Diversity of photosynthetic picoeukaryotes in eutrophic shallow lakes as assessed by combining flow cytometry cell-sorting and high throughput sequencing

Photosynthetic picoeukaryotes (PPE) are key components of primary production in marine and freshwater ecosystems. In contrast with those of marine environments, freshwater PPE groups have received little attention. In this work, we used flow cytometry cell sorting, microscopy and metabarcoding to investigate the composition of small photosynthetic eukaryote communities from six eutrophic shallow lakes in South America, Argentina. We compared the total molecular diversity obtained from PPE sorted populations as well as from filtered total plankton samples (FTP). Most reads obtained from sorted populations belonged to the classes: Trebouxiophyceae, Chlorophyceae and Bacillariophyceae. We retrieved sequences from non-photosynthetic groups, such as Chytridiomycetes and Ichthyosporea which contain a number of described parasites, indicating that these organisms were probably in association with the autotrophic cells sorted. Dominant groups among sorted PPEs were poorly represented in FTP and their richness was on average lower than in the sorted samples. A significant number of operational taxonomic units (OTUs) were exclusively found in sorting samples, emphasizing that sequences from FTP underestimate the diversity of PPE. Moreover, 22% of the OTUs found among the dominant groups had a low similarity (<95%) with reported sequences in public databases, demonstrating a high potential for novel diversity in these lakes.Fil: Metz, Sebastián Darío. Universidad Nacional de San Martin. Instituto Tecnológico de Chascomús - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Tecnológico de Chascomús; ArgentinaFil: Dos Santos, Adriana Lopes. Sorbonne University; Francia. Centre National de la Recherche Scientifique; Francia. Nanyang Technological University; SingapurFil: Castro Berman, Manuel. Universidad Nacional de San Martin. Instituto Tecnológico de Chascomús - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Tecnológico de Chascomús; ArgentinaFil: Bigeard, Estelle. Sorbonne University; Francia. Centre National de la Recherche Scientifique; Francia. Nanyang Technological University; SingapurFil: Licursi, Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Not, Fabrice. Sorbonne University; Francia. Centre National de la Recherche Scientifique; FranciaFil: Lara, Enrique. University of Neuchatel; SuizaFil: Unrein, Fernando. Universidad Nacional de San Martin. Instituto Tecnológico de Chascomús - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Tecnológico de Chascomús; Argentin

Cryptic species in the parasitic Amoebophrya species complex revealed by a polyphasic approach

As critical primary producers and recyclers of organic matter, the diversity of marine protists has been extensively explored by high-throughput barcode sequencing. However, classification of short metabarcoding sequences into traditional taxonomic units is not trivial, especially for lineages mainly known by their genetic fingerprints. This is the case for the widespread Amoebophrya ceratii species complex, parasites of their dinoflagellate congeners. We used genetic and phenotypic characters, applied to 119 Amoebophrya individuals sampled from the same geographic area, to construct practical guidelines for species delineation that could be applied in DNA/RNA based diversity analyses. Based on the internal transcribed spacer (ITS) regions, ITS2 compensatory base changes (CBC) and genome k-mer comparisons, we unambiguously defined eight cryptic species among closely related ribotypes that differed by less than 97% sequence identity in their SSU rDNA. We then followed the genetic signatures of these parasitic species during a three-year survey of Alexandrium minutum blooms. We showed that these cryptic Amoebophrya species co-occurred and shared the same ecological niche. We also observed a maximal ecological fitness for parasites having narrow tointermediate host ranges, reflecting a high cost for infecting a broader host range. This study suggests that a complete taxonomic revision of these parasitic dinoflagellates is long overdue to understand their diversity and ecological role in the marine plankton

Symbiont Chloroplasts Remain Active During Bleaching-Like Response Induced by Thermal Stress in Collozoum pelagicum (Collodaria, Retaria)

