BACKGROUND: Polinton-like viruses (PLVs) are diverse eukaryotic DNA viral elements (14-40 kb) that often undergo significant expansion within protist genomes through repeated insertion events. Emerging evidence indicates they function as antiviral defense systems in protists, reducing the progeny yield of their infecting giant viruses (phylum Nucleocytoviricota) and influencing the population dynamics and evolution of both viruses and their hosts. While many PLVs have been identified within the genomes of sequenced protists, most were recovered from metagenomic data. Even with the large number of PLVs identified from metagenomic data, their host-virus linkages remain unknown owing to the scarcity of ecologically relevant protist genomes. Additionally, the extent of PLV diversification within abundant freshwater taxa remains undetermined. In order to tackle these questions, high-quality genomes of abundant and representative taxa that bridge genomic and metagenomic PLVs are necessary. In this regard, cryptophytes, which are among the most widely distributed, abundant organisms in freshwaters and have remained largely out of bounds of genomic and metagenomic approaches, are ideal candidates for investigating the diversification of such viral elements both in cellular and environmental context. RESULTS: We leveraged long-read sequencing to recover large (200-600 Mb), high-quality, and highly repetitive (> 60%) genomes of representative freshwater and marine photosynthetic cryptophytes. We uncovered over a thousand complete PLVs within these genomes, revealing vast lineage-specific expansions, particularly in the common freshwater cryptophyte Rhodomonas lacustris. By combining deep sequence homology annotation with biological network analyses, we discern well-defined PLV groups defined by characteristic gene-sharing patterns and the use of distinct strategies for replication and integration within host genomes. Finally, the PLVs recovered from these cryptophyte genomes also allow us to assign host-virus linkages in environmental sequencing data. CONCLUSIONS: Our findings provide a primer for understanding the evolutionary history, gene content, modes of replication and infection strategies of cryptophyte PLVs, with special emphasis on their expansion as endogenous viral elements (EVEs) in freshwater bloom-forming R. lacustris. Video Abstract.

• Keywords
• Chrysophytes, Cryptophytes, Polinton-like viruses, Rhodomonas, Virophages,
• MeSH
• Cryptophyta * virology   genetics   MeSH
• DNA Viruses * genetics   classification   MeSH
• Photosynthesis MeSH
• Phylogeny MeSH
• Genome, Viral MeSH
• Metagenomics MeSH
• Giant Viruses * genetics   classification   MeSH
• Publication type
• Journal Article MeSH

BACKGROUND: Protists are essential contributors to eukaryotic diversity and exert profound influence on carbon fluxes and energy transfer in freshwaters. Despite their significance, there is a notable gap in research on protistan dynamics, particularly in the deeper strata of temperate lakes. This study aimed to address this gap by integrating protists into the well-described spring dynamics of Římov reservoir, Czech Republic. Over a 2-month period covering transition from mixing to established stratification, we collected water samples from three reservoir depths (0.5, 10 and 30 m) with a frequency of up to three times per week. Microbial eukaryotic and prokaryotic communities were analysed using SSU rRNA gene amplicon sequencing and dominant protistan groups were enumerated by Catalysed Reporter Deposition-Fluorescence in situ Hybridization (CARD-FISH). Additionally, we collected samples for water chemistry, phyto- and zooplankton composition analyses. RESULTS: Following the rapid changes in environmental and biotic parameters during spring, protistan and bacterial communities displayed swift transitions from a homogeneous community to distinct strata-specific communities. A prevalence of auto- and mixotrophic protists dominated by cryptophytes was associated with spring algal bloom-specialized bacteria in the epilimnion. In contrast, the meta- and hypolimnion showcased a development of a protist community dominated by putative parasitic Perkinsozoa, detritus or particle-associated ciliates, cercozoans, telonemids and excavate protists (Kinetoplastida), co-occurring with bacteria associated with lake snow. CONCLUSIONS: Our high-resolution sampling matching the typical doubling time of microbes along with the combined microscopic and molecular approach and inclusion of all main components of the microbial food web allowed us to unveil depth-specific populations' successions and interactions in a deep lentic ecosystem.

• Keywords
• 18S and 16S amplicon sequencing, CARD-FISH, Epilimnion, Freshwater, Hypolimnion, Metalimnion, Microbial food webs, Protists, Spring succession,
• Publication type
• Journal Article MeSH

Telonemia are one of the oldest identified marine protists that for most part of their history have been recognized as a distinct incertae sedis lineage. Today, their evolutionary proximity to the SAR supergroup (Stramenopiles, Alveolates, and Rhizaria) is firmly established. However, their ecological distribution and importance as a natural predatory flagellate, especially in freshwater food webs, still remain unclear. To unravel the distribution and diversity of the phylum Telonemia in freshwater habitats, we examined over a thousand freshwater metagenomes from all over the world. In addition, to directly quantify absolute abundances, we analyzed 407 samples from 97 lakes and reservoirs using Catalyzed Reporter Deposition-Fluorescence in situ Hybridization (CARD-FISH). We recovered Telonemia 18S rRNA gene sequences from hundreds of metagenomic samples from a wide variety of habitats, indicating a global distribution of this phylum. However, even after this extensive sampling, our phylogenetic analysis did not reveal any new major clades, suggesting current molecular surveys are near to capturing the full diversity within this group. We observed excellent concordance between CARD-FISH analyses and estimates of abundances from metagenomes. Both approaches suggest that Telonemia are largely absent from shallow lakes and prefer to inhabit the colder hypolimnion of lakes and reservoirs in the Northern Hemisphere, where they frequently bloom, reaching 10%-20% of the total heterotrophic flagellate population, making them important predatory flagellates in the freshwater food web.

• Keywords
• CARD-FISH, Telonemia, freshwater lakes, metagenomics, microbial food webs, predatory flagellate,
• MeSH
• Biodiversity MeSH
• Phylogeny * MeSH
• In Situ Hybridization, Fluorescence * MeSH
• Lakes microbiology   parasitology   MeSH
• Metagenome MeSH
• Metagenomics MeSH
• RNA, Ribosomal, 18S * genetics   MeSH
• Fresh Water * microbiology   parasitology   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• RNA, Ribosomal, 18S * MeSH

Photosynthetic cryptophytes are ubiquitous protists that are major participants in the freshwater phytoplankton bloom at the onset of spring. Mortality due to change in environmental conditions and grazing have been recognized as key factors contributing to bloom collapse. In contrast, the role of viral outbreaks as factors terminating phytoplankton blooms remains unknown from freshwaters. Here, we isolated and characterized a cryptophyte virus contributing to the annual collapse of a natural cryptophyte spring bloom population. This viral isolate is also representative for a clade of abundant giant viruses (phylum Nucleocytoviricota) found in freshwaters all over the world.

• Keywords
• Imitervirales, NCLDVs, Nucleocytoviricota, cryptophytes, freshwater giant viruses, giant viruses, phytoplankton spring bloom, viral isolation,
• MeSH
• Cryptophyta genetics   MeSH
• Eukaryota MeSH
• Phytoplankton MeSH
• Humans MeSH
• Giant Viruses * MeSH
• Viruses * MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH

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.

• MeSH
• Bacteria MeSH
• Eukaryota genetics   MeSH
• Metagenome * MeSH
• Plankton MeSH
• Fresh Water MeSH
• Viruses * genetics   MeSH
• Water MeSH
• Publication type
• Video-Audio Media MeSH
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Water MeSH