Toxicity of coastal coccolithophores (Prymnesiophyceae, Haptophyta)

Over the last decade, certain coccolithophores have been thesubject of extensive multidisciplinary research. Several speciesof coccolithophore, belonging mainly to the families Pleurochrysidaceaeand Hymenomonadaceae, inhabit inshore coastal waters where theymay occasionally bloom and hence impact aquaculture resources.The toxicity to Artemia salina larvae of 11 species of coccolithophore(nine coastal and two oceanic members of the order Coccolithales)was tested. For the nine coastal species, tests were conductedwith rapidly growing and stationary phase cultures at a rangeof cellular concentrations and for two different exposure times(24 and 48 h). Five of the coastal species (four in the genusPleurochrysis as well as Jomonlithus littoralis) were foundto be toxic to A. salina nauplii. Allelopathic effects of acell-free filtrate of a culture of a toxic coccolithophore werealso tested on three flagellate microalgal species: Scrippsiellatrochoidea, Tetraselmis sp. and Isochrysis galbana. Negativeeffects of the filtrate on growth rates and motility of S. trochoideaand Tetraselmis sp. were recorded, suggesting that the toxinof the coccolithophore tested could be an exotoxin similar tothat produced by other non-calcifying members of the Prymnesiophyceae.The fact that certain coccolithophores were found to be toxicto invertebrates and were shown to exhibit allelopathic activitycould imply negative effects at different trophic levels incoastal areas.     

Holococcolithophore‐heterococcolithophore (Haptophyta) life cycles: Flow cytometric analysis of relative ploidy levels

Several coccolithophore species are known to exhibit heteromorphic life cycles. In certain species, notably Emiliania huxleyi, the heterococcolith‐bearing phase alternates with a non‐calcifying stage, whereas in others the heterococcolith‐bearing phase alternates with a holococcolith‐bearing phase. Heterococcolithophore‐holococcolithophore life cycles have previously been observed for only one species in culture, but have also been inferred from an increasing number of observations of combination coccospheres. 18S rDNA sequences from pure cultures of both the heterococcolith‐bearing and holococcolith‐bearing phases of Coccolithus pelagicus were identical, providing an additional indication of their identity as different life cycle stages of the same species. Flow cytometric analyses have been undertaken on SybrGreen‐stained nuclei isolated from pure cultures of the two phases of four coccolithophore species (Coccolithus pelagicus, Calcidiscus leptoporus, Coronosphaera mediterranea and Emiliania huxleyi) in order to determine relative DNA content. Results confirm the hypothesis that holococcolithophore‐heterococcolithophore life cycles are haplo‐diploid in nature. Light microscope observations of the processes of sexual fusion and meiosis are reported for two of the experimental species. The results are discussed in the context of the evolution of bio‐mineralization in the coccolithophores and the possible ubiquity of haplo‐diploidy in the haptophytes.     

THE CONTRIBUTION OF SUB‐SAHARAN AFRICAN STRAINS TO THE PHYLOGENY OF CYANOBACTERIA: FOCUSING ON THE NOSTOCACEAE (NOSTOCALES,CYANOBACTERIA)1

To date, phylogenies have been based on known gene sequences accessible at GenBank, and the absence of many cyanobacterial lineages from collections and sequence databases has hampered their classification. Investigating new biotopes to isolate more genera and species is one way to enrich strain collections and subsequently enhance gene sequence databases. A polyphasic approach is another way of improving our understanding of the details of cyanobacterial classification. In this work, we have studied phylogenetic relationships in strains isolated from freshwater bodies in Senegal and Burkina Faso to complement existing morphological and genetic databases. By comparing 16S rDNA sequences of African strains to those of other cyanobacteria lineages, we placed them in the cyanobacterial phylogeny and confirmed their genus membership. We then focused on the Nostocaceae family by concatenated analysis of four genes (16S rDNA, hetR, nifH, and rpoC1 genes) to characterize relationships among Anabaena morphospecies, in particular, Anabaena sphaerica var. tenuis G. S. West. Using a polyphasic approach to the Nostocaceae family, we demonstrate that A. sphaerica var. tenuis is more closely related to Cylindrospermospsis/Raphidiopsis than to other planktonic Anabaena/Aphanizomenon. On the basis of phylogeny and morphological data, we propose that these three significantly different clusters should be assigned to three genera.