Atlantic Meridional Transect Research Articles

Air-sea transfer of gas phase controlled compounds

Gases in the atmosphere/ocean have solubility that spans several orders of magnitude. Resistance in the molecular sublayer on the waterside limits the air-sea exchange of sparingly soluble gases such as SF6 and CO2. In contrast, both aerodynamic and molecular diffusive resistances on the airside limit the exchange of highly soluble gases (as well as heat). Here we present direct measurements of air-sea methanol and acetone transfer from two open cruises: the Atlantic Meridional Transect in 2012 and the High Wind Gas Exchange Study in 2013. The transfer of the highly soluble methanol is essentially completely airside controlled, while the less soluble acetone is subject to both airside and waterside resistances. Both compounds were measured concurrently using a proton-transfer-reaction mass spectrometer, with their fluxes quantified by the eddy covariance method. Up to a wind speed of 15 m s-1, observed air-sea transfer velocities of these two gases are largely consistent with the expected near linear wind speed dependence. Measured acetone transfer velocity is ∼30% lower than that of methanol, which is primarily due to the lower solubility of acetone. From this difference we estimate the “zero bubble” waterside transfer velocity, which agrees fairly well with interfacial gas transfer velocities predicted by the COARE model. At wind speeds above 15 m s-1, the transfer velocities of both compounds are lower than expected in the mean. Air-sea transfer of sensible heat (also airside controlled) also appears to be reduced at wind speeds over 20 m s-1. During these conditions, large waves and abundant whitecaps generate large amounts of sea spray, which is predicted to alter heat transfer and could also affect the air-sea exchange of soluble trace gases. We make an order of magnitude estimate for the impacts of sea spray on air-sea methanol transfer.

A neural network‐based method for merging ocean color and Argo data to extend surface bio‐optical properties to depth: Retrieval of the particulate backscattering coefficient

AbstractThe present study proposes a novel method that merges satellite ocean color bio‐optical products with Argo temperature‐salinity profiles to infer the vertical distribution of the particulate backscattering coefficient (bbp).This neural network‐based method (SOCA‐BBP for Satellite Ocean‐Color merged with Argo data to infer the vertical distribution of the Particulate Backscattering coefficient) uses three main input components: (1) satellite‐based surface estimates of bbpand chlorophyllaconcentration matched up in space and time with (2) depth‐resolved physical properties derived from temperature‐salinity profiles measured by Argo profiling floats and (3) the day of the year of the considered satellite‐Argo matchup. The neural network is trained and validated using a database including 4725 simultaneous profiles of temperature‐salinity and bio‐optical properties collected by Bio‐Argo floats, with concomitant satellite‐derived products. The Bio‐Argo profiles are representative of the global open‐ocean in terms of oceanographic conditions, making the proposed method applicable to most open‐ocean environments. SOCA‐BBP is validated using 20% of the entire database (global error of 21%). We present additional validation results based on two other independent data sets acquired (1) by four Bio‐Argo floats deployed in major oceanic basins, not represented in the database used to train the method; and (2) during an AMT (Atlantic Meridional Transect) field cruise in 2009. These validation tests based on two fully independent data sets indicate the robustness of the predicted vertical distribution of bbp. To illustrate the potential of the method, we merged monthly climatological Argo profiles with ocean color products to produce a depth‐resolved climatology of bbpfor the global ocean.

Temporal changes in total and size-fractioned chlorophyll-a in surface waters of three provinces in the Atlantic Ocean (September to November) between 2003 and 2010