Collodaria (Retaria) are important contributors to planktonic communities and biogeochemical processes (e.g., the biologic pump) in oligotrophic oceans. Similarly to corals, Collodaria live in symbiosis with dinoflagellate algae, a relationship that is thought to explain partly their ecological success. In the context of global change, the robustness of the symbiotic interaction, and potential subsequent bleaching events are of primary interest for oceanic ecosystems functioning. In the present study, we compared the ultrastructure, morphology, symbiont density, photosynthetic capacities and respiration rates of colonial Collodaria exposed to a range of temperatures corresponding to natural conditions (21°C), moderate (25°C), and high (28°C) thermal stress. We showed that symbiont density immediately decreased when temperature rose to 25°C, while the overall Collodaria holobiont metabolic activity increased. When temperature reached 28°C, the holobiont respiration nearly stopped and the host morphological structure was largely damaged, as if the host tolerance threshold has been crossed. Over the course of the experiment, the photosynthetic capacities of remaining algal symbionts were stable, chloroplasts being the last degraded organelles in the microalgae. These results contribute to a better characterization and understanding of temperature-induced bleaching processes in planktonic photosymbioses

First Viruses Infecting the Marine Diatom Guinardia delicatula

The marine diatom Guinardia delicatula is a cosmopolitan species that dominates seasonal blooms in the English Channel and the North Sea. Several eukaryotic parasites are known to induce the mortality of this species. Here, we report the isolation and characterization of the first viruses that infect G. delicatula. Viruses were isolated from the Western English Channel (SOMLIT-Astan station) during the late summer bloom decline of G. delicatula. A combination of laboratory approaches revealed that these lytic viruses (GdelRNAV) are small tailless particles of 35–38 nm in diameter that replicate in the host cytoplasm where both unordered particles and crystalline arrays are formed. GdelRNAV display a linear single-stranded RNA genome of ~9 kb, including two open reading frames encoding for replication and structural polyproteins. Phylogenetic relationships based on the RNA-dependent-RNA-polymerase gene marker showed that GdelRNAV are new members of the Bacillarnavirus, a monophyletic genus belonging to the order Picornavirales. GdelRNAV are specific to several strains of G. delicatula. They were rapidly and largely produced (&lt;12 h, 9.34 × 104 virions per host cell). Our analysis points out the host's variable viral susceptibilities during the early exponential growth phase. Interestingly, we consistently failed to isolate viruses during spring and early summer while G. delicatula developed important blooms. While our study suggests that viruses do contribute to the decline of G. delicatula's late summer bloom, they may not be the primary mortality agents during the remaining blooms at SOMLIT-Astan. Future studies should focus on the relative contribution of the viral and eukaryotic pathogens to the control of Guinardia's blooms to understand the fate of these prominent organisms in marine systems

Rapid protein evolution, organellar reductions, and invasive intronic elements in the marine aerobic parasite dinoflagellate Amoebophrya spp

Background: Dinoflagellates are aquatic protists particularly widespread in the oceans worldwide. Some are responsible for toxic blooms while others live in symbiotic relationships, either as mutualistic symbionts in corals or as parasites infecting other protists and animals. Dinoflagellates harbor atypically large genomes (similar to 3 to 250 Gb), with gene organization and gene expression patterns very different from closely related apicomplexan parasites. Here we sequenced and analyzed the genomes of two early-diverging and co-occurring parasitic dinoflagellate Amoebophrya strains, to shed light on the emergence of such atypical genomic features, dinoflagellate evolution, and host specialization. Results: We sequenced, assembled, and annotated high-quality genomes for two Amoebophrya strains (A25 and A120), using a combination of Illumina paired-end short-read and Oxford Nanopore Technology (ONT) MinION long-read sequencing approaches. We found a small number of transposable elements, along with short introns and intergenic regions, and a limited number of gene families, together contribute to the compactness of the Amoebophrya genomes, a feature potentially linked with parasitism. While the majority of Amoebophrya proteins (63.7% of A25 and 59.3% of A120) had no functional assignment, we found many orthologs shared with Dinophyceae. Our analyses revealed a strong tendency for genes encoded by unidirectional clusters and high levels of synteny conservation between the two genomes despite low interspecific protein sequence similarity, suggesting rapid protein evolution. Most strikingly, we identified a large portion of non-canonical introns, including repeated introns, displaying a broad variability of associated splicing motifs never observed among eukaryotes. Those introner elements appear to have the capacity to spread over their respective genomes in a manner similar to transposable elements. Finally, we confirmed the reduction of organelles observed in Amoebophrya spp., i.e., loss of the plastid, potential loss of a mitochondrial genome and functions. Conclusion: These results expand the range of atypical genome features found in basal dinoflagellates and raise questions regarding speciation and the evolutionary mechanisms at play while parastitism was selected for in this particular unicellular lineage

