Giant viruses of the polar regions: diversity, endemism, adaptation and ecological structuring.

The poles represent Earth's most climate-sensitive biomes, where microbial communities and viruses drive fundamental ecological processes. Within these extreme environments, giant viruses of the phylum Nucleocytoviricota have emerged as key regulators of microbial mortality and biogeochemical cycling. This review synthesizes current knowledge on polar giant viruses, emphasizing their diversity, endemism, genomic adaptations, and ecological roles across polar habitats. Polar systems harbour highly structured, habitat-specific viral assemblages characterized by significant endemism and sharp ecological boundaries, shaped by strong environmental filtering, host biogeography, virus-virus interactions and spatial isolation. Genomic analyses show that these viruses possess unique adaptations to persistent cold, including proteomic shifts consistent with psychrophily and the enrichment of auxiliary metabolic genes. Interactions with giant virus parasites (virophages) further contribute to the complexity of polar giant virus ecology. However, rapid warming and the loss of perennial ice threaten to destabilize these ancient refugia and their giant virus populations. Changes in temperature, hydrological connectivity and ecosystem structure may alter virus-host dynamics and weaken the strong viral endemism. These environmental shifts risk the extinction of unique lineages and the disruption of the critical biogeochemical roles they perform, highlighting the urgent need to understand viral dynamics in rapidly changing polar and cryospheric ecosystems.