Distribution and host diversity of Amoebophryidae parasites across oligotrophic waters of the Mediterranean Sea

Abstract

Abstract. Sequences affiliated to Syndiniales (Marine alveolate, MALV) regularly dominate 18S rDNA genetic libraries of nearly all marine ecosystems investigated so far. Among them, Amoebophryidae (MALV group II) is composed of numerous and genetically distant environmental sequences, where Amoebophrya is the only known and formally described genus. Amoebophrya species include virulent pathogens for a wide range of dinoflagellate species. Beside their regular occurrence in marine ecosystems, their quantitative distribution and the environmental factors triggering host infection have barely been studied in open oligotrophic waters. In order to understand the functional role of these parasites in natural environments, we studied the distribution and contribution to the eukaryotic community of the small free-living stage of Amoebophryidae (the dinospores) along a transect in the Mediterranean Sea, as well as their host diversity at three oligotrophic stations. Dinospores were more abundant at a coastal station (max. 1.5 × 10 3 cells ml −1 ) than in oligotrophic waters (max. 51 ± 16.3 cells ml −1 ), where they represented 10.3 to 34.9% of the total eukaryotic community at 40 and 30 m depth, respectively and 21.2% on average along the water column. Positive correlation was found between dinospore occurrence and higher concentration of NO 3 + NO 2 at the coastal station. At selected stations, out of 38 different dinoflagellates taxa identified, 15 were infected, among which a majority were not recognized as Amoebophryidae host so far. Prevalences (percentage of infected cells) generally varied between 1% and 10%, with a notable exception for Blepharocysta paulsenii for which 25% of cells were infected Correspondence to: L. Guillou (laure.guillou@sb-roscoff.fr) at the most oligotrophic station. The present study shows that dinospores are able to thrive and infect dinoflagellates both in coastal and ultra-oligotrophic open waters. Our results emphasize the role of parasitism in microbial food web dynamics and ultimately on biogeochemical cycles.

Environmental Sequencing Provides Reasonable Estimates of the Relative Abundance of Specific Picoeukaryotes

“…This is probably due to a particularly high number of rDNAoperon copies in MALV groups (2,30,32). The 18S rDNA copy number can vary by orders of magnitude among protist taxa, from a few copies per cell in some green algae (53) to about 30 copies in MAST-4 (54) or several thousand copies in some dinoflagellates (53), depending on the cell size and genome size (55).…”

“…This can be achieved with fluorescence in situ hybridization (FISH), which enables the visualization and quantification of specific cells in natural assemblages by using oligonucleotide probes as phylogenetic stains (25). FISH has served to identify the cells from novel environmental clades (11,26,27) and has been applied in a few marine surveys (28)(29)(30)(31). However, this approach is relatively time consuming and targets only one taxon at a time.…”

Protistan Diversity in Environmental Molecular Surveys

“…Combined with single-cell analysis of host cells, it turns out that MALV-II cells parasitize mostly dinofl agellates, whereas MALV-I cells have a wider host spectrum, including ciliates, radiolarians, and fi sh eggs (Harada et al 2007 ). Furthermore, FISH confi rms the large MALV overestimation in DNA surveys, as counts of MALV-II dinospores are generally low (0.4-3.1 % of picoeukaryotes) in open sea samples (Siano et al 2010 ). Still, these moderate counts, together with high prevalence (2-10 % of dinofl agellate cells appear infected) and a wide range of host species, suggests that MALV may be important parasites in the marine realm.…”

Infection dynamics of Amoebophryidae parasitoids on harmful dinoflagellates in a southern Chilean fjord dominated by diatoms

“…The different Amoebophryidae life stages were detected by fluorescent in situ hybridization coupled with tyramide signal amplification (FISH-TSA) using the ALV01 oligonucleotide probe (5'-GCC TGC CGT GAA CAC TCT-3') specific for Amoebophryidae ) labeled with horseradish peroxidase (Thermo fisher). The FISH procedure was performed as described by Siano et al (2011). After hybridization, the filters were covered with calcofluor (for visualization of dinoflagellate theca; 100 ng ml −1 ) for 10 min and washed in distilled water.…”

“…(20 to 80%) is usually observed during annual dinoflagellate blooms in estuarine systems, where high nutrient concentrations and water mass stability favor the growth of dinoflagellates. However, recent studies in oligotrophic coastal waters of Brazil (Salomon et al 2009) and ultra-oligotrophic waters of the Mediterranean Sea (Siano et al 2011) have reported Amoebophrya spp. infections of dinoflagellates with prevalences up to 7 and 25%, respectively, suggesting that these parasitoids could also be relevant at low host concentrations.…”