Rapid protein evolution, organellar reductions, and invasive intronic elements in the marine aerobic parasite dinoflagellate Amoebophrya spp

BACKGROUND : Dinoflagellates are aquatic protists particularly widespread in the oceans worldwide. Some are responsible for toxic blooms while others live in symbiotic relationships, either as mutualistic symbionts in corals or as parasites infecting other protists and animals. Dinoflagellates harbor atypically large genomes (~ 3 to 250 Gb), with gene organization and gene expression patterns very different from closely related apicomplexan parasites. Here we sequenced and analyzed the genomes of two early-diverging and co-occurring parasitic dinoflagellate Amoebophrya strains, to shed light on the emergence of such atypical genomic features, dinoflagellate evolution, and host specialization. RESULTS : We sequenced, assembled, and annotated high-quality genomes for two Amoebophrya strains (A25 and A120), using a combination of Illumina paired-end short-read and Oxford Nanopore Technology (ONT) MinION long-read sequencing approaches. We found a small number of transposable elements, along with short introns and intergenic regions, and a limited number of gene families, together contribute to the compactness of the Amoebophrya genomes, a feature potentially linked with parasitism. While the majority of Amoebophrya proteins (63.7% of A25 and 59.3% of A120) had no functional assignment, we found many orthologs shared with Dinophyceae. Our analyses revealed a strong tendency for genes encoded by unidirectional clusters and high levels of synteny conservation between the two genomes despite low interspecific protein sequence similarity, suggesting rapid protein evolution. Most strikingly, we identified a large portion of non-canonical introns, including repeated introns, displaying a broad variability of associated splicing motifs never observed among eukaryotes. Those introner elements appear to have the capacity to spread over their respective genomes in a manner similar to transposable elements. Finally, we confirmed the reduction of organelles observed in Amoebophrya spp., i.e., loss of the plastid, potential loss of a mitochondrial genome and functions. CONCLUSION : These results expand the range of atypical genome features found in basal dinoflagellates and raise questions regarding speciation and the evolutionary mechanisms at play while parastitism was selected for in this particular unicellular lineage.ADDITIONAL FILE 1: FIGURE S1. Phylogeny of Alveolata. Proteomes from 89 alveolates genomes and transcriptome assemblies from the MMETSP project (https://zenodo.org/record/257026/files/) were used to create orthologous groups using orthofinder v2.2 with the diamond BLAST similarity search. Single ortholog alignments were pruned using PhyloTreePruner v.1.0 (minimum taxa to keep 44 and support value 0.9) and realigned using mafft v7 and filtered with Gblocks v.0.91b (−b5 = a -p = n). Filtered alignments were concatenated using seqCat.pl and a phylogenetic tree was produced under Maximum Likelihood framework using RAxML v8.2.9 with the PROTGAMMALGF model of sequence evolution and 101 bootstraps. Asterics represent support values of 95 and above. A detailed method can be found in Kayal et al. 2018 BMC Evol. Biol. (https://doi.org/10.1186/s12862-018-1142-0). The full tree can be found at http://mmo.sb-roscoff.fr/jbrowseAmoebophrya/. FIGURE S2. SSU rDNA sequence identity (in percentage, relative to A25 and A120 compared to other species). FIGURE S3. Distribution of k-mer in A25 and A120 genomes. FIGURE S4. Classification of repeated elements in 3 Amoebophrya genomes (AT5, A25, and A120) using REPET. The x-axis represents the cumulated number