Diversity and taxonomy of giant algal viruses: chloroviruses lead the way

ABSTRACT Viruses are the most diverse and abundant biological entities on Earth. Their impact on life is undeniable, and viruses of microorganisms, in particular, exert a profound impact on the planet by shaping biogeochemical cycles and driving the evolution of living organisms. Among them, the giant viruses of the family Phycodnaviridae are major players in aquatic environments, infecting many groups of algae from inland and marine ecosystems. The discovery of chloroviruses over 40 years ago started a new era in virology, paving the way for fundamental discoveries into the diversity and evolution of giant viruses, and ultimately leading to the establishment of the order Algavirales, included in the phylum Nucleocytoviricota . In recent years, dozens of chlorovirus genomes have been sequenced, greatly expanding our understanding of the extent of their diversity and evolution. This review provides an overview of the diversity of giant algal viruses of the order Algavirales and compiles the most recent data on the genomic landscape of chloroviruses based on 130 sequenced viruses. Genetics reflects biological diversity, as evidenced by the distinct plaque morphotypes, which range from tiny (<1 mm) and diffuse plaques to very large (>4 mm) and well-defined plaques. Such diversity is likely to be present in other algal viruses, and the mechanisms behind this remain to be evaluated. Moreover, challenges related to the taxonomy of phycodnaviruses are discussed, and a new perspective for Algavirales is presented based on comparative genomics and phylogenetics, suggesting the existence of new virus species yet to be discovered.