Biological and genomic characterization of a new chlorovirus isolate expands the diversity and complexity of giant algal viruses in Brazil

Abstract

A novel giant virus was isolated from a freshwater sample collected in Para de Minas city, Minas Gerais state, Brazil. The new isolate, named BR-AMG2, is a chlorovirus infecting the microalga Chlorella variabilis. Members of the family Phycodnaviridae, the chloroviruses have large linear dsDNA genomes (285-410 kbp), encoding hundreds of proteins and tRNAs. Despite their ecological and evolutionary importance, chloroviruses remain poorly studied, especially in South America. This study aimed to expand the understanding of this viral group by isolating and fully characterizing new chloroviruses in Brazil. After inoculating alga cultures with environmental samples, cytopathic effects, marked by lysis plaque formation, were observed 72 hours post-sampling. Chlorovirus BR-AMG2 remained stable across a temperature range of - 30 {degrees}C to 37 {degrees}C, yet was highly sensitive to UV exposure. This virus has a 160 nm icosahedral capsid and establishes a large viral factory in the host cytoplasm. Whole genome sequencing revealed a 338,043 bp genome, containing 11 tRNAs and 388 protein-coding sequences. We observed an extensive DNA methylation machinery, accounting for [~]5% of gene content in this isolate, suggesting a highly methylated genome. Phylogenetic reconstruction and Average Nucleotide Identity analyses confirmed that BR-AMG2 is a new member of the species Chlorovirus americanus, the first isolate recovered from a tropical region of the planet. Additionally, we observed a slight expansion of the species pangenome, suggesting ongoing genetic innovation with new isolates. Altogether, our results expand our knowledge about the viral diversity in Brazil and provide valuable insights into the biology of giant algae-infecting viruses.