of bases of repeated elements in the genome. FIGURE S5. Conserved motif of the putative splice leader (SL) in A25 and A120. FIGURE S6. Alignments of gene encoding the putative spliced leader (SL) gene in A25 and A120. FIGURE S7. Gene orientation change rate in 3 Amoebophrya genomes. FIGURE S8. Number of orthologs genes shared by selected taxa. FIGURE S9. Boxplot of the dN/dS ratios of orthologous genes between A25 and A120, calculated using the model average method (MA). FIGURE S10. Synteny dot-plot obtained by comparison between Amoebophrya A25 and AT5 genomes. FIGURE S11. Synteny dot-plot obtained by comparison between Amoebophrya A120 and AT5 genomes. FIGURE S12. Intron length distribution. FIGURE S13. GC content distribution. FIGURE S14. Multiple alignments of U2 snRNAs. FIGURE S15. Multiple alignments of U4 snRNAs. FIGURE S16. Multiple alignments of U5 snRNAs. FIGURE S17. Multiple alignments of U6 snRNAs. FIGURE S18. Secondary structure of Amoebophrya snRNA. FIGURE S19. Example of introner elements (IEs) in Amoebophrya. FIGURE S20. Distribution the direct repeats with size ranging between 3 and 8 nucleotides in A25. FIGURE S21. Distribution of the direct repeats with size ranging between 3 and 8 nucleotides in A120. FIGURE S22. Composition of direct repeats in introners elements. The diversity in composition of the three (a, b, c) most abundant of direct repeats in introner elements in A25 (up) and A120 (down). FIGURE S23. Terminal inverted repeat locations around the splicing sites in A25 and A120. The position of inverted repeats according to the location of the splice sites in A25 and A120. Left, the inverted repeats of A120 are located at 1–5 the nucleotides upstream and downstream of the splice sites. Right, the inverted repeats of A25 are located at the 1–6 nucleotides in upstream and downstream of the splice sites. FIGURE S24. The flowchart for the in silico search of introner elements. FIGURE S25. Hierarchical clustering analysis (pairwise similarity and OrthoMCL) of all intron families and of the inverted repeats in A25 and A120. FIGURE S26. Percentage of genes with assigned functions in relation with introns composition. FIGURE S27. Difference in the proportion of IEs-containing-genes compared to their KEGG assignment in A25 and A120. FIGURE S28. Distribution of conserved introns. TABLE S1. RCC number, date and site of isolation of strains considered in this study. TABLE S2. Metrics of Nanopore runs for the two Amoebophrya strains. TABLE S3. Search for pathways involved in plastidial functions that are entirely independent of plastid-encoded gene content. TABLE S4. Number of the different types of introns identified in A25 and A120 genomes. TABLE S5. Search for RNA editing in A25 and A120 introns. TABLE S6. Putative Amoebophrya A25 and A120 snRNP homologs. TABLE S7. Classification into families of non-canonical introns in A25 and A120. TABLE S8. RNAseq read assembly statistics of Amoebophrya A25 and A120 corresponding samples from the different time of infection and to the freeliving stage (dinospore only). TABLE S9. Total number of contigs belonging to samples from different stages of infection and the proportion of them that were aligned against the genomes of both Amoebophrya A25 and A120. ND corresponds to “not determined” when no measurement was done. TABLE S10. Metabolic pathway screened in A25 and A120 proteomes.This research was funded by the ANR (Agence Nationale de la Recherche) Grant ANR-14-CE02-0007 HAPAR, the CEA and the Région Bretagne (RC doctoral grant ARED PARASITE 9450 and EK postdoctoral grant SAD HAPAR 9229), and the CNRS (X-life SEAgOInG).http://www.mdpi.com/journal/biomedicinesam2022BiochemistryGeneticsMicrobiology and Plant Patholog