Phytoplankton total chlorophyll concentration (TCHLa) and phytoplankton size structure are two important ecological indicators in biological oceanography. Using high performance liquid chromatography (HPLC) pigment data, collected from surface waters along the Atlantic Meridional Transect (AMT), we examine temporal changes in TCHLa and phytoplankton size class (PSC: micro-, nano- and pico-phytoplankton) between 2003 and 2010 (September to November cruises only), in three ecological provinces of the Atlantic Ocean. The HPLC data indicate no significant change in TCHLa in northern and equatorial provinces, and an increase in the southern province. These changes were not significantly different to changes in TCHLa derived using satellite ocean-colour data over the same study period. Despite no change in AMT TCHLa in northern and equatorial provinces, significant differences in PSC were observed, related to changes in key diagnostic pigments (fucoxanthin, peridinin, 19′-hexanoyloxyfucoxanthin and zeaxanthin), with an increase in small cells (nano- and pico-phytoplankton) and a decrease in larger cells (micro-phytoplankton). When fitting a three-component model of phytoplankton size structure — designed to quantify the relationship between PSC and TCHLa to each AMT cruise, model parameters varied over the study period. Changes in the relationship between PSC and TCHLa have wide implications in ecology and marine biogeochemistry, and provide key information for the development and use of empirical ocean-colour algorithms. Results illustrate the importance of maintaining a time-series of in-situ observations in remote regions of the ocean, such as that acquired in the AMT programme.

An Instrument for Rapid Mesozooplankton Monitoring at Ocean Basin Scale

The development and testing of a new imaging and classification system for mesozooplankton sampling over very large spatial and temporal scales is reported. The system has been evaluated on the Atlantic Meridional Transect (AMT), acquiring nearly one million images of planktonic particles over a transect of 13,500 km. These images have been acquired at a flow rate of 12.5 L per minute, in near-continuous underway mode from the ships seawater supply and in discrete mode using integrated vertical net haul samples. The aim of this development is to produce an instrument capable of delivering autonomously acquired and processed data on the biomass and taxonomic distribution of mesozooplankton over ocean-basin scales, in or near real-time, so that data are immediately available without the need for significant amounts of post-cruise processing and analysis. The hardware and image acquisition and processing software system implemented to support this development, together with some preliminary results from AMT21, are described. The images acquired during this Atlantic cruise comprise microplankton, mesoplankton, fish larvae and sampling artefacts (air bubbles, detritus, etc.), and were classified to one of 7 pre-defined taxonomic categories with 67% success.

Air–sea fluxes of oxygenated volatile organic compounds across the Atlantic Ocean

Abstract. We present air–sea fluxes of oxygenated volatile organics compounds (OVOCs) quantified by eddy covariance (EC) during the Atlantic Meridional Transect cruise in 2012. Measurements of acetone, acetaldehyde, and methanol in air as well as in water were made in several different oceanic provinces and over a wide range of wind speeds (1–18 m s−1). The ocean appears to be a net sink for acetone in the higher latitudes of the North Atlantic but a source in the subtropics. In the South Atlantic, seawater acetone was near saturation relative to the atmosphere, resulting in essentially zero net flux. For acetaldehyde, the two-layer model predicts a small oceanic emission, which was not well resolved by the EC method. Chemical enhancement of air–sea acetaldehyde exchange due to aqueous hydration appears to be minor. The deposition velocity of methanol correlates linearly with the transfer velocity of sensible heat, confirming predominant airside control. We examine the relationships between the OVOC concentrations in air as well as in water, and quantify the gross emission and deposition fluxes of these gases.

A multicomponent model of phytoplankton size structure

AbstractSize‐fractionated filtration (SFF) is a direct method for estimating pigment concentration in various size classes. It is also common practice to infer the size structure of phytoplankton communities from diagnostic pigments estimated by high‐performance liquid chromatography (HPLC). In this paper, the three‐component model of Brewin et al. (2010) was fitted to coincident data from HPLC and from SFF collected along Atlantic Meridional Transect cruises. The model accounted for the variability in each data set, but the fitted model parameters differed for the two data sets. Both HPLC and SFF data supported the conceptual framework of the three‐component model, which assumes that the chlorophyll concentration in small cells increases to an asymptotic maximum, beyond which further increase in chlorophyll is achieved by the addition of larger celled phytoplankton. The three‐component model was extended to a multicomponent model of size structure using observed relationships between model parameters and assuming that the asymptotic concentration that can be reached by cells increased linearly with increase in the upper bound on the cell size. The multicomponent model was verified using independent SFF data for a variety of size fractions and found to perform well (0.628 ≤ r ≤ 0.989) lending support for the underlying assumptions. An advantage of the multicomponent model over the three‐component model is that, for the same number of parameters, it can be applied to any size range in a continuous fashion. The multicomponent model provides a useful tool for studying the distribution of phytoplankton size structure at large scales.

