorococcus. In addition, the flux of Synechococcus clade III, and Prochlorococcus MIT9312 was enhanced relative to their standing stock in a cyclonic eddy sampled during winter 2012. Using quantification of genetic markers, this study establishes distinct clade‐and strain specific patterns in POC flux among the most abundant picophytoplankton groups in the Atlantic Ocean.

Seasonal variation in the copepod gut microbiome in the subtropical North Atlantic Ocean.

Characterisation of marine copepod gut microbiome composition and its variability provides information on function of marine food webs, biogeochemical cycles and copepod health. Copepod gut microbiomes were investigated quarterly over two years at the Bermuda Atlantic Time-series Station in the North Atlantic Subtropical Gyre, while assessing seasonal shifts in stable and transient communities. Microbial communities were analysed using amplicon sequencing targeting the bacterial 16S rRNA V3-V4 region and the cyanobacterial ntcA gene. Persistent bacterial groups belonging to Firmicutes, Bacteroidetes and Actinobacteria were present in the copepod guts throughout the year, and showed synchronous changes, suggesting a link to variability in copepod nutritional content. The gut communities were separate from those in the seawater, suggesting the copepod gut hosts long-term, specialized communities. Major temporal variations in the gut communities during the early winter and spring, specifically a high relative abundance of Synechococcus (up to 65%), were attributed to bacterioplankton shifts in the water column, and copepod grazing on these picoplanktonic cyanobacteria. The presence of obligate and facultative anaerobes, including Clostridiales year round, suggests that anaerobic bacterial processes are common in these dynamic microhabitats in the oligotrophic open ocean.

A new statistical approach for interpreting oceanic fCO2 data

Surface ocean CO2 fugacity (fCO2) often exhibits large fluctuations and heterogeneity because of multiple controlling factors that pose a challenge for trend analysis, especially in the ocean margins. We propose a new statistical approach, Generalized Additive Mixed Modeling (GAMM), to interpret oceanic fCO2 data in two ocean margins (Japan and Europe) and two open ocean areas. The latter included areas near the Hawaii Ocean Time-series (HOT) and the Bermuda Atlantic Time-series (BATS). This method utilizes day of year, sea surface salinity (SSS), sea surface temperature (SST), and sampling date as predictors. Using this method, we were able to derive multidecadal fCO2 trends with both improved precision and greater robustness to data gaps compared to an existing deseasonalization method used in the open ocean. The fCO2 trend derived by our method for the Japanese margin (1992–2013), the European margin (1989–2014), and the open ocean near HOT (1983–2013) were 2.1±0.6, 1.9±0.7, and 2.0±0.5μatmyear−1 (mean±standard deviation of multiple 1°×1° grids in margins and 5°×5° grids in open ocean), respectively, and the fCO2 trends were all close to the atmosphere CO2 trend (1.7–1.9μatmyear−1). Our analysis produced generally smaller standard errors (paired t-test, p≪0.001) than those obtained using the existing method based on the same dataset. In addition, our method was less sensitive to data gaps compared to this existing method. However, for regularly spaced fCO2 times-series data, for example, discrete bottle data collected in the BATS station in 1991–2011, this method was not advantageous over the existing method (1.9±0.2 vs. 2.0±0.2μatmyear−1). To test the broader applicability of this method, we compared fCO2 trends in the Southern Ocean derived using our method with those from a recently reported Markov Chain Monte Carlo method and found no significant difference between the two sets of values. Therefore, we recommend the application of our method in interpreting fCO2 data in different oceanic environments.

Mesoscale and sub-mesoscale variability in phytoplankton community composition in the Sargasso Sea

