A giant virus in a small lake: characterization of a new tropical algal virus expands the geographical and genetic diversity of the Brazilian virosphere

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ABSTRACT Chloroviruses are large double-stranded DNA (dsDNA) viruses that infect Chlorella -like green algae in freshwater environments. Despite being extensively studied in the Northern Hemisphere, little is known about their diversity and biology in tropical regions. In this study, we report the isolation and characterization of a novel chlorovirus, named Chlorovirus BR-MG-N01, infecting Chlorella variabilis NC64A. The virus was recovered from a freshwater lake in Guanhães, Minas Gerais, Brazil, located in a transition zone between the Atlantic Forest and Cerrado biomes. Electron microscopy revealed icosahedral particles of approximately 160 nm in diameter. The viral growth profile showed a 2-log increase after 24 h of infection. BR-MG-N01 demonstrated high thermal stability, remaining viable from −80°C to 56°C, but showed low tolerance to ultraviolet radiation, with a rapid decrease in titer after 3 min of exposure. Genome sequencing revealed a 318 kbp dsDNA genome containing 384 coding sequences. Gene annotation identified a high number of endonucleases, two of which are putatively novel, as well as genes related to transcription and oxidative stress resistance. Phylogenetic analysis based on the DNA polymerase gene placed BR-MG-N01 within the Alphachlorovirus subgenus, closely related to the species Chlorovirus vanettense . Comparative genomic analysis revealed strong synteny and high sequence similarity with other members of this species. The isolation of Chlorovirus BR-MG-N01 expands the known distribution of chloroviruses and highlights the need for further exploration of tropical environments to uncover the ecological and evolutionary diversity of these enigmatic aquatic viruses. IMPORTANCE Chloroviruses are large dsDNA viruses that infect algae, with limited available research in tropical environments. This work helps address that gap by presenting the first detailed genomic and biological characterization of a native chlorovirus from Brazil. We show that Brazilian environments harbor viral lineages as complex and diverse as those described in well-studied regions of the Northern Hemisphere. By documenting the isolate’s morphology, stability, replication profile, genomic architecture, and evolutionary placement, this study establishes the basis for future ecological and evolutionary research on algal viruses in the country. Overall, the discovery highlights a hidden viral diversity in Brazilian ecosystems and reinforces the importance of investigating microorganisms from tropical and understudied habitats.

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