SHORT‐TERM TEMPORAL VARIABILITY OF AMMONIUM AND UREA UPTAKE BY ALEXANDRIUM CATENELLA (DINOPHYTA) IN CULTURES1

In batch cultures of four Mediterranean strains (from France, Italy, and Spain) of Alexandrium catenella (Whedon et Kof.) Balech growing on a daily light cycle, ammonium and urea uptake were estimated by the ¹⁵N tracer technique. Ammonium uptake could be described by Michaelis–Menten kinetics along a substrate gradient of 0.1–10 μgat N · L^?1 for the four strains, while two different patterns were observed for urea uptake with Michaelis–Menten kinetics for one strain and linear kinetics for the others. In all cases, an increase in uptake rates with time was noted over the daylight period. This trend led to a net increase in the maximum uptake rate (V_max; for saturable kinetics) and in the initial slope α. For ammonium, V_max increased by a factor of 2–10 depending on the strain, and, for urea, the maximal uptake rates measured increased by a factor of 2–18. Temporal variations of half‐saturation constants (K_s) for both nutrients did not show a clear trend. Increases in V_max and α showed an acclimation of the cells’ uptake system over time to a N pulse, which may be explained by the light periodicity. For two strains, extensive ammonium release was observed during urea assimilation. This mechanism removes urea from the medium, so it is no longer available to other potential competitors, but supplies N back to the medium in the form of ammonium. From a methodological point of view, the phenomenon leads to considerable underestimates of the contribution of urea to phytoplankton growth.     

Diversity surveys and evolutionary relationships of aoxB genes in aerobic arsenite-oxidizing bacteria

A new primer set was designed to specifically amplify ca. 1,100 bp of aoxB genes encoding the As(III) oxidase catalytic subunit from taxonomically diverse aerobic As(III)-oxidizing bacteria. Comparative analysis of AoxB protein sequences showed variable conservation levels and highlighted the conservation of essential amino acids and structural motifs. AoxB phylogeny of pure strains showed well-discriminated taxonomic groups and was similar to 16S rRNA phylogeny. Alphaproteobacteria-, Betaproteobacteria-, and Gammaproteobacteria-related sequences were retrieved from environmental surveys, demonstrating their prevalence in mesophilic As-contaminated soils. Our study underlines the usefulness of the aoxB gene as a functional marker of aerobic As(III) oxidizers.     

Bacterial Community Structure and Functional arrA Gene Diversity Associated with Arsenic Reduction and Release in an Industrially Contaminated Soil

This study aimed at evaluating potential arsenic (As) mobility in an industrially contaminated soil (64 mg/kg of As) of the Meuse River basin, and at identifying key bacterial groups that drive soil As dynamics. Both speciation and release of As from this soil was followed under anaerobic conditions using a laboratory batch experiment. In the presence of exogenous carbon sources, As^V initially present in the soil matrix and/or adsorbed on synthetic hydrous ferric oxides were solubilized and mainly reduced to As^III by indigenous soil microflora. After a 1-month incubation period in these biotic conditions, As^III accounted for 80–85% of the total dissolved As and more than 60% of the solid-phase As. Bacterial community structure (i.e., 16S rDNA-based capillary electrophoresis single-strand conformation polymorphism profiles) changed with incubation time and As amendment. The detection of distantly related arsenate respiratory reductase genes (arrA), as functional markers of As^V respirers, indicates that novel dissimilatory As^V-reducing bacteria may be involved in As biotransformation and mobility in anoxic soils. Since As and iron were concomitantly released, a crucial role of indirect As-mobilizing bacteria on As behavior was also revealed. Our results show that the majority of As within the soil matrix was bioavailable and bioaccessible for heterotrophic As^V reduction to As^III, which may increase As toxicity and mobility in the contaminated soils.     

Assessment of the allelochemical activity of Ostreopsis cf. ovata and the ovatoxins towards competitive benthic microalgae

Aquatic Ecology - Recurrent blooms of the toxic dinoflagellate Ostreopsis cf. ovata are frequently reported in the Northwestern Mediterranean Sea. The impact of these proliferations on other...     

Variable inter and intraspecies alkaline phosphatase activity within single cells of revived dinoflagellates

Adaptation of cell populations to environmental changes is mediated by phenotypic variability at the single-cell level. Enzyme activity is a key factor in cell phenotype and the expression of the alkaline phosphatase activity (APA) is a fundamental phytoplankton strategy for maintaining growth under phosphate-limited conditions. Our aim was to compare the APA among cells and species revived from sediments of the Bay of Brest (Brittany, France), corresponding to a pre-eutrophication period (1940’s) and a beginning of a post-eutrophication period (1990’s) during which phosphate concentrations have undergone substantial variations. Both toxic marine dinoflagellate Alexandrium minutum and the non-toxic dinoflagellate Scrippsiella acuminata were revived from ancient sediments. Using microfluidics, we measured the kinetics of APA at the single-cell level. Our results indicate that all S. acuminata strains had significantly higher APA than A. minutum strains. For both species, the APA in the 1990’s decade was significantly lower than in the 1940’s. For the first time, our results reveal both inter and intraspecific variabilities of dinoflagellate APA and suggest that, at a half-century timescale, two different species of dinoflagellate may have undergone similar adaptative evolution to face environmental changes and acquire ecological advantages.Subject terms: Climate-change ecology, Microbial ecology     