The Sargasso Sea is a dynamic physical environment in which strong seasonal variability combines with forcing by mesoscale (~100km) eddies. These drivers determine nutrient, light, and temperature regimes and, ultimately, the composition and productivity of the phytoplankton community. On four cruises (2011 and 2012; one eddy per cruise), we investigated links between water column structure and phytoplankton community composition in the Sargasso at a range of time and space scales. On all cruises, cyanobacteria (Prochlorococcus and Synechococcus) dominated the phytoplankton numerically, while haptophytes were the dominant eukaryotes (up to 60% of total chl-a). There were substantial effects of mesoscale and sub-mesoscale forcing on phytoplankton community composition in both spring and summer. Downwelling (in anticyclones) resulted in Prochlorococcus abundances that were 22−66% higher than at ‘outside’ stations. Upwelling (in cyclones) was associated with significantly higher abundances and POC biomass of nanoeukaryotes. In general, however, each eddy had its own unique characteristics. The center of anticyclone AC1 (spring 2011) had the lowest phytoplankton biomass (chl-a) of any eddy we studied and had lower nitrate+nitrite (N+N <5mmolm−2) and eukaryote chl-a biomass as compared to its edge and to the Bermuda Atlantic Time-Series station (BATS). At the center of cyclone C1 (summer 2011), we observed uplift of the 26.5kgm−3 isopycnal and high nutrient inventories (N+N=74±46mmolm−2). We also observed significantly higher haptophyte chl-a (non-coccolithophores) and lower cyanobacterial chl-a at the center and edge of C1 as compared to outside the eddy at BATS. Cyclone C2 (spring 2012) exhibited a deep mixed layer, yet had relatively low nutrient concentrations. We observed a shift in the taxonomic composition of haptophytes between a coccolithophore-dominated community in C2 (98% of total haptophyte chl-a) and a non-coccolithophore community at BATS. In summer 2012, downwelling associated with anticyclone AC2 occurred at the edge of the eddy (not at the center), where AC2 interacted with a nearby cyclone. At the edge, we found significantly lower Synechococcus abundances and higher eukaryote chl-a compared to the center of AC2 and BATS. These along-transect nuances demonstrate the significance of small-scale perturbations that substantially alter phytoplankton community structure. Therefore, while seasonality in the North Atlantic is the primary driver of broad-scale trends in phytoplankton community composition, the effects of transient events must be considered when studying planktonic food webs and biogeochemical cycling in the Sargasso Sea.

Meta-omic signatures of microbial metal and nitrogen cycling in marine oxygen minimum zones.

Iron (Fe) and copper (Cu) are essential cofactors for microbial metalloenzymes, but little is known about the metalloenyzme inventory of anaerobic marine microbial communities despite their importance to the nitrogen cycle. We compared dissolved O2, NO, NO, Fe and Cu concentrations with nucleic acid sequences encoding Fe and Cu-binding proteins in 21 metagenomes and 9 metatranscriptomes from Eastern Tropical North and South Pacific oxygen minimum zones and 7 metagenomes from the Bermuda Atlantic Time-series Station. Dissolved Fe concentrations increased sharply at upper oxic-anoxic transition zones, with the highest Fe:Cu molar ratio (1.8) occurring at the anoxic core of the Eastern Tropical North Pacific oxygen minimum zone and matching the predicted maximum ratio based on data from diverse ocean sites. The relative abundance of genes encoding Fe-binding proteins was negatively correlated with O2, driven by significant increases in genes encoding Fe-proteins involved in dissimilatory nitrogen metabolisms under anoxia. Transcripts encoding cytochrome c oxidase, the Fe- and Cu-containing terminal reductase in aerobic respiration, were positively correlated with O2 content. A comparison of the taxonomy of genes encoding Fe- and Cu-binding vs. bulk proteins in OMZs revealed that Planctomycetes represented a higher percentage of Fe genes while Thaumarchaeota represented a higher percentage of Cu genes, particularly at oxyclines. These results are broadly consistent with higher relative abundance of genes encoding Fe-proteins in the genome of a marine planctomycete vs. higher relative abundance of genes encoding Cu-proteins in the genome of a marine thaumarchaeote. These findings highlight the importance of metalloenzymes for microbial processes in oxygen minimum zones and suggest preferential Cu use in oxic habitats with Cu > Fe vs. preferential Fe use in anoxic niches with Fe > Cu.

Depth-dependent photodegradation of marine dissolved organic matter

Marine dissolved organic matter (DOM) in surface and deep waters of the eastern Atlantic Ocean and Sargasso Sea was analyzed by excitation emission matrix (EEM) fluorescence spectroscopy and parallel factor analysis (PARAFAC). Photo-degradation with semi-continuous monitoring of EEMs and absorbance spectra was used to measure the photo-degradation kinetics and changes of the PARAFAC components in a depth profile of DOM at the Bermuda Atlantic Time Series (BATS) station in the Sargasso Sea. A five component model was fit to the EEMs, which included traditional terrestrial-like, marine-like, and protein-like components. Terrestrial-like components showed the expected high photo-reactivity, but surprisingly, the traditional marine-like peak showed slight photo-production in surface waters, which may account for its prevalence in marine systems. Surface waters were depleted in photo-labile components while protein-like fluorescent components were enriched, consistent with previous studies. Ultra-high resolution mass spectrometry detected unique aliphatic compounds in the surface waters at the BATS site, which may be photo-produced or photo-stable. Principle component and canonical analysis showed strong correlations between relative contributions of unsaturated/aromatic molecular formulas and depth, with aliphatic compounds more prevalent in surface waters and aromatic compounds in deep waters. Strong correlations were seen between these aromatic compounds and humic-like fluorescent components. The rapid photo-degradation of the deep-sea fluorescent DOM in addition to the surface water relative depletion of aromatic compounds suggests that deep-sea fluorescent DOM is very unlikely to survive overturning circulation.

