Genetics & Molecular Biology
Deep-Sea Giant Viruses Thrive and Adapt in the Ocean's Dark Zones
Original Authors: Liu, Nagasaka, Wu, Ban, Mimick, Meng, Neches, Moniruzzaman, Yoshida, Nishimura, Endo, Okazaki, Ogata
Giant viruses (GVs) are recognized for their impact on marine ecosystems in the sunlit surface waters, infecting a variety of eukaryotic hosts and influencing biogeochemical cycles. However, their presence, activity, and adaptive strategies in the vast, dark aphotic layers of the ocean have remained largely unknown. To shed light on this mystery, researchers undertook a comprehensive study, performing eight seasonal time-series samplings of surface and mesopelagic layers at a coastal site in Muroto, Japan, integrating advanced 18S metabarcoding, metagenomic, and metatranscriptomic data.
This intensive local investigation at Muroto successfully identified 48 GV genomes, with six of these being exclusively found in the mesopelagic layer. Remarkably, these mesopelagic-specific GVs demonstrated persistent transcriptional activity across all seasons, underscoring their active presence and potential involvement in these environments. This initial discovery highlighted the potential for unique viral dynamics beneath the ocean's surface.
To expand on these findings, the team conducted a global-scale analysis, compiling 4,473 species-level GV genomes and examining 1,890 marine metagenomes. This broader perspective unveiled 101 deep-sea-specific GVs distributed across the viral phylogenetic tree, indicating that adaptation to deep-sea conditions has occurred in multiple viral lineages. These deep-sea adapted GVs showed enrichment in specific gene functions related to the ubiquitin system, energy metabolism, and nitrogen acquisition, pointing to a specialized genetic toolkit for survival in resource-limited, dark habitats.
As lead author Liu notes in the paper, "These findings support the scenario that distinct GV lineages have adapted to hosts in aphotic marine environments by altering their gene repertoire to thrive in this unique habitat." This research significantly expands our understanding of giant virus ecology and their previously underestimated importance in the complex web of life within the ocean's vast and mysterious dark zones.