Discrepancies between net particulate carbon production and (13) C-labelled bicarbonate uptake by Alexandrium catenella (Dinophyceae): grazing controls the balance between autotrophic and non autotrophic carbon acquisition(1).

Inorganic carbon uptake by Alexandrium catenella estimated from incorporation of (13) C 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 (13) C 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.

Genetic diversity of Amoebophryidae (Syndiniales) during Alexandrium catenella/tamarense (Dinophyceae) blooms in the Thau lagoon (Mediterranean Sea, France).

During toxic spring and fall blooms produced by the dinoflagellate Alexandrium in the Thau lagoon (Mediterranean Sea), we monitored the presence of Amoebophryidae (Syndiniales), a group of parasites virulent toward a wide range of dinoflagellate hosts. A PCR-biased approach unveiled the presence of at least 10 different parasitic groups during Alexandrium proliferation. However, fluorescent in situ hybridization failed to reveal parasitic infection inside Alexandrium cells in field populations. In contrast, several co-occurring, less abundant thecate dinoflagellate species were infected by Amoebophryidae, showing up to 10% of infected cells. We concluded that Alexandrium populations were not infected by these local parasites, at least during our survey. In order to check this resistance capacity on a more global scale, we cross-infected several Alexandrium strains isolated from the Thau lagoon with one strain of the parasite Amoebophrya sp. originating from Salt Pond, MA, USA. All of these hosts were strongly infected by the North American parasite, leading to the conclusion that blooming Alexandrium in the Thau lagoon were not particularly resistant to this kind of parasite. These results provide additional evidence that dinoflagellates may become invasive when they successfully escaped their natural enemies in time and/or space (the "enemy release" hypothesis).

A simple and innovative method for species identification of phytoplankton cells on minute quantities of DNA.

Dinoflagellates belonging to the genus Alexandrium are often involved in harmful algal blooms. Their ecological exploration is thus essential to increase our knowledge on these toxic events. Yet, population genetic studies, taxonomic identification and environmental monitoring are hampered by major constraints: the necessity to establish monoclonal cultures from environmental samples and the sensitivity of available molecular tools. The present work describes a very simple and sensitive method for extraction and amplification of DNA at the infra-single-cell level. Its on-slide format allows for easy visual control of both quality and quantity of the templates. Combined with a semi-multiplex PCR protocol designed on the 18S-28S rDNA-ITS region of Alexandrium catenella and Alexandrium tamarense, this procedure allowed the identification and discrimination of these species from both monoclonal cultures and natural samples.

Unexpected genetic diversity among and within populations of the toxic dinoflagellate Alexandrium catenella as revealed by nuclear microsatellite markers.

Since 1998, blooms of Alexandrium catenella associated with paralytic shellfish poisoning have been repeatedly reported for Thau Lagoon (French Mediterranean coast). Based on data obtained for rRNA gene markers, it has been suggested that the strains involved could be closely related to the Japanese temperate Asian ribotype of the temperate Asian clade. In order to gain more insight into the origin of these organisms, we carried out a genetic analysis of 61 Mediterranean and 23 Japanese strains using both ribosomal and microsatellite markers. Whereas the phylogeny based on ribosomal markers tended to confirm the previous findings, the analysis of microsatellite sequences revealed an unexpected distinction between the French and Japanese populations. This analysis also highlighted great intraspecific diversity that was not detected with the classical rRNA gene markers. The Japanese strains are divided into two differentiated A. catenella lineages: the Sea of Japan lineage and the east coast lineage, which includes populations from the Inland Sea and the Pacific Ocean. A. catenella strains isolated from Thau Lagoon belong to another lineage. These findings indicate that microsatellite markers are probably better suited to investigations of the population genetics of this species that is distributed worldwide. Finally, application of the population genetics concepts available for macroorganisms could support new paradigms for speciation and migration in phytoplankton assemblages.