Intercomparison of dissolved trace elements at the Bermuda Atlantic Time Series station

The international GEOTRACES programme is emphasising the use of the GEOTRACES reference sample programme and the importance of cross-over or baseline stations where all aspects of sample collection, filtration, processing and analytical methods can be compared. These intercomparison efforts are of crucial importance to merge or link data from different origins together. The occupation of the Bermuda Atlantic Time Series (BATS) station (31°45.92′N, 64°04.95′W) by the Netherlands GEOTRACES (GA02; 13 June 2010) as well as US GEOTRACES (GA03; 19–21 November 2011) Atlantic section expeditions provided an intercomparison opportunity to confirm the compatibility of the 2 different sampling systems as well as different analytical techniques used. In order to compare the data from different sampling systems or analytical techniques, a new statistical approach was developed to include the analytical uncertainty. Furthermore, modifications to an existing multi-element technique (Biller and Bruland, 2012) were made that increased the number of elements analysed and that allows the time consuming extractions to be done shipboard. Overall, we show excellent agreement between data generated by different sampling systems and analytical techniques. However, while both sampling systems are capable of collecting uncontaminated samples for all the GEOTRACES key elements, some apparent outliers are present. The intercomparison between the different analytical techniques also reveals subtle differences that would have gone unnoticed if only reference samples would have been used, underlining the importance and power of intercomparison stations. The results show that cross-over station data can be used to assess consistency between datasets if these stations have been carefully planned and analysed in combination with (internal) seawater reference samples to assure intra-dataset consistency.

Dynamic variability of dissolved Pb and Pb isotope composition from the U.S. North Atlantic GEOTRACES transect

This study presents dissolved Pb concentration and isotopic composition distributions from GEOTRACES GA03, the U.S. North Atlantic Transect. Pb in the ocean is primarily derived from anthropogenic sources and Pb fluxes into the North Atlantic Ocean have been steadily decreasing following the phase-out of alkyl leaded gasoline usage in North America and Europe between 1975 and 1995. A compilation of dissolved Pb profiles from three stations occupied repeatedly during the last three decades reveals a dramatic decrease in concentrations within the surface layers and the thermocline maxima, although elevated concentrations greater than 60pmol/kg are still observed in the center of the North Atlantic gyre where ventilation timescales are longer than at the western boundary. The evolution of stable Pb isotopes at these stations shows a shift from dominantly North American-like composition in surface waters in the early 1980s towards a more European-like composition in later years. The most recent shallow signatures at the Bermuda Atlantic Time Series station (BATS) show an even more recent trend returning to higher 206Pb/207Pb ratios after the completed phase-out of leaded gasoline in Europe, presumably because recently deposited Pb is more strongly influenced by industrial and incineration Pb than by residual alkyl leaded gasoline utilization. In surface waters, trends toward a more prominent European influence are also found in the middle of the basin and toward the European coast, coincident with higher concentrations of surface dissolved Pb. Scavenging of anthropogenic Pb is observed within the TAG hydrothermal plume, and it is unclear if there is any significant contribution to deep water by basaltic Pb leached by hydrothermal fluids. In the upper water column, many stations along the transect show Pb concentration maxima at ~100m depth, coincident with a low 206Pb/207Pb isotopic signature that is typical of European emission sources. Although Pb ores from the United States historically tend to carry 206Pb/207Pb signatures >1.17 (Hurst, 2002), subsurface signatures as low as 1.1563 in 206Pb/207Pb were observed in this feature. This signature appears to be carried westward within saline Subtropical Underwater (STUW), that ventilates from the Central Eastern part of the North Atlantic Subtropical Gyre where the lowest surface isotope 206Pb/207Pb ratios are observed. Along the western boundary, deep water masses of different ages carry distinct isotope ratios corresponding to their respective times of ventilation. Finally, a low 206Pb/207Pb signature in bottom water along the Eastern margin suggests that there may be some mobilization of European-derived anthropogenic Pb from recent surface deposits on the ocean floor.

