Daniel Delmas

Growth and phosphorus uptake by the toxic dinoflagellate Alexandrium catenella (Dinophyceae) in response to phosphate limitation.

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 (PO43−) concentrations in this environment over the 1970–1995 period. To determine if this dinoflagellate species has a competitive advantage in PO43−‐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 33PO43− uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (QP and QC). Shifts in trends of QP/QC and QP per cell (QP·cell−1) along the DR gradient allowed the definition of successive P‐stress thresholds for A. catenella cells. The maximal uptake rate of 33PO43− increased strongly with the decrease in DR and the decrease in QP/QC, displaying physiological acclimations to PO43− 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 PO43− limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO43− threshold between 0.4 and 1 μM. As lower PO43− 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.

Potential Effect of Freshwater Virus on the Structure and Activity of Bacterial Communities in the Marennes-Oléron Bay (France)

Batch culture experiments using viral enrichment were conducted to test the response of a coastal bacterial community to autochthonous (i.e., co-existing) or allochthonous riverine viruses. The effects of viral infections on bacterial dynamics and activity were assessed by epifluorescence microscopy and thymidine incorporation, respectively, whereas the effect of viral infection on bacterial community composition was examined by polymerase chain reaction-single strand conformation polymorphism 16S ribosomal RNA fingerprinting. The percentages of high nucleic acid-containing cells, evaluated by flow cytometry, were significantly correlated (r2 = 0.91, n = 12, p < 0.0001) to bacterial production, making this value a good predictor of active cell dynamics along the study. While confinement and temperature were the two principal experimental factors affecting bacterial community composition and dynamics, respectively, additions of freshwater viruses had significant effects on coastal bacterial communities. Thus, foreign viruses significantly reduced net bacterial population increase as compared to the enrichment treated with inactivated virus. Moreover, freshwater viruses recurrently and specifically affected bacterial community composition, as compared to addition of autochthonous viruses. In most cases, the combined treatment viruses and freshwater dissolved organic matter helped to maintain or even enhance species richness in coastal bacterial communities in agreement to the ‘killing the winner’ hypothesis. Thus, riverine virus input could potentially influence bacterial community composition of the coastal bay albeit with modest modification of bulk bacterial growth.

Microbial interactions in marine water amended by eroded benthic biofilm: A case study from an intertidal mudflat

Abstract In shallow macrotidal ecosystems with large intertidal mudflats, the sediment–water coupling plays a crucial role in structuring the pelagic microbial food web functioning, since inorganic and organic matter and microbial components (viruses and microbes) of the microphytobenthic biofilm can be suspended toward the water column. Two experimental bioassays were conducted in March and July 2008 to investigate the importance of biofilm input for the pelagic microbial and viral loops. Pelagic inocula (

Régulation de l'activité exoprotéolytique bactérienne aux concentrations naturelles en matière organique dissoute dans un marais maritime

Abstract The regulation of the bacterial exoproteolytic activity, at natural substrate concentrations, was studied during the survey of an Atlantic coastal marine pond (France). The regulation of this activity occurs at two different levels: on the one hand, at the cellular level, the ectoenzyme synthesis is regulated by hydrolysis substrates, dissolved combined amino acids (DCAA), and end products, dissolved free amino acids (DFAA), in terms of the relative amounts available to the cell, and on the other hand, at the ecosystem level, i.e. the hydrolytic activity, by the total amounts of DCAA and DFAA in situ. The DFAA acts as an inhibitor in enzymatic synthesis; in contrast, dissolved proteins induce the enzymatic synthesis and the exoproteolytic activitiy. These results, obtained in natural concentration conditions, confirm the functioning in situ of the ectoenzymatic activity regulation model of Chrost, until now only validated in an enriched experimental medium.