Abstract
Parasites are key drivers of phytoplankton bloom dynamics and related aquatic ecosystem processes. Yet, the dearth of morphological and molecular information hinders the assessment of their diversity and ecological role. Using single-cell techniques, we characterise morphologically and molecularly, intracellular parasitoids infecting four potentially toxin-producing Pseudo-nitzschia and one Melosira species on the North Atlantic coast. These sequences define two, morphologically indistinguishable clades within the phylum Oomycota, related to the genera of algal parasites Anisolpidium and Olpidiopsis and the diatom parasitoid species Miracula helgolandica. Our morphological data are insufficient to attribute either clade to the still unsequenced genus Ectrogella; hence it is proposed to name the clades OOM_1 and OOM_2. A screening of global databases of the barcode regions V4 and V9 of the 18S rDNA demonstrate the presence of these parasitoids beyond the North Atlantic coastal region. During a biweekly metabarcoding survey (Concarneau Bay, France), reads associated with one sequenced parasitoid coincided with the decline of Cerataulina pelagica bloom, whilst the other parasitoids co-occurred at low abundance with Pseudo-nitzschia. Our data highlight a complex and unexplored diversity of the oomycete parasitoids of diatoms and calls for the investigation of their phenology, evolution, and potential contribution in controlling their host spatial-temporal dynamics.
Highlights
Parasitism is regarded as one of the most widespread ecological strategies among heterotrophs (Lafferty et al, 2006, 2008)
Endobiotic parasitoids infecting the diatoms Pseudonitzschia spp. and Melosira cf. nummuloides were observed in plankton samples collected in Brittany (France) and Scotland (United Kingdom) between 2010 and 2016
The clearest observable stages of infection consist in swollen Pseudo-nitzschia cells symptomatic of the parasite thallus growing inside the diatom and pushing the valves apart
Summary
Parasitism is regarded as one of the most widespread ecological strategies among heterotrophs (Lafferty et al, 2006, 2008). All main phytoplankton groups are targeted by viruses, algicidal bacteria and protistan parasites (Gachon et al, 2010) These organisms are often referred as parasitoids since they usually kill their unicellular host, and directly influence the dynamics of their host population (Kühn et al, 2004). Similar parasitoids of Pseudo-nitzschia were reported in the United States and Scotland (Gachon et al, 2010; Lelong et al, 2012; Trainer et al, 2012), suggesting a wide dispersal of these oomycetes across the Northern hemisphere. Two oomycete parasitoids infecting Pseudo-nitzschia pungens (Prince Edwards, Canada and Helgoland, Germany) and Rhizosolenia imbricata (Helgoland, Germany) were described as two new species, Miracula helgolandica and Olpidiopsis drebesii on the basis of short sequences We report the morphological and molecular characterisation of seven oomycete parasitoids of Pseudo-nitzschia spp. and Melosira, and use environmental DNA sequences to assess their widespread occurrence and their dynamics in the plankton community
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