Photochemistry of marine and fresh waters: A role for copper–dissolved organic matter ligands

The photodegradation of marine dissolved organic matter (DOM) plays a critical role in the global carbon cycle. The photodegradation of both DOM and particulate organic matter (POM) leads to the formation of carbon monoxide and/or the production of labile organic matter that can be rapidly biodegraded. DOM also regulates metal speciation, particularly important for the bioavailability of nutrient metals required by phytoplankton. Copper is one of these essential nutrients but is also toxic in elevated concentrations. Its speciation is regulated by DOM photodegradation however, DOM–copper ligands are not well characterized because of their low concentrations in natural waters. Copper immobilized metal affinity chromatography (IMAC) was used to isolate high and low affinity DOM ligands from both Pacific near-shore seawater and freshwater from the Black River (NC). Laser flash photolysis (LFP) was used to characterize excited state species from these fractions and to identify excited state species in the open ocean through a depth profile from 5 to 4532m (North Atlantic, Bermuda Atlantic Time Series Station (BATS)). LFP generated transients characteristic of the solvated electron (τ=3.5±0.1μs), a microbial-sourced triplet excited state (τ=1.1μs±0.1μs), and a long-lived transient, DOM*, with a millisecond lifetime. Unlike the seawater triplet excited state, a copper ligand from Black River water generated a triplet excited state with a lifetime of 3.4±0.1μs. Fractions were also characterized by excitation–emission matrix fluorescence spectroscopy (EEMs), UV–visible, and 1H NMR spectroscopy. EEMs spectra correlated the marine excited state species with the protein-like, microbial fraction while the freshwater transients were associated with chromophoric DOM (CDOM). 1H NMR with spectral database matching identified ninety-seven compounds as potential sources of these excited state species in the Pacific seawater IMAC fractions. Aqueous extracts of marine macro-algae (Sargassum natans (Atlantic Ocean), Macrocystis pyrifera), and surf grass (Phyllospadix torreyi) were analyzed to investigate potential additional sources of these excited state transients. To our knowledge, this is the first study to characterize DOM–copper ligands in seawater using a combination of EEMs, NMR and laser flash photolysis.

Phytoplankton growth formulation in marine ecosystem models: Should we take into account photo-acclimation and variable stoichiometry in oligotrophic areas?

The aim of this study is to evaluate the consequences of accounting for variable Chl:C (chlorophyll:carbon) and C:N (carbon:nitrogen) ratios in the formulation of phytoplankton growth in biogeochemical models. We compare the qualitative behavior of a suite of phytoplankton growth formulations with increasing complexity: 1) a Redfield formulation (constant C:N ratio) without photo-acclimation (constant Chl:C ratio), 2) a Redfield formulation with diagnostic chlorophyll (variable and empirical Chl:C ratio), 3) a quota formulation (variable C:N ratio) with diagnostic chlorophyll, and 4) a quota formulation with prognostic chlorophyll (dynamic variable). These phytoplankton growth formulations are embedded in a simple marine ecosystem model in a 1D framework at the Bermuda Atlantic Time-series (BATS) station. The model parameters are tuned using a stochastic assimilation method (micro-genetic algorithm) and skill assessment techniques are used to compare results. The lowest misfits with observations are obtained when photo-acclimation is taken into account (variable Chl:C ratio) and with non-Redfield stoichiometry (variable C:N ratio), both under spring and summer conditions. This indicates that the most flexible models (i.e., with variable ratios) are necessary to reproduce observations. As seen previously, photo-acclimation is essential in reproducing the observed deep chlorophyll maximum and subsurface production present during summer. Although Redfield and quota formulations of C:N ratios can equally reproduce chlorophyll data the higher primary production that arises from the quota model is in better agreement with observations. Under the oligotrophic conditions that typify the BATS site no clear difference was detected between quota formulations with diagnostic or prognostic chlorophyll.

Controls on the Redox Potential of Rainwater

Hydrogen peroxide acting as a reductant affects the redox potential of rainwater collected at the Bermuda Atlantic Time Series Station, the South Island of New Zealand, the contiguous USA, and the primary study site in Wilmington, NC. Analytical measurements of both halves of redox couples for dissolved iron, mercury, and the nitrate-nitrite-ammonium system can predict the rainwater redox potential measured directly by a platinum electrode. Measurements of these redox couples along with the pH in rain yields pe⁻ between 8 and 11; the half reaction for hydrogen peroxide acting as a reductant using typical rainwater conditions of 15 μM H₂O₂ at pH 4.7 gives pe⁻ = 9.12, where pe⁻ = negative log of the activity of hydrated electrons. Of the six rainwater redox systems investigated, only manganese speciation appeared to be controlled by molecular oxygen (pe⁻ = 15.90). Copper redox speciation was consistent with superoxide acting as a reductant (pe⁻ = 2.7). The concentration of H₂O₂ in precipitation has more than doubled over the preceding decade due to a decrease in SO₂ emissions, which suggests the redox chemistry of rainwater is dynamic and changing, potentially altering the speciation of many organic compounds and trace metals in atmospheric waters.