Comparison of two methods to derive the size-structure of natural populations of phytoplankton

Various methods have been proposed to estimate the size structure of phytoplankton in situ, each exhibiting limitations and advantages. Two common approaches are size-fractionated filtration (SFF) and analysis of pigments derived from High Performance Liquid Chromatography (HPLC), and yet these two techniques have rarely been compared. In this paper, size-fractionated chlorophylls for pico- (<2μm), nano- (2–20μm) and micro-phytoplankton (>20μm) were estimated independently from concurrent measurements of HPLC and SFF data collected along Atlantic Meridional Transect cruises. Three methods for estimating size-fractionated chlorophyll from HPLC data were tested. Size-fractionated chlorophylls estimated from HPLC and SFF data were significantly correlated, with HPLC data explaining between 40 and 88% of the variability in the SFF data. However, there were significant biases between the two methods, with HPLC methods overestimating nanoplankton chlorophyll and underestimating picoplankton chlorophyll when compared with SFF. Uncertainty in both HPLC and SFF data makes it difficult to ascertain which is more reliable. Our results highlight the importance of using multiple methods when determining the size-structure of phytoplankton in situ, to reduce uncertainty and facilitate interpretation of data.

Refining Globigerinoides ruber Mg/Ca paleothermometry in the Atlantic Ocean

The Mg/Ca ratio of the planktonic foraminifera Globigerinoides ruber (white) has become a widely used proxy for reconstructing sea surface temperature (SST), as numerous studies have shown that temperature is the primary control on Mg/Ca ratios in foraminiferal calcite. However, a recent study of core-top sediments across an Atlantic meridional transect suggests that salinity might have a stronger control on foraminiferal Mg/Ca ratios than previously thought. By analyzing Mg/Ca ratios and δO18 values in G. ruber (white), Arbuszewski et al. (2010) found a 27% increase in G. ruber Mg/Ca ratios per 1 salinity unit increase for seawater salinities above 35.5. Here, we use shell weight analyses and SEM images from a subset of the core-tops used in the Arbuszewski et al. (2010) study across a narrow depth range (3197–3733 m) to show that G. ruber shells from the equatorial region are highly dissolved compared to those from the subtropical North and South Atlantic gyres, significantly impacting their Mg/Ca-SSTs. Shell weights from the higher-productivity equatorial region of the Atlantic are on average 20% and 15% lower than those from the oligotrophic North and South Atlantic gyres, respectively. Given the large preservation gradient along the transect studied by Arbuszewski et al. (2010), application of a single dissolution-corrected Mg/Ca:SST calibration equation (Dekens et al., 2002) on cores from the subtropical gyres and the equatorial region is not appropriate. When regional differences in preservation are considered, as well as realistic habitat depths and calcification seasons for G. ruber in temperate latitudes, we find a strong correlation between observational SSTs and calculated G. ruber Mg/Ca-SSTs in core-top samples spanning 43°N to 25°S in the Atlantic. In addition, our re-calibrated Mg/Ca-SSTs are more strongly correlated with isotopic calcification temperatures across the transect than originally reported by Arbuszewski et al. (2010). This study provides evidence that regional differences in preservation, rather than salinity, significantly affects G. ruber Mg/Ca-SSTs in the Atlantic, and supports culturing experiments that found salinity only has a minor influence on foraminiferal Mg/Ca ratios.

Composition of dissolved organic matter along an Atlantic Meridional Transect from fluorescence spectroscopy and Parallel Factor Analysis