Temperature Affects the Biological Control of Dinoflagellates by the Generalist Parasitoid Parvilucifera rostrata

International audienceThe increase in emerging harmful algal blooms in the last decades has led to an extensive concern in understanding the mechanisms behind these events. In this paper, we assessed the growth of two blooming dinoflagellates (Alexandrium minutum and Heterocapsa triquetra) and their susceptibility to infection by the generalist parasitoid Parvilucifera rostrata under a temperature gradient. The growth of the two dinoflagellates differed across a range of temperatures representative of the Penzé Estuary (13 to 22 °C) in early summer. A. minutum growth increased across this range and was the highest at 19 and 22 °C, whereas H. triquetra growth was maximal at intermediate temperatures (15–18 °C). Interestingly, the effect of temperature on the parasitoid infectivity changed depending on which host dinoflagellate was infected with the dinoflagellate responses to temperature following a positive trend in A. minutum (higher infections at 20–22 °C) and a unimodal trend in H. triquetra (higher infections at 18 °C). Low temperatures negatively affected parasitoid infections in both hosts (i.e., “thermal refuge”). These results demonstrate how temperature shifts may not only affect bloom development in microalgal species but also their control by parasitoids

Biogeography and diversity of Collodaria (Radiolaria) in the global ocean

Collodaria are heterotrophic marine protists that exist either as large colonies composed of hundreds of cells or as large solitary cells. All described species so far harbour intracellular microalgae as photosymbionts. Although recent environmental diversity surveys based on molecular methods demonstrated their consistently high contribution to planktonic communities and their worldwide occurrence, our understanding of their diversity and biogeography is still very limited. Here we estimated the 18S ribosomal DNA (rDNA) gene copies per collodarian cell for solitary (5770 +/- 1960 small subunit (SSU) rDNA copies) and colonial specimens (37 474 +/- 17 799 SSU rDNA copies, for each individual cell within a colony) using single-specimen quantitative PCR. We then investigated the environmental diversity of Collodaria within the photic zone through the metabarcoding survey from the Tara Oceans expedition and found that the two collodarian families Collosphaeridae and Sphaerozoidae contributed the most to the collodarian diversity and encompassed mostly cosmopolitan taxa. Although the biogeographical patterns were homogeneous within each biogeochemical biome considered, we observed that coastal biomes were consistently less diverse than oceanic biomes and were dominated by the Sphaerozoidae while the Collosphaeridae were dominant in the open oceans. The significant relationships with six environmental variables suggest that collodarian diversity is influenced by the trophic status of oceanic provinces and increased towards more oligotrophic regions

Time Calibrated Morpho-molecular Classification of Nassellaria (Radiolaria)

International audienceNassellaria are marine protists belonging to the Radiolaria lineage (Rhizaria). Their skeleton, made of opaline silica, exhibit an excellent fossil record, extremely valuable in micro-paleontological studies for paleo-environmental reconstruction. Yet, to date very little is known about the extant diversity and ecology of Nassellaria in contemporary oceans, and most of it is inferred from their fossil record. Here we present an integrative classification of Nassellaria based on taxonomical marker genes (18S and 28S ribosomal DNA) and morphological characteristics obtained by optical and scanning electron microscopy imaging. Our phylogenetic analyses distinguished 11 main morpho-molecular clades relying essentially on the overall morphology of the skeleton and not on internal structures as previously considered. Using fossil calibrated molecular clock we estimated the origin of Nassellaria among radiolarians primitive forms in the Devonian (ca. 420 Ma), that gave rise to living nassellarian groups in the Triassic (ca. 250 Ma), during the biggest diversification event over their evolutionary history. This morpho-molecular framework provides both a new morphological classification easier to identify under light microscopy and the basis for future molecular ecology surveys. Altogether, it brings a new standpoint to improve our scarce understanding of the ecology and worldwide distribution of extant nassellarians

Redescription and phylogenetic analyses of Durchoniella spp. (Ciliophora, Astomatida) associated with the polychaete Cirriformia tentaculata (Montagu, 1808)

Microscopic and phylogenetic analyses were performed on endocommensal astome ciliates retrieved from the middle intestine of a marine cirratulid polychaete, Cirriformia tentaculata, collected in the bay of Roscoff (English Channel, Northwest French coast) and on the Southwest English coast. Three morphotypes of the astome genus Durchoniella were identified, two corresponding to described species (the type species Durchoniella brasili (Léger and Duboscq, 1904) de Puytorac, 1954 and Durchoniella legeriduboscqui de Puytorac, 1954) while a third morphotype remains undescribed. Their small subunit (SSU) rRNA gene sequences showed at least 97.2% identity and phylogenetic analyses grouped them at the base of the subclass Scuticociliatia (Oligohymenophorea), as a sister lineage to all astomes from terrestrial oligochaete annelids. Ultrastructural examination by transmission electron microscopy and fluorescence in situ hybridization analyses revealed the presence of endocytoplasmic cocci and rod-shaped bacteria surrounded by a very thin membrane. These endocytoplasmic bacteria may play a role in the association between endocommensal astome ciliates and cirratulid polychaetes inhabiting in anoxic coastal sediments