Nitrogen Fixation in Mesoscale Eddies of the North Pacific Subtropical Gyre: Patterns and Mechanisms

Mesoscale eddies have been shown to support elevated dinitrogen (N2) fixation rates (NFRs) and abundances of N2-fixing microorganisms (diazotrophs), but the mechanisms underlying these observations are not well understood. We sampled two pairs of mesoscale cyclones and anticyclones in the North Pacific Subtropical Gyre in 2017 and 2018 and compared our observations with seasonal patterns from the Hawaii Ocean Time-series (HOT) program. Consistent with previous reports, we found that NFRs were anomalously high for this region (up to 3.7-fold above previous monthly HOT observations) in the centers of both sampled anticyclones. In 2017, these elevated rates coincided with high concentrations of the diazotroph Crocosphaera. We then coupled our field-based observations, together with transcriptomic analyses of nutrient stress marker genes and ecological models, to evaluate the role of biological (via estimates of growth and grazing rates) and physical controls on populations of Crocosphaera, Trichodesmium, and diatom symbionts at the mesoscale. Our results suggest that increased Crocosphaera abundances in the 2017 anticyclone resulted from the alleviation of phosphate limitation, allowing cells to grow at rates exceeding grazing losses. In contrast, distributions of larger, buoyant taxa (Trichodesmium and diatom symbionts) appeared less affected by eddy-driven biological controls. Instead, they appeared driven by physical dynamics along frontal boundaries that separate cyclonic and anticyclonic eddies. No examined controls were able to explain our 2018 findings of higher NFRs in the anticyclone. A generalized explanation of elevated NFRs in mesoscale eddies remains challenging due to the interplay of eddy-driven bottom-up, top-down, and physical control mechanisms.This work was funded by the Simons Foundation (Award # 721252 to DMK, 721256 to AEW, 721223 to EFD, 721221 to MJC, 721244 to EVA, 721225 to STD, 329108 to SJ, and 724220 to JPZ) and expedition funding from the Schmidt Ocean Institute for R/V Falkor Cruise FK180310 in 2018.Peer reviewe

Phytoplankton dynamics in the Mediterranean Sea : approaches by high frequency sampling, modelling and Bayesian inference

L'ensemble des écosystèmes marins repose sur le phytoplancton pour convertir le carbone atmosphérique en matière organique par le processus de photosynthèse. Deux approches sont présentées pour mesurer la productivité du phytoplancton en tenant compte de l'évolution temporelle de la taille des cellules. Elles traduisent, par essence, l'assimilation progressive du carbone inorganique au cours du cycle de vie d'une cellule et sa réallocation d'une génération à l'autre au moment de la division cellulaire. Dans le milieu naturel, ce flux de carbone dépend des communautés phytoplanctoniques et de leurs sensibilités. L'observation du phytoplancton dans un milieu perturbé à faible échelle de temps et/ou d'espace est essentielle pour anticiper le changement climatique. La Méditerranée en particulier est amenée à modifier rapidement son climat et les populations qu'elle abrite. En mer Méditerranée comme dans l'océan global, les campagnes de mesures sont à la base des scénarios qui traduisent l'impact de l'environnement sur le fonctionnement et la capacité du phytoplancton à tamponner les émissions de gaz issues de l'activité humaine.The conversion of atmospheric carbon into organic matter by photosynthesis is important for all marine ecosystems. Two approaches are presented to measure phytoplankton productivity from the temporal evolution of cells' size. They traduce the incremental assimilation of inorganic carbon during cells' lifespan, before its reallocation to the next generation of cells produced by division. In the sea, the carbon flux depends on phytoplankton communities and their inherent sensitivity. The short time/spatial scale monitoring of phytoplankton in disturbed ecosystems is essential to foresee the Global change. Climate and inhabiting populations will especially reacts to Global change in the Mediterranean Sea. In the Mediterranean and in the global Ocean, observations programs are fundamental for climatic scenario used to predict the effect of environmental changes on the buffering capacity of CO₂ emissions induced by phytoplankton productivity

Seasonal distribution of ultraphytoplankton and heterotrophic prokaryotes in relation to abiotic variables on the north coast of Sfax after restoration

The Taparura project was set up to restore the north Sfax coast (Tunisia) by shutting down the northernphosphate plant responsible for chronic pollution and uncontrolled phosphogypsum dumping. The restorationeffect on coastal ultraphytoplankton (<10 lm) and heterotrophic prokaryotes was investigatedusing conventional flow cytometry over four successive seasons during 2009–2010. Cell concentrationswere generally higher than values reported for the open sea, both in the western and eastern Mediterraneanbasins. One striking point was that chl a concentration on the north Sfax coast was unchanged afterrestoration but was still one order of magnitude higher than in the Gulf of Gabès. Restoration of pH, followingthe shutdown of the phosphate processing plants on the north coast, appeared to reach normallevels for seawater during the study, whereas seawater acidification persisted on the south coast whereplants are still in operation. The largest ultraphytoplankton biomass was from an unknown cell group,whose identity and role needs to be established

