A novel protist parasite, Salmoxcellia vastator n. gen., n. sp. (Xcelliidae, Perkinsozoa), infecting farmed salmonids in Norway

Global perspective of environmental distribution and diversity of Perkinsea (Alveolata) explored by a meta-analysis of eDNA surveys

Perkinsea constitutes a lineage within the Alveolata eukaryotic superphylum, mainly composed of parasitic organisms. Some described species represent significant ecological and economic threats due to their invasive ability and pathogenicity, which can lead to mortality events. However, the genetic diversity of these described species is just the tip of the iceberg. Environmental surveys targeting this lineage are still scarce and mainly limited to the Northern Hemisphere. Here, we aim to conduct an in depth exploration of the Perkinsea group, uncovering the diversity across a variety of environments, including those beyond freshwater and marine ecosystems. We seek to identify and describe putative novel organisms based on their genetic signatures. In this study, we conducted an extensive analysis of a metabarcoding dataset, focusing on the V4 region of the 18S rRNA gene (the EukBank dataset), to investigate the diversity, distribution and environmental preferences of the Perkinsea. Our results reveal a remarkable diversity within the Perkinsea, with 1568 Amplicon Sequence Variants (ASVs) identified across thousands of environmental samples. Surprisingly, we showed a substantial diversity of Perkinsea within soil samples (269 ASVs), challenging the previous assumption that this group is confined to marine and freshwater environments. In addition, we revealed that a notable proportion of Perkinsea ASVs (428 ASVs) could correspond to putative new organisms, encompassing the well-established taxonomic group Perkinsidae. Finally, our study shed light on previously unveiled taxonomic groups, including the Xcellidae, and revealed their environmental distribution. These findings demonstrate that Perkinsea exhibits far greater diversity than previously detected and surprisingly extends beyond marine and freshwater environments. The meta-analysis conducted in this study has unveiled the existence of previously unknown clusters within the Perkinsea lineage, solely identified based on their genetic signatures. Considering the ecological and economic importance of described Perkinsea species, these results suggest that Perkinsea may play a significant, yet previously unrecognized, role across a wide range of environments, spanning from soil environments to the abyssal zone of the open ocean with important implications for ecosystem functioning.

Perkinsus marinus in bioreactor: growth and a cost-reduced growth medium

Perkinsus marinus (Perkinsea) is an osmotrophic facultative intracellular marine protozoan responsible for "Dermo" disease in the eastern oyster, Crassostrea virginica. In 1993 in vitro culture of P. marinus was developed in the absence of host cells. Compared to most intracellular protozoan parasites, the availability of P. marinus to grow in the absence of host cells has provided the basis to explore its use as a heterologous expression system. As the genetic toolbox is becoming available, there is also the need for larger-scale cultivation and lower-cost media formulations. Here, we took an industrial approach to scaled-up growth from a small culture flask to bioreactors, which required developing new cultivation parameters, including aeration, mixing, pH, temperature control, and media formulation. Our approach also enabled more real-time data collection on growth. The bioreactor cultivation method showed similar or accelerated growth rates of P. marinus compared to culture in T-flasks. Redox measurements indicated sufficient oxygen availability throughout the cultivation. Replacing fetal bovine serum with chicken serum showed no differences in the growth rate and a 60% reduction in the medium cost. This study opens the door to furthering P. marinus as a valid heterologous expression system by showing the ability to grow in bioreactors.

ONE-SENTENCE SUMMARY

Perkinsus marinus, a microbial parasite of oysters that could be useful for developing vaccines for humans, has been shown to grow well in laboratory equipment that can be expanded to commercial scale using a less expensive growth formula than usual laboratory practice.

High-resolution metagenomic reconstruction of the freshwater spring bloom

BACKGROUND

The phytoplankton spring bloom in freshwater habitats is a complex, recurring, and dynamic ecological spectacle that unfolds at multiple biological scales. Although enormous taxonomic shifts in microbial assemblages during and after the bloom have been reported, genomic information on the microbial community of the spring bloom remains scarce.

RESULTS

We performed a high-resolution spatio-temporal sampling of the spring bloom in a freshwater reservoir and describe a multitude of previously unknown taxa using metagenome-assembled genomes of eukaryotes, prokaryotes, and viruses in combination with a broad array of methodologies. The recovered genomes reveal multiple distributional dynamics for several bacterial groups with progressively increasing stratification. Analyses of abundances of metagenome-assembled genomes in concert with CARD-FISH revealed remarkably similar in situ doubling time estimates for dominant genome-streamlined microbial lineages. Discordance between quantitations of cryptophytes arising from sequence data and microscopic identification suggested the presence of hidden, yet extremely abundant aplastidic cryptophytes that were confirmed by CARD-FISH analyses. Aplastidic cryptophytes are prevalent throughout the water column but have never been considered in prior models of plankton dynamics. We also recovered the first metagenomic-assembled genomes of freshwater protists (a diatom and a haptophyte) along with thousands of giant viral genomic contigs, some of which appeared similar to viruses infecting haptophytes but owing to lack of known representatives, most remained without any indication of their hosts. The contrasting distribution of giant viruses that are present in the entire water column to that of parasitic perkinsids residing largely in deeper waters allows us to propose giant viruses as the biological agents of top-down control and bloom collapse, likely in combination with bottom-up factors like a nutrient limitation.

CONCLUSION

We reconstructed thousands of genomes of microbes and viruses from a freshwater spring bloom and show that such large-scale genome recovery allows tracking of planktonic succession in great detail. However, integration of metagenomic information with other methodologies (e.g., microscopy, CARD-FISH) remains critical to reveal diverse phenomena (e.g., distributional patterns, in situ doubling times) and novel participants (e.g., aplastidic cryptophytes) and to further refine existing ecological models (e.g., factors affecting bloom collapse). This work provides a genomic foundation for future approaches towards a fine-scale characterization of the organisms in relation to the rapidly changing environment during the course of the freshwater spring bloom. Video Abstract.