Absorption spectra and induced fluorescence excitation emission matrices of colored dissolved organic matter were measured in water samples collected along the Atlantic Meridional Transect in different bio-geographic provinces of the Atlantic Ocean from October to November 2010. The highest values of CDOM absorption coefficient at 305nm (aCDOM(305)), were recorded at the continental margins of the English Channel and Patagonian Shelf. The lowest values of aCDOM(305) were observed in the mixed layer of both North and South Atlantic subtropical oligotrophic gyres. The DOM composition was assessed using fluorescence spectroscopy, Excitation Emission Matrix spectra (EEMs) and the Parallel Factor Analysis (PARAFAC) model in addition to spectral indices calculated from CDOM absorption spectrum and EEMs. Six different components were identified in the EEMs by PARAFAC: Two components were similar to the humic-like fraction of DOM, associated with basin scale microbial mineralization processes. These components represent allochthonous DOM in the biogeographic provinces studied. One component of marine humic-like material of autochthonous origin was associated with DOM production from marine phytoplankton. Three components were associated with protein-like DOM. Two protein-like components had the spectral characteristics of pure tryptophan and tyrosine. There was a significant difference in DOM composition both between bio-geographical provinces and above and below the mixed layer. In the mixed layer in all provinces, except the waters of the Western European Shelf, the DOM was dominated by protein-like components. At the Western European Shelf, it was dominated by humic-like components. Fluorescence intensities of humic-like components were high at the Patagonian Shelf, but were up to 40% lower compared to northern hemisphere shelf waters. Humic-like components made a significant contribution to the DOM composition of the upper mesopelagic layer in all provinces, with the highest values at the Equatorial Upwelling Zone. There was a significant inverse relationship between humic-like components and salinity and temperature and a positive relationship with Apparent Oxygen Utilization. The humification index (HIX) was linearly correlated with the intensity of the humic-like DOM components. These trends suggest that the humic-like components are in dynamic equilibrium between likely microbial production in the deep ocean and photochemical degradation in the mixed layer.

Methanol, acetaldehyde, and acetone in the surface waters of the Atlantic Ocean

[1] Oceanic methanol, acetaldehyde, and acetone concentrations were measured during an Atlantic Meridional Transect (AMT) cruise from the UK to Chile (49°N to 39°S) in 2009. Methanol (48–361 nM) and acetone (2–24 nM) varied over the track with enrichment in the oligotrophic Northern Atlantic Gyre. Acetaldehyde showed less variability (3–9 nM) over the full extent of the transect. These oxygenated volatile organic compounds (OVOCs) were also measured subsurface, with methanol and acetaldehyde mostly showing homogeneity throughout the water column. Acetone displayed a reduction below the mixed layer. OVOC concentrations did not consistently correlate with primary production or chlorophyll-a levels in the surface Atlantic Ocean. However, we did find a novel and significant negative relationship between acetone concentration and bacterial leucine incorporation, suggesting that acetone might be removed by marine bacteria as a source of carbon. Microbial turnover of both acetone and acetaldehyde was confirmed. Modeled atmospheric data are used to estimate the likely air-side OVOC concentrations. The direction and magnitude of air-sea fluxes vary for all three OVOCs depending on location. We present evidence that the ocean may exhibit regions of acetaldehyde under-saturation. Extrapolation suggests that the Atlantic Ocean represents an overall source of these OVOCs to the atmosphere at 3, 3, and 1 Tg yr−1 for methanol, acetaldehyde, and acetone, respectively.

Functional inferences of environmental coccolithovirus biodiversity

The cosmopolitan calcifying alga Emiliania huxleyi is one of the most abundant bloom forming coccolithophore species in the oceans and plays an important role in global biogeochemical cycling. Coccolithoviruses are a major cause of coccolithophore bloom termination and have been studied in laboratory, mesocosm and open ocean studies. However, little is known about the dynamic interactions between the host and its viruses, and less is known about the natural diversity and role of functionally important genes within natural coccolithovirus communities. Here, we investigate the temporal and spatial distribution of coccolithoviruses by the use of molecular fingerprinting techniques PCR, DGGE and genomic sequencing. The natural biodiversity of the virus genes encoding the major capsid protein (MCP) and serine palmitoyltransferase (SPT) were analysed in samples obtained from the Atlantic Meridional Transect (AMT), the North Sea and the L4 site in the Western Channel Observatory. We discovered nine new coccolithovirus genotypes across the AMT and L4 site, with the majority of MCP sequences observed at the deep chlorophyll maximum layer of the sampled sites on the transect. We also found four new SPT gene variations in the North Sea and at L4. Their translated fragments and the full protein sequence of SPT from laboratory strains EhV-86 and EhV-99B1 were modelled and revealed that the theoretical fold differs among strains. Variation identified in the structural distance between the two domains of the SPT protein may have an impact on the catalytic capabilities of its active site. In summary, the combined use of 'standard' markers (i.e. MCP), in combination with metabolically relevant markers (i.e. SPT) are useful in the study of the phylogeny and functional biodiversity of coccolithoviruses, and can provide an interesting intracellular insight into the evolution of these viruses and their ability to infect and replicate within their algal hosts.

