Biology subjects

Schvarcz, C. R.

Publications and source records attributed to Schvarcz, C. R..

8 recordsLinked to original sources

bioRxiv2026

First Evidence of Dicistroviruses Infecting Protists

Molecular surveys suggest that RNA viruses are abundant and diverse in the ocean, but the hosts for most of these viruses are unknown because so few have been cultivated. Here, we present the genomes of five positive-sense, single-stranded RNA (+ssRNA) viruses isolated from tropical seawater that infect green algae in the genus Tetraselmis (order Chlorodendrales). Phylogenetic analyses of multiple genes placed these closely related viruses within the family Dicistroviridae (order Picornavirales) making them the first viruses within the bounds of the Dicistroviridae family demonstrated to infect an organism other than arthropods. The RNA-dependent and capsid gene sequences of the Tetraselmis RNA viruses (TetRNAV01-05) cluster with others recovered from diverse environmental water samples or aquatic invertebrate tissues, and together they form a strongly supported sister clade to viruses in the genus Triatovirus. The TetRNAVs harbor a unique intergenic internal ribosome entry site (IRES), suggesting a translation strategy distinct from that described in arthropod-infecting dicistroviruses. Our results suggest that many uncultivated viruses presumed to infect invertebrates, because they were detected in invertebrate-derived samples and their genes cluster within the family Dicistroviridae, may instead be protist-infecting viruses.

microbiology↗

bioRxiv2025

Strain-level diversity of giant viruses infecting chlorarachniophyte algae in the subtropical North Pacific

Giant DNA viruses are ubiquitous among unicellular eukaryotes and occur in marine, freshwater, and terrestrial environments. Despite intense metagenomic data mining, their strain-level diversity remains largely unexplored. Here we introduce a model system comprising four isolates of a giant virus called ChlorV, which infects marine microalgae of the class Chlorarachniophyceae (Rhizaria) from station ALOHA, Hawaii. The ChlorV genomes are 469 kbp to 493 kbp long and encode approximately 400 proteins, at least 106 of which are present n purified virions. Although the four viral genomes are highly syntenic, they differ by several insertions and deletions that often encode methyltransferases. Interestingly, we found that some of these methyltransferase genes correlated with specific DNA methylation patterns in the ChlorV strain Our study describes the first giant viruses infecting the eukaryotic supergroup Rhizaria and demonstrates how viral strain-level variation in gene content and epigenetic features may affect eco-evolutionary processes in marine microalgae.

microbiology↗

bioRxiv2025

A dinoflagellate-infecting giant virus with a micron-length tail

Viral infection is a ubiquitous source of marine plankton mortality, but relatively few viruses that infect phytoplankton have been characterized. Here we describe a virus, PelV-1, with unusual morphological and genomic features that infects a dinoflagellate, Pelagodinium sp. Both host and virus were isolated from the epipelagic zone in the North Pacific Subtropical Gyre. PelV-1 has a [~]200 nm capsid size, and the virion variably exhibits two appendages, the presence and length of which may reflect different stages of virion maturity or artifacts of sample preparation. The appendages are a thinner 30 nm-wide tail-like structure that can extend to 2.3 {micro}m -- the longest virus appendage described to date-- and a shorter, thicker (>40-70 nm) protrusion, which appears to emerge from a star-shaped capsid opening directly opposite the attachment point of the long, thin tail. Sequencing and assembly of material in a purified lysate generated a high-coverage (> 4,000x) genome of 459 kb (33.8% GC). A second, distinct genome of 504 kb (25.8% GC) was also assembled, but had low read coverage (< 24x), suggesting the presence of a low-abundance, co-cultured virus (co-PelV). Phylogenetic analysis indicates that both PelV-1 and co-PelV are members of Mesomimiviridae. They contain various genes for the metabolism of amino acids (e.g., asparagine synthase), carbohydrates (e.g., epimerase, glycosyl hydrolase, aconitate hydratase, succinate dehydrogenase of the TCA cycle), and lipids (e.g., phospholipases), as well as other noteworthy genes (e.g., light-harvesting complex, rhodopsin, ion channel, sugar transporters, aquaporin). PelV-1 also has ORFs most similar to tail fiber genes of Synechococcus phage and other tail domain-containing protein homologs. The ecological advantages that might be conferred by the extraordinarily long tail and metabolic genes of PelV-1 is unknown, but this isolate expands the scope of morphological and metabolic diversity of viruses and suggests many more unusual marine viruses await discovery. Author summaryGiant viruses challenged our traditional views of virology due to their large size and the presence of hundreds of auxiliary metabolic genes. But despite the immense giant virus diversity discovered through sequencing, few isolates were described, and those were primarily viruses that infect amoeba host and rarely from phytoplankton. This hampers our understanding of marine host-virus interaction and thus the impact of viruses on the ocean ecosystem. Here we provide genomic and morphological characterization of a novel dinoflagellate giant virus (PelV-1) and a second co-occurring, albeit low abundance, virus (co-PelV). Dinoflagellates are vital in marine symbiosis and algal blooms but only two giant virus isolates have been described with no available genomic resources to date. Thus, this is a significant contribution to the literature on dinoflagellate viruses. Among the notable features of PelV-1 are its unique micron-length tail appendage, phagocytosis-like entry mechanism and its varied auxiliary metabolic genes including photosynthesis and energy generating genes.

