Depth-driven decline in viral diversity unveils potential novel viruses in global deep-sea ecosystems.

Deep-sea ecosystems remain poorly understood due to exploration challenges. Despite the advancements metagenomics have brought to the understanding of the ocean microbiome, the diversity of marine viruses, particularly in the deep sea, is still not well characterized. In this study, we analysed the impact of depth on the composition and diversity of marine viruses in deep-sea waters at a global scale. Raw reads from deep-sea shotgun DNA sequences were retrieved from the Tara and Malaspina expeditions, encompassing depths from 270 to 4,005 m. A total of 80 samples containing viral reads were identified and analysed through a comprehensive bioinformatics pipeline, including quality assessment, taxonomic classification and metabolic annotation. The analysis reveals that microbial viral diversity significantly decreases with depth, with shallower waters exhibiting higher species richness. We determined that a substantial proportion of deep-sea viral sequences remains unclassified - up to 31.9% at depths of 270-1,000 m and 9.6% at 2,400-4,005 m. Additionally, a higher abundance of auxiliary metabolic genes was observed at shallower depths, indicating potential roles in host metabolism and adaptation. Our findings reveal the deep ocean as a vast, largely unexplored source of microbial viral diversity. This research emphasizes how depth influences viral diversity and community makeup in deep-sea environments, underscoring the need for further exploration to fully grasp their complexity and ecological roles.