GROWTH AND PHOSPHORUS UPTAKE BY THE TOXIC DINOFLAGELLATE ALEXANDRIUM CATENELLA (DINOPHYCEAE) IN RESPONSE TO PHOSPHATE LIMITATION1

Alexandrium catenella (Whedon et Kof.) Balech has exhibited seasonal recurrent blooms in the Thau lagoon (South of France) since first reported in 1995. Its appearance followed a strong decrease (90%) in phosphate (PO₄^3?) concentrations in this environment over the 1970–1995 period. To determine if this dinoflagellate species has a competitive advantage in PO₄^3?‐limited conditions in terms of nutrient acquisition, semicontinuous cultures were carried out to characterize phosphorus (P) uptake by A. catenella cells along a P‐limitation gradient using different dilution rates (DRs). Use of both inorganic and organic P was investigated from measurements of ³³PO₄^3? uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (Q_P and Q_C). Shifts in trends of Q_P/Q_C and Q_P per cell (Q_P·cell?1) along the DR gradient allowed the definition of successive P‐stress thresholds for A. catenella cells. The maximal uptake rate of ³³PO₄^3? increased strongly with the decrease in DR and the decrease in Q_P/Q_C, displaying physiological acclimations to PO₄^3? limitation. Concerning maximal APA per cell, the observation of an all‐or‐nothing pattern along the dilution gradient suggests that synthesis of AP was induced and maximized at the cellular scale as soon as PO₄^3? limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO₄^3? threshold between 0.4 and 1 μM. As lower PO₄^3? concentrations are regularly observed during A. catenella blooms in Thau lagoon, a significant portion of P uptake by A. catenella cells in the field may come from organic compounds.     

Discrepancies between net particulate carbon production and 13C‐labelled bicarbonate uptake by Alexandrium catenella (Dinophyceae): grazing controls the balance between autotrophic and non autotrophic carbon acquisition1

Inorganic carbon uptake by Alexandrium catenella estimated from incorporation of ¹³C labelled bicarbonate (an estimate of carbon gain by autotrophy) was compared to increases in particulate carbon (PC) that integrate all processes leading to carbon gain by cells (autotrophy, heterotrophy, mixotrophy). During blooms of A. catenella in the field, the ¹³C tracer technique could account for only 47% (range 29%–59%) of the increase in PC in conventional 24 h incubations. From dilution experiments, the ratio of PC increases to bicarbonate uptake was related significantly and positively to the grazing rate, indicating that dissolved organic carbon contributes to growth as a direct function of grazing activity. In addition, as grazing rate increases, the contribution of dissolved inorganic carbon uptake to carbon‐based growth decreases in a linear way (from 56% to 33% of total C acquisition) and the contribution of non autotrophic processes increases (from 54% to 67%). Thus, grazing appears to closely control the balance between autotrophic and non autotrophic processes leading to carbon acquisition by natural populations of A. catenella.     

Stress‐related Responses in Alexandrium tamarense Cells Exposed to Environmental Changes

Organisms tend to be sensitive to drastic changes in environmental conditions. For unicellular microorganisms, variations in physico‐chemical conditions are particularly challenging and may result in acclimation, entrance into quiescence, or death through necrotic or autocatalytic pathways. This study focuses on the thecate dinoflagellate Alexandrium tamarense. Cellular responses to oxidative, thermal, and nutrient stress were characterized using stress indicators, such as pigment content, efficiency of photosystem II or production of reactive oxygen species (ROS), as well as hallmarks of apoptosis including activity of caspase‐like enzymes and expression of a metacaspase gene homolog. The formation of temporary cysts, a survival strategy of short‐term quiescence, was also monitored. Cellular responses appeared to depend on multifactorial influences where type and intensity of stimulus as well as position in cell cycle may act in combination. Sequences of events observed implicate ROS production as a key determinant of stress‐related pathways, playing potential roles in intracellular signaling, formation of temporary cysts, or cellular damage. Variations observed in caspase‐like activities and metacaspase gene expression did not appear to be associated with programmed cell death pathways; our results suggest a wider range of functions for these proteases in phytoplankton cells, including roles in survival pathways and cell cycle progression.     

Current distribution and potential expansion of the harmful benthic dinoflagellate Ostreopsis cf. siamensis towards the warming waters of the Bay of Biscay,North-East Atlantic

In a future scenario of increasing temperatures in North-Atlantic waters, the risk associated with the expansion of the harmful, benthic dinoflagellate Ostreopsis cf. siamensis has to be evaluated and monitored. Microscopy observations and spatio-temporal surveys of environmental DNA (eDNA) were associated with Lagrangian particle dispersal simulations to: (i) establish the current colonization of the species in the Bay of Biscay, (ii) assess the spatial connectivity among sampling zones that explain this distribution, and (iii) identify the sentinel zones to monitor future expansion. Throughout a sampling campaign carried out in August to September 2018, microscope analysis showed that the species develops in the south-east of the bay where optimal temperatures foster blooms. Quantitative PCR analyses revealed its presence across almost the whole bay to the western English Channel. An eDNA time-series collected on plastic samplers showed that the species occurs in the bay from April to September. Due to the water circulation, colonization of the whole bay from the southern blooming zones is explained by inter-site connectivity. Key areas in the middle of the bay permit continuous dispersal connectivity towards the north. These key areas are proposed as sentinel zones to monitor O. cf. siamensis invasions towards the presumably warming water of the North-East Atlantic.