microbiology↗

bioRxiv2025

Evidence for the acquisition of a proteorhodopsin-like rhodopsin by a chrysophyte-infecting giant virus

Chrysophytes are nanoflagellate protists widespread in aquatic ecosystems with diverse trophic roles as primary producers and bacterivores. Molecular evidence suggests that chrysophytes are commonly infected by giant viruses but isolates of such virus-host systems have not been reported. Here, we describe the first cultivated chrysophyte-infecting virus, Chrysophyceae Clade H virus SA1 (ChrysoHV), isolated along with its phago-mixotrophic host alga from surface waters in the tropical North Pacific Ocean. The ChrysoHV capsid (290 {+/-} 40 nm diameter) is associated with a loose, sac-like membrane that extends its effective diameter (720 {+/-} 120 nm) and presents a long (1,200 {+/-} 240 nm), thin (20 {+/-} 2), flexible tail, a morphology unlike any virion yet described. The assembled genome is 1.19 Mbp. Phylogenetic analysis places ChrysoHV as the third cultivated member of the Aliimimivirinae subfamily in the Mimiviridae family of giant viruses. The ChrysoHV genome encodes two heliorhodopsins and one proteorhodopsin. Proteorhodopsins are well known light-driven proton pumps in bacteria but have not been previously reported in a viral genome. The predicted viral proteorhodopsin structure suggests it may not have a functional retinal binding site implying a light-independent function. The genome also encodes two ribosomal proteins and nine genes with closest known homologs in marine cyanobacteria, most annotated as encoding for proteins involved in nutrient uptake. This unusual virus could serve as a model system for exploring viral rhodopsin functions and its genome suggests that phagotrophic protists may serve as an intracellular market for gene exchange between infecting viruses and ingested bacterial prey. ImportanceChrysophytes are abundant eukaryotic phytoplankton with trophic strategies ranging from photosynthesis to phagotrophy. They serve as models of mixotrophy among aquatic protists, but no chrysophyte-infecting viruses had been isolated, leaving a gap in experimental virus-host systems for a major class of protists. This study reports on the characterization of the first isolated chrysophyte-infecting virus, ChrysoHV. The virion morphology is unusual, having a loose membranous sac around a large capsid and a long filamentous tail. The genome contains genes for ribosomal proteins, a rarity in eukaryotic viruses, and multiple genes with homologs in common marine bacteria, one of which is a type of rhodopsin never before reported in a virus. We hypothesize that phago-mixotrophs, through infections and ingestion, may facilitate lateral gene exchange between eukaryote-infecting viruses and bacteria, entities that might not otherwise directly interact. The results expand the observed morphological diversity among viruses and the catalog of known virus genes.

microbiology↗

bioRxiv2024

Genomic diversity and global distribution of four new prasinoviruses from the tropical North Pacific

Viruses that infect phytoplankton are an integral part of marine ecosystems, but the vast majority of viral diversity remains uncultivated. Here we introduce four near-complete genomic assemblies of viruses that infect the widespread marine picoeukaryote Micromonas commoda, doubling the number of published genomes of Micromonas dsDNA viruses. All host and virus isolates were obtained from tropical waters of the North Pacific, a first for viruses infecting green algae in the order Mamiellales. Genome length of the new isolates ranges from 205-212 kb, and phylogenetic analysis shows that all four are members of the genus Prasinovirus. Three of the viruses form a clade that is adjacent to previously sequenced Micromonas viruses, while the fourth virus is relatively divergent from previously sequenced prasinoviruses. We identified 61 putative genes not previously found in prasinoviruses, including a phosphate transporter from a family novel to viruses, and a potential apoptosis inhibitor novel to marine viruses. Forty-eight genes in the new viruses are also found in host genome(s) and may have been acquired through horizontal gene transfer. By analyzing the coding sequences of all published prasinoviruses we found that [~]25% of prasinovirus gene content is significantly correlated with host genus identity (i.e., Micromonas, Ostreococcus, or Bathycoccus), and the functions of these genes suggest that much of the viral life cycle is differentially adapted to the three host genera. Mapping of metagenomic reads from global survey data indicates that one of the new isolates, McV-SA1, is relatively common in multiple ocean basins. Importance statementThe genomes analyzed here represent the first viruses from the tropical North Pacific that infect the abundant phytoplankton order Mamiellales. Comparing isolates from the same location demonstrates high genomic diversity among viruses that co-occur and presumably compete for hosts. Comparing all published prasinovirus genomes highlights gene functions that are likely associated with adaptation to different host genera. Metagenomic data indicates these viruses are globally distributed, and one of the novel isolates may be among the most abundant marine viruses.