Biogeographical patterns of phytoplankton community size structure in the oceans

AbstractAimDevelop a biogeographical classification of phytoplankton size distributions for the Atlantic Ocean and predict the global phytoplankton size composition based on prevailing environmental conditions.LocationAtlantic Ocean and Global OceanMethodsUsing phytoplankton size composition data, nutrient concentrations (nitrite+nitrate, phosphate, and silicate), irradiance, temperature and zooplankton abundances of the Atlantic Meridional Transect programme, we derived and tested an environmental classification method of phytoplankton size distribution with a k‐means clustering technique. We then used principal component and Dirichlet multivariate regression analyses to disentangle the relative influence of different environmental conditions on the phytoplankton size composition. Subsequently, we evaluated different probabilisitic models and selected the most parsimonious one to estimate the global phytoplankton size distributions in the world oceans based on global climatology data of the World Ocean Atlas 2009.ResultsBased only on prevailing environmental conditions and without a priori knowledge concerning, for example, the position of oceanic fronts, the primary productivity, the distribution of organisms or any geographical information, our classification method captures the size structures of phytoplankton communities across the Atlantic. We find a strong influence of temperature and nitrite+nitrate concentration on the prevalence of the different size classes, and we present evidence that both factors may act independently on structuring phytoplankton communities. While at low nitrite+nitrate concentrations temperature has a major structuring impact, at high nitrite+nitrate concentrations its influence is reduced. Finally, we show that the global distribution of phytoplankton community size structure can be predicted by a probabilistic model based only on temperature and nitrite+nitrate.Main conclusionThe global distribution of phytoplankton community size structure can be predicted with good approximation using a parsimonious probabilistic model forced by only temperature and nitrite+nitrate data.

Can we detect oceanic biodiversity hotspots from space?

Understanding the variability of marine biodiversity is a central issue in microbiology. Current observational programs are based on in situ studies, but their implementation at the global scale is particularly challenging, owing to the ocean extent, its temporal variability and the heterogeneity of the data sources on which compilations are built. Here, we explore the possibility of identifying phytoplanktonic biodiversity hotspots from satellite. We define a Shannon entropy index based on patchiness in ocean color bio-optical anomalies. This index provides a high resolution (1 degree) global coverage. It shows a relation to temperature and mid-latitude maxima in accordance with those previously evidenced in microbiological biodiversity model and observational studies. Regional maxima are in remarkable agreement with several known biodiversity hotspots for plankton organisms and even for higher levels of the marine trophic chain, as well as with some in situ planktonic biodiversity estimates (from Atlantic Meridional Transect cruise). These results encourage to explore marine biodiversity with a coordinated effort of the molecular, ecological and remote sensing communities.

Deriving phytoplankton size classes from satellite data: Validation along a trophic gradient in the eastern Atlantic Ocean

In recent years, the global distribution of phytoplankton functional types (PFT) and phytoplankton size classes (PSC) has been determined by remote sensing. Many of these methods rely on interpretation of phytoplankton size or type from pigment data, but independent validation has been difficult due to lack of appropriate in situ data on cell size.This work uses in situ data (photosynthetic pigments concentration and cell abundances) from the north-east Atlantic, along a trophic gradient, sampled from 2005 to 2010, as well as Atlantic Meridional Transect (AMT) data for the same region, to test a previously developed conceptual model, which calculates the fractional contributions of pico-, nano- and micro-plankton to total phytoplankton chlorophyll biomass (Brewin et al., 2010). The application of the model proved to be successful, as shown by low mean absolute error between data and model fit. However, regional values obtained for the model parameters had some effect on the relative distribution of size classes as a function of chlorophyll-a, compared with the results according to the original model. The regional parameterisation yielded a dominance of micro-plankton contribution for chlorophyll-a concentrations greater than 0.5mgm−3, rather than from 1.3mgm−3 in the original model. Intracellular chlorophyll-a (Chla) per cell, for each size class, was computed from the cell enumeration results (microscope counts and flow cytometry) and the chlorophyll-a concentration for that size class given by the model. The median intracellular chlorophyll-a values computed were 0.004, 0.224 and 26.78pg Chlacell−1 for pico-, nano-, and micro-plankton respectively. This is generally consistent with the literature, thereby providing an indirect validation of the method based on pigments to assign size classes. Using a satellite-derived composite image of chlorophyll-a for the study area, a map of cell abundance was generated based on the computed intracellular chlorophyll-a for each size-class, thus extending the remote-sensing method for mapping size classes of phytoplankton from chlorophyll-a concentration to mapping cell numbers in each class. The map reveals the ubiquitous presence of pico-plankton, and shows that all size classes are more abundant in more productive areas.