A diversified and segregated mRNA spliced-leader system in the parasitic Perkinsozoa

Spliced-leader trans-splicing (SLTS) has been described in distantly related eukaryotes and acts to mark mRNAs with a short 5′ exon, giving different mRNAs identical 5′ sequence-signatures. The function of these systems is obscure. Perkinsozoa encompasses a diversity of parasitic protists that infect bivalves, toxic-tide dinoflagellates, fish and frog tadpoles. Here, we report considerable sequence variation in the SLTS-system across the Perkinsozoa and find that multiple variant SLTS-systems are encoded in parallel in the ecologically important Perkinsozoa parasite Parvilucifera sinerae. These results demonstrate that the transcriptome of P. sinerae is segregated based on the addition of different spliced-leader (SL) exons. This segregation marks different gene categories, suggesting that SL-segregation relates to functional differentiation of the transcriptome. By contrast, both sets of gene categories are present in the single SL-transcript type sampled from Maranthos, implying that the SL-segregation of the Parvilucifera transcriptome is a recent evolutionary innovation. Furthermore, we show that the SLTS-system marks a subsection of the transcriptome with increased mRNA abundance and includes genes that encode the spliceosome system necessary for SLTS-function. Collectively, these data provide a picture of how the SLTS-systems can vary within a major evolutionary group and identify how additional transcriptional-complexity can be achieved through SL-segregation.

A parasite outbreak in notothenioid fish in an Antarctic fjord

Climate changes can promote disease outbreaks, but their nature and potential impacts in remote areas have received little attention. In a hot spot of biodiversity on the West Antarctic Peninsula, which faces among the fastest changing climates on Earth, we captured specimens of two notothenioid fish species affected by large skin tumors at an incidence never before observed in the Southern Ocean. Molecular and histopathological analyses revealed that X-cell parasitic alveolates, members of a genus we call Notoxcellia, are the etiological agent of these tumors. Parasite-specific molecular probes showed that xenomas remained within the skin but largely outgrew host cells in the dermis. We further observed that tumors induced neovascularization in underlying tissue and detrimentally affected host growth and condition. Although many knowledge gaps persist about X-cell disease, including its mode of transmission and life cycle, these findings reveal potentially active biotic threats to vulnerable Antarctic ecosystems.

Emerging Parasitic Protists: The Case of Perkinsea

The last century has witnessed an increasing rate of new disease emergence across the world leading to permanent loss of biodiversity. Perkinsea is a microeukaryotic parasitic phylum composed of four main lineages of parasitic protists with broad host ranges. Some of them represent major ecological and economical threats because of their geographically invasive ability and pathogenicity (leading to mortality events). In marine environments, three lineages are currently described, the Parviluciferaceae, the Perkinsidae, and the Xcellidae, infecting, respectively, dinoflagellates, mollusks, and fish. In contrast, only one lineage is officially described in freshwater environments: the severe Perkinsea infectious agent infecting frog tadpoles. The advent of high-throughput sequencing methods, mainly based on 18S rRNA assays, showed that Perkinsea is far more diverse than the previously four described lineages especially in freshwater environments. Indeed, some lineages could be parasites of green microalgae, but a formal nature of the interaction needs to be explored. Hence, to date, most of the newly described aquatic clusters are only defined by their environmental sequences and are still not (yet) associated with any host. The unveiling of this microbial black box presents a multitude of research challenges to understand their ecological roles and ultimately to prevent their most negative impacts. This review summarizes the biological and ecological traits of Perkinsea-their diversity, life cycle, host preferences, pathogenicity, and highlights their diversity and ubiquity in association with a wide range of hosts.

A Novel Parasitoid of Marine Dinoflagellates, Pararosarium dinoexitiosum gen. et sp. nov. (Perkinsozoa, Alveolata), Showing Characteristic Beaded Sporocytes

The phylum Perkinsozoa is known as an exclusively parasitic group within alveolates and is widely distributed in various aquatic environments from marine to freshwater environments. Nonetheless, their morphology, life cycle, the identity of the host, and physiological characteristics remain still poorly understood. During intensive sampling along the west coast of Korea in October and November 2017, a new parasitoid, which shares several characteristics with the extant families Perkinsidae and Parviluciferaceae, was discovered and three strains of the new parasitoid were successfully established in cultures. Cross-infection experiments showed that among the examined planktonic groups, only dinoflagellates were susceptible to the new parasitoid, with infections observed in species belonging to eight genera. Even though the new parasitoid shared many morphological and developmental characteristics with other Perkinsozoan parasites, it differed from them by its densely packed trophocyte structure without a large vacuole or hyaline material during the growth stage. These characteristics are common among Parviluciferaceae members. Furthermore, through palintomic extracellular sporogenesis, it produced characteristic interconnected sporocytes resembling a string of beads. Phylogenetic analyses based on the small subunit and large subunit ribosomal DNA sequences revealed that the new parasitoid was distantly related to the family Parviluciferaceae and was more closely related to the families Perkinsidae and Xcellidae. Morphological, ultrastructural, and molecular data on the new parasitoid raised the need to erect a new family, i.e., Pararosariidae, within the phylum Perkinsozoa with Pararosarium dinoexitiosum gen. et sp. nov. as the type species. The isolation and establishment in culture of the new parasitoid outside the family Parviluciferaceae in the present study would contribute to the better understanding of the diversity of Perkinsozoan parasites and provide useful material for comparisons to other parasite species in the further study.