Intercalibration of selected anthropogenic radionuclides for the GEOTRACES Program

As part of the GEOTRACES Program, six laboratories participated in an intercalibration exercise on several anthropogenic radionuclides of interest. The effort was successful for 239,240Pu activity, 240Pu/239Pu isotope ratio, and 137Cs activity measured in filtered seawater samples from the Bermuda Atlantic Time Series station (BATS) and a site on the continental slope of the Northeastern U.S. A limited number of analyses were reported for 237Np, 241Am, 90Sr, and 238Pu in filtered seawater. Intercalibration of any of the isotopes of interest in filtered particulate matter was unsuccessful due to insufficient size of the samples distributed. Methods used were based on traditional radio‐counting techniques and inductively coupled plasma mass spectrometry (ICP‐MS). Although the majority of analyses were performed on samples ≥ 60 L, one lab demonstrated the ability to analyze several of the anthropogenic radionuclides on 10–20 L sample volumes using ICP‐MS.

GEOTRACES intercalibration of 230Th, 232Th, 231Pa, and prospects for 10Be

Nineteen labs representing nine nations participated in the GEOTRACES intercalibration initiative that determined concentrations of 232Th, 230Th, 231Pa, or 10Be in seawater, suspended particles or sediments. Results generally demonstrated good agreement among labs that analyzed marine sediments. Two sets of seawater samples, aliquots of particulate material filtered in situ, and/or aliquots of biogenic sediments were distributed to participating labs. Internal consistency among participating labs improved substantially between the first and second set of seawater samples. Contamination was a serious problem for 232Th. Standard Niskin bottles introduced no detectable contamination, whereas sample containers, reagents, and labware were implicated as sources of contamination. No detectable differences in concentrations of dissolved 232Th, 230Th, or 231Pa were observed among samples of seawater filtered through Nuclepore, Supor, or QMA (quartz) filters with pore diameters ranging between 0.4 and 1.0 µm. Isotope yield monitors equilibrate with dissolved Th in seawater on a time scale of much less than 1 day. Samples of filtered seawater acidified to a pH between 1.7 and 1.8 experienced no detectable loss of dissolved Th or Pa during storage for up to 3 years. The Bermuda Atlantic Time Series station will serve as a GEOTRACES baseline station for future intercalibration of 232Th and 230Th concentrations in seawater. Efforts to improve blanks and standard calibration are ongoing, as is the development of methods to determine concentrations of particulate nuclides, tests of different filtration methods, and an increasing awareness of the need to define protocols for reporting uncertainties.

Complete Genome Sequence of Croceibacter atlanticus HTCC2559 T

Here we announce the complete genome sequence of Croceibacter atlanticus HTCC2559(T), which was isolated by high-throughput dilution-to-extinction culturing from the Bermuda Atlantic Time Series station in the Western Sargasso Sea. Strain HTCC2559(T) contained genes for carotenoid biosynthesis, flavonoid biosynthesis, and several macromolecule-degrading enzymes. The genome confirmed physiological observations of cultivated Croceibacter atlanticus strain HTCC2559(T), which identified it as an obligate chemoheterotroph.

Simulating dimethylsulphide seasonality with the Dynamic Green Ocean Model PlankTOM5

We study the dynamics of dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) using the global ocean biogeochemistry model PlankTOM5, which includes three phytoplankton and two zooplankton functional types (PFTs). We present a fully prognostic DMS module describing intracellular particulate DMSP (DMSPp) production, concentrations of dissolved DMSP (DMSPd), and DMS production and consumption. The model produces DMS fields that compare reasonably well with the observed annual mean DMS fields, zonal mean DMS concentrations, and its seasonal cycle. Modeled ecosystem composition and modeled total chlorophyll influenced mean DMS concentrations and DMS seasonality at mid‐ and high latitudes, but did not control the seasonal cycle in the tropics. The introduction of a direct, irradiation‐dependent DMS production term (exudation) in the model improved the match between modeled and observed DMS seasonality, but deteriorated simulated zonal mean concentrations. In PlankTOM5, exudation was found to be most important for DMS seasonality in the tropics, and a variable DMSP cell quota as a function of light and nutrient stress was more important than a PFT‐specific minimal DMSPp cell quota. The results suggest that DMS seasonality in the low latitudes is mostly driven by light. The agreement between model and data for DMS, DMSPp, and DMSPd is reasonable at the Bermuda Atlantic Time Series Station, where the summer paradox is observed.