Particulate optical scattering coefficients along an Atlantic Meridional Transect

The particulate optical backscattering coefficient (bbp) is a fundamental optical property that allows monitoring of marine suspended particles both in situ and from space. Backscattering measurements in the open ocean are still scarce, however, especially in oligotrophic regions. Consequently, uncertainties remain in bbp parameterizations as well as in satellite estimates of bbp. In an effort to reduce these uncertainties, we present and analyze a dataset collected in surface waters during the 19th Atlantic Meridional Transect. Results show that the relationship between particulate beam-attenuation coefficient (cp) and chlorophyll-a concentration was consistent with published bio-optical models. In contrast, the particulate backscattering per unit of chlorophyll-a and per unit of cp were higher than in previous studies employing the same sampling methodology. These anomalies could be due to a bias smaller than the current uncertainties in bbp. If that was the case, then the AMT19 dataset would confirm that bbp:cp is remarkably constant over the surface open ocean. A second-order decoupling between bbp and cp was, however, evident in the spectral slopes of these coefficients, as well as during diel cycles. Overall, these results emphasize the current difficulties in obtaining accurate bbp measurements in the oligotrophic ocean and suggest that, to first order, bbp and cp are coupled in the surface open ocean, but they are also affected by other geographical and temporal variations.

Mixotrophic basis of Atlantic oligotrophic ecosystems

Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO(2) fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO(2) fixers in the oligotrophic gyres.

Distribution of Mycosporine-Like Amino Acids Along a Surface Water Meridional Transect of the Atlantic

The composition and abundance of mycosporine-like amino acids (MAAs) were investigated in the surface waters along a 13,000-km meridional transect (52° N to 45° S) in the Atlantic Ocean (Atlantic Meridional Transect programme: Cruise AMT 18: 4/10/2008-10/11/2008). MAAs were ubiquitous along the transect, although the composition of the MAAs was variable. Highest concentrations were in the far south (below 40° S; MAA >1 μg L(-1)) and in north subtropical equatorial region (NER: 0-25° N; MAA up to 0.8 μg L(-1)). Highest MAA relative to chlorophyll-a occurred in the NER (MAA/chl-a ratio between 2 and 5). MAA/chl-a significantly correlated with the preceding month's mean daily UV dose and with UV-B/UV-A. In the far south, high MAA concentrations coincided with high phytoplankton biomass, high nutrients and a deep mixed layer associated with the austral spring. Here, the phytoplankton community was dominated by micro- and nano-eukaryotes. At the NER, the high MAA/chl-a coincided with low nutrient concentrations, a shallow mixed layer depth (20-70 m) and to a lesser extent to a shallow nitracline (40-90 m). Here, the phytoplankton consisted primarily of picophytoplankton (0-0.2 μm), dominated by the pico-cyanobacteria Synechococcus sp. and Prochlorococcus sp. and by the nitrogen fixing filamentous cyanobacterium Trichodesmium. The low nitrate concentrations (<0.1 μmol L(-1)) at the NER suggest that nitrogen fixation was required for MAA production. Specific MAAs could not easily be assigned to particular groups of phytoplankton and we could not rule out the possibility that MAAs were associated with symbiotic cyanobacteria contained within heterotrophic dinoflagellates or diatoms.

