In this review, I describe recent findings on the molecular architecture and genomic characterization of giant viruses that infect microbial eukaryotes (protists) across diverse ecosystems and ecological niches. Giant viruses are distinguished by their large and complex genomes, which encode a wide range of functions, including protein translation, carbohydrate and lipid metabolism, nitrogen cycling, light assimilation, and key metabolic pathways such as glycolysis and the tricarboxylic acid cycle. Additionally, these genomes feature unique genes, often acquired through horizontal gene transfer, that are not found in other viruses and contribute to the viruses’ ability to manipulate host metabolism and evade host defenses. A core set of genes conserved across different families of giant viruses is highlighted, serving as essential components for key life-cycle processes and providing valuable phylogenetic markers. The review also discusses the role of ORFans and virophages in contributing to the genetic diversity and evolutionary adaptation of these viruses. These findings are crucial for understanding the diversity, evolutionary mechanisms, and complex virus–host interactions of giant viruses, as well as for developing more advanced classification systems. Furthermore, the potential biotechnological applications of unique viral genes and pathways are explored, underscoring the importance of ongoing research in this field.
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