PHYLOGENETIC CHARACTERISATION OF OOMYCETES INFECTING TOXIC SPECIES OF THE MARINE DIATOM PSEUDO-NITZSCHIA

• Main Authors: Garvetto, A., Nezan, E., Badis, Y., Bilien, G., Arce, P., Bresnan, E., Gachon, C., Siano, R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: North Atlantic
• Online Access: https://pure.uhi.ac.uk/en/publications/c84b648f-afcd-4020-8a19-f6e743650bbf; https://doi.org/10.2216/0031

In all ecosystems, parasites of primary producers can influence the ‘whole community composition’ and thus affect downstream ecological processes. Marine plankton sampling problems, the limited number of cultivable species, and the dearth of morphological information about microalgae parasites hinders the assessment of their diversity, phylogenetic position and ecological importance. Using morphological observations, a parasite of the marine toxigenic diatom Pseudo-nitzschia pungens has been tentatively affiliated to the oomycete genus Ectrogella, which has been reported to infect both marine and freshwater diatoms. No phylogenetic affiliation was obtained, opening the debate about its systematic position. By single-cell genetic analyses, 18s-rDNA sequences of six distinct intracellular eukaryotic parasites were obtained, infecting four toxic Pseudo-nitzschia and one Melosira species on the North Atlantic coast. Robust phylogenetic analyses demonstrate that these sequences cluster into two separate clades within the phylum Oomycota, relating to the seaweed parasite genera Anisolpidium and Olpidiopsis. Morphological features were insufficient to unambiguously attribute these parasites to any Ectrogella species. These two Oomycota clades have been named OOM_1 and OOM_2 awaiting further morphological and molecular information. A screening of global databases of the regions V4 and V9 of the 18s-rDNA, demonstrated the presence of these parasites beyond the North Atlantic coastal region. In a biweekly metabarcoding survey of the diatom communities in the Concarneau Bay (France, Brittany), high abundances of OOM_2 coincided once with the decline of Pseudo-nitzschia spp. and then with that of Cerataulina pelagica. This finding, together with the genetic identification of the same oomycete infecting both Pseudo-nitzschia australis and Melosira sp. supports the hypothesis of a lack of host specificity of the studied parasites. These data highlight a complex and still unexplored genetic diversity within oomycete parasites ...