Mimick, E. | bioRxiv Science

bioRxiv2026

Expansive Diversity and Temporal Dynamics of Emerging Polinton-like Viruses in a Marine Ecosystem

Polinton-like viruses (PLVs) are an emerging group of double-stranded DNA viruses of microbial eukaryotes whose ecological roles in marine ecosystems remain poorly understood. Using a five-year monthly viromic time series from the San Pedro Ocean Time-series (SPOT), we investigated the diversity, temporal dynamics, and functional potential of PLVs in a coastal marine ecosystem. We identified 2,355 distinct PLV populations, revealing PLVs to be a highly abundant and diverse component of the marine virosphere. Phylogenetic analyses resolved multiple major PLV clades, including abundant Group X/Trimcap PLVs characterized by triplicate major capsid proteins, supporting the widespread occurrence of this unusual viral architecture in marine PLVs. Approximately half of PLV populations exhibited significant repeatable seasonal dynamics, partitioning into numerous chronotypes that reflect highly modular temporal niches. PLV abundance correlated positively with multiple productivity-linked environmental variables, including nitrate, particulate organic carbon, and primary productivity, suggesting close coupling between PLVs and seasonal ecosystem productivity. Functional analyses further identified diverse auxiliary metabolic genes in many PLV genomes involved in carbohydrate, lipid, redox, and nucleotide metabolism, with strong phylogenetic structuring across PLV clades. Together, these findings demonstrate that PLVs are abundant, functionally diverse, and ecologically dynamic members of marine viral communities, and suggest they are important yet underappreciated regulators of protist ecology and evolution in marine ecosystems.

microbiology↗

bioRxiv2025

Giant viruses specific to deep oceans show persistent presence and activity

Giant viruses (GVs) of the phyla Nucleocytoviricota and Mirusviricota are large double-stranded DNA viruses that infect diverse eukaryotic hosts and impact biogeochemical cycles. Their diversity and ecological roles have been well studied in the photic layer of the ocean, but less is known about their activity, population dynamics, and adaptive strategies in the aphotic layers. Here, we conducted eight seasonal time-series samplings of the surface and mesopelagic layers at a coastal site in Muroto, Japan, and integrated 18S metabarcoding, metagenomic, and metatranscriptomic data to investigate deep-sea GVs and their potential hosts. The analysis identified 48 GV genomes including six that were exclusively detected in the mesopelagic layer. Notably, these mesopelagic-specific GVs showed persistent activity across seasons. To investigate the global deep-sea-specific GV distribution, we compiled GV reference genomic data from the OceanDNA MAG project and other resources, and analyzed 1,890 marine metagenomes. This revealed 101 deep-sea-specific GVs, distributed across the GV phylogenetic tree, indicating that adaptation to deep-sea environments has occurred in multiple lineages. One clade enriched with deep-sea-specific GVs included one GV identified in our Muroto sampling, which displayed a wide geographic distribution. Seventy-six KEGG orthologs and 74 Pfam domains were specifically enriched in deep-sea-specific GVs, encompassing functions related to the ubiquitin system, energy metabolism, and nitrogen acquisition. These findings support the scenario that distinct GV lineages have adapted to hosts in aphotic marine environments by altering their gene repertoire to thrive in this unique habitat. IMPORTANCEGiant viruses are widespread in the ocean surface and are key in shaping marine ecosystems by infecting phytoplankton and other protists. However, little is known about their activity and adaptive strategies in deep-sea environments. In this study, we performed metagenomic and metatranscriptomic analyses of seawater samples collected from a coastal site in Japan and discovered giant virus genomes showing persistent transcriptional activity across seasons in the deep-sea water. Using a global marine dataset, we further uncovered the widespread existence of deep-sea-specific giant viruses and characterized their unique gene repertoire, which likely facilitates adaptation to the limited availability of light and organic compounds in the aphotic zone. These findings expand our understanding of giant virus ecology in the dark ocean.

genomics↗