Recent research developments in automated flow cytometry unravel multiple drivers of aquatic microbial diversity

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Consequence of a sudden wind event on the dynamics of a coastal phytoplankton community: an insight into specific population growth rates using a single cell high frequency approach

International audiencePhytoplankton is a key component in marine ecosystems. It is responsible for most of the marine primary production, particularly in eutrophic lagoons, where it frequently blooms. Because they are very sensitive to their environment, the dynamics of these microbial communities has to be observed over different time scales, however, assessment of short term variability is often out of reach of traditional monitoring methods. To overcome these limitations, we set up a Cytosense automated flow cytometer (Cytobuoy b.v.), designed for high frequency monitoring of phytoplankton composition, abundance, cell size, and pigment content, in one of the largest Mediterranean lagoons, the Berre lagoon (South-Eastern France). During October 2011, it recorded the cell optical properties of 12 groups of pico-, nano-, and microphytoplankton. Daily variations in the cluster optical properties were consistent with individual changes observed using microscopic imaging, during the cell cycle. We therefore used an adaptation of the size-structured matrix population model, developed by Sosik et al. (2003) to process the single cell analysis of the clusters and estimate the division rates of 2 dinoflagellate populations before, during, and after a strong wind event. The increase in the estimated in situ daily cluster growth rates suggest that physiological changes in the cells can prevail over the response of abundance

Zooplankton size distribution in the Atlantic Ocean: recent estimates from a global pelagic size structure database and relationships with environmental factors

International audiencePlankton ecologists have long used the Normalized Biomass Size Spectrum (NBSS) as a common framework to study the size distribution of aquatic bacteria, phytoplankton, and zooplankton across various temporal and spatial scales. Systematic NBSS measurements have shown that its shape varies across ecosystems and could be used as an indicator of the state of the ecosystem. Indeed, NBSS slopes typically indicate how efficiently biomass is transferred across sizes, impacting the throughput of the biological carbon pump. Zooplankton, in particular, influences the carbon pump by feeding on and repackaging phytoplankton production, as well as respiring O2 at rates that may exceed its replenishment, contributing to the existence of Oxygen Minimum Zones (OMZ) located in the Pacific, Indian, and Atlantic Oceans. Using a range of non-intrusive imaging devices, which produce community composition datasets along with complementary size measurements of individual organisms, we investigate the size distribution of zooplankton in the Atlantic Ocean. As part of our ongoing efforts to create a Pelagic Size Structure database (PSSdb, https://www.st.nmfs.noaa.gov/copepod/pssdb/), we will present a regional assessment of zooplankton NBSSs computed at different scales and compare them to estimates generated at the global scale. We will also investigate the response of zooplankton to environmental factors, including low concentrations of dissolved O2, to foresee how the gatekeepers of the marine biological carbon pump may be impacted by future anthropogenic pressures

Automatic recognition of flow cytometric phytoplankton functional groups using convolutional neural networks

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Standard vocabulary, consensual functional groups and automated classification for phytoplankton high throughput datasets using automated flow cytometry

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Combining laser diffraction, flow cytometry and optical microscopy to characterize a nanophytoplankton bloom in the Northwestern Mediterranean

International audienceThe study of particle size distribution (PSD) gives insights on the dynamics of distinct pools of particles in the ocean, which reflect the functioning of the marine ecosystem and the efficiency of the carbon pump. In this study, we combined continuous particle size estimations and discrete measurements focused on phytoplankton to describe a spring bloom in the North West Mediterranean Sea. During April 2013, about 90 continuous profiles of PSD quantified through in situ laser diffraction and transmissiometry (the Laser in situ Scattering and Transmissiometry Deep (LISST-Deep), Sequoia Sc) were complemented by Niskin bottle samples for flow cytometry analyses, taxonomic identification by optical microscopy and pigments quantification. In the euphotic zone, the PSD shape seen by the LISST was fairly stable with two particle volume peaks covering the 2–11 µm and 15–109 µm size fractions. The first pool strongly co-varied with the chlorophyll fluorescence emitted by phytoplankton cells. In addition, over the 2–11 µm fraction, the LISST derived abundance was highly correlated with the abundance of nanophytoplankton counted by flow cytometry. Microscopy identified a phytoplankton community dominated by nanodiatoms and nanoflagellates. High correlation of LISST derived particle carbon and Particulate Organic Carbon and high nitrogen in the Particulate Organic Matter also supported a dominance of actively growing phytoplankton cells in this pool. The second, broader pool of particles covering sizes 15–109 µm was possibly microflocs coming from rivers and/or sediments. This study demonstrates the complementarity of continuous measurements of PSD combined with discrete measurements to better quantify size, abundance, biomass, and spatial (both vertical and horizontal) distribution of phytoplankton in open ocean environments