Invariable biomass‐specific primary production of taxonomically discrete picoeukaryote groups across the Atlantic Ocean

Oceanic photosynthetic picoeukaryotes (< 3 µm) are responsible for > 40% of total primary production at low latitudes such as the North-Eastern tropical Atlantic. In the world ocean, warmed by climate changes, the expected gradual shift towards smaller primary producers could render the role of photosynthetic picoeukaryotes even more important than they are today. Little is still known, however, about how the taxonomic composition of this highly diverse group affects primary production at the basin scale. Here, we combined flow cytometric cell sorting, NaH¹⁴CO₃ radiotracer incubations and class-specific fluorescence in situ hybridization (FISH) probes to determine cell- and biomass-specific inorganic carbon fixation rates and taxonomic composition of two major photosynthetic picoeukaryote groups on a ∼7500-km-long latitudinal transect across the Atlantic Ocean (Atlantic Meridional Transect, AMT19). We show that even though larger cells have, on average, cell-specific CO₂ uptake rates ∼5 times higher than the smaller ones, the average biomass-specific uptake is statistically similar for both groups. On the other hand, even at a high taxonomic level, i.e. class, the contributions to both groups by Prymnesiophyceae, Chrysophyceae and Pelagophyceae are significantly different (P < 0.001 in all cases). We therefore conclude that these group's carbon fixation rates are independent of the taxonomic composition of photosynthetic picoeukaryotes across the Atlantic Ocean. Because the above applies across different oceanic regions the diversity changes seem to be a secondary factor determining primary production.

Comparison of phosphate uptake rates by the smallest plastidic and aplastidic protists in the North Atlantic subtropical gyre

The smallest phototrophic protists (<3 μm) are important primary producers in oligotrophic subtropical gyres - the Earth's largest ecosystems. In order to elucidate how these protists meet their inorganic nutrient requirements, we compared the phosphate uptake rates of plastidic and aplastidic protists in the phosphate-depleted subtropical and tropical North Atlantic (4-29°N) using a combination of radiotracers and flow cytometric sorting on two Atlantic Meridional Transect cruises. Plastidic protists were divided into two groups according to their size (<2 and 2-3 μm). Both groups of plastidic protists showed higher phosphate uptake rates per cell than the aplastidic protists. Although the phosphate uptake rates of protist cells were on average seven times (P<0.001) higher than those of bacterioplankton, the biomass-specific phosphate uptake rates of protists were one fourth to one twentieth of an average bacterioplankton cell. The unsustainably low biomass-specific phosphate uptake by both plastidic and aplastidic protists suggests the existence of a common alternative means of phosphorus acquisition - predation on phosphorus-rich bacterioplankton cells.

Basin‐scale distribution patterns of photosynthetic picoeukaryotes along an Atlantic Meridional Transect

Photosynthetic picoeukaryotes (PPEs) of a size < 3 µm play a crucial role in oceanic primary production. However, little is known of the structure of the PPE community over large spatial scales. Here, we investigated the distribution of various PPE classes along an Atlantic Meridional Transect sampled in boreal autumn 2004 that encompasses a range of ocean provinces (gyres, upwelling, temperate regions), using dot blot hybridization technology targeting plastid 16S rRNA gene amplicons. Two algal classes, Prymnesiophyceae and Chrysophyceae, dominated the PPE community throughout the Atlantic Ocean, over a range of water masses presenting different trophic profiles. However, these classes showed strongly complementary distributions with Chrysophyceae dominating northern temperate waters, the southern gyre and equatorial regions, while prymnesiophytes dominated the northern gyre. Phylogenetic analyses using both plastid and nuclear rRNA genes revealed a high diversity among members of both classes, including sequences contained in lineages with no close cultured counterpart. Other PPE classes were less prevalent along the transect, with members of the Cryptophyceae, Pelagophyceae and Eustigmatophyceae essentially restricted to specific regions. Multivariate statistical analyses revealed strong relationships between the distribution patterns of some of these latter PPE classes and temperature, light intensity and nutrient concentrations. Cryptophyceae, for example, were mostly found in the upwelling region and associated with higher nutrient concentrations. However, the key classes of Prymnesiophyceae and Chrysophyceae were not strongly influenced by the variables measured. Although there appeared to be a positive relationship between Chrysophyceae distribution and light intensity, the complementary distributions of these classes could not be explained by the variables recorded and this requires further explanation.