ecology↗

bioRxiv2024

Ultrastructural and transcriptional changes during a giant virus infection of a green alga

The complete genome sequence of the Oceanusvirus kaneohense strain (Tetraselmis virus 1; TetV-1) was previously reported, but little is known about the virus infection cycle. Using a permissive Tetraselmis isolate (UHM1315), we estimated the eclipse period (4-8 hours), latent period (16 hrs), and burst size (800-1000) of the virus and documented ultrastructural and transcriptional changes in the host during infection. Putative viral factories and electron-dense inclusion bodies appeared in the cytoplasm of infected cells by 8 and 16 h post-infection, respectively. The nucleus and chloroplasts appeared to remain intact but reduced in size after 8 h. Transcriptome sequencing suggests that the viral genome codes for 830 transcripts. Those expressed early in infection (eclipse period at 0.25 and 4 hr) were related to the initiation of transcription, DNA synthesis, translation, and host immune repression. During the later, post-eclipse period (8, 12, 16 hr), virus structural genes were expressed. For the algal host, transcripts related to lipid metabolism and endocytosis were upregulated during the early phase, while those for protein modification/ turnover/ transport were downregulated. In the later period, host transcripts associated with basic cellular processes were upregulated, while genes related to morphogenesis/development were downregulated. Many of the most highly expressed virus and host genes were of unknown function, highlighting a need for additional functional studies.

microbiology↗

bioRxiv2022

Trophic strategies explain the ocean niches of small eukaryotic phytoplankton

A large fraction of marine primary production is performed by diverse small protists, and many of these phytoplankton are mixotrophs that also consume bacterial prey. However, the mechanisms structuring this diversity and its biogeochemical consequences remain poorly understood. Here we use isolates from seven major taxa to demonstrate tradeoffs between phototrophic and phagotrophic abilities. We then show that trophic strategy along the autotrophy-mixotrophy spectrum correlates strongly with global niche differences, across depths and across gradients of stratification and chlorophyll a. A model of competition shows that community shifts can be explained by greater fitness of faster-grazing mixotrophs when nutrients are scarce and light is plentiful. Our results illustrate how basic physiological constraints and principles of resource competition can organize complexity in the surface ocean ecosystem.

ecology↗

bioRxiv2021

Broad phylogenetic and functional diversity among mixotrophic consumers of Prochlorococcus

Small eukaryotic phytoplankton are major contributors to global primary production and marine biogeochemical cycles. Many taxa are thought to be mixotrophic, but quantitative studies of phagotrophy exist for very few. In addition, little is known about consumers of Prochlorococcus, the abundant cyanobacterium at the base of oligotrophic ocean food webs. Here we describe thirty-nine new phytoplankton isolates from the North Pacific Subtropical Gyre (Station ALOHA), all flagellates [~]2-5 um diameter, and we quantify their ability to graze Prochlorococcus. The mixotrophs are from diverse classes (dictyochophytes, haptophytes, chrysophytes, bolidophytes, a dinoflagellate, and a chlorarachniophyte), many from previously uncultured clades. Grazing ability varied substantially, with specific clearance rate (volume cleared per body volume) varying over ten-fold across isolates and six-fold across genera. Slower grazers tend to create more biovolume per prey biovolume consumed. Using qPCR we found that the haptophyte Chrysochromulina was most abundant among the isolated mixotrophs at Station ALOHA, with 76-250 cells mL-1 across depths in the upper euphotic zone. Our results show that within a single ecosystem the phototrophs that ingest bacteria come from many branches of the eukaryotic tree, and are functionally diverse, indicating a broad range of strategies along the spectrum from phototrophy to phagotrophy.

ecology↗