Unveiling the Ocean Viromes of Challenger Deep, the Deepest Place in the World’s Oceans

Background: The hadal ocean biosphere, i.e the deepest part of the world’s oceans, harbors a unique community of cellular microbes, implying that there is also a novel assemblage of co-occurring viruses. However, the diversity and lifestyle of viroplanktons in hadal zone is poorly studied. Results: Herein, we show the unique nature of the Mariana Trench DNA virome (MTV) comprising a dataset of 95,813 non-redundant assembled viral sequences (contigs) from the ocean’s surface (2m) to the hadal zone at 8727 m below the surface. We assigned 29% of the contigs into 236 viral taxa, of which all but 3% belonged to groups with no cultured representatives. The remaining 71% of the contigs could not be assigned to a taxonomic group. The dominant viral contigs were almost more abundant in deep even hadal zone than in surface zone. The MTV is unlike any other viral assemblage described to date, and presents a novel viral ecological zone in the global ocean. The dominant viral contigs in the MTV were abundant and significantly specific in the “Trench” zone, and were associated with viruses that had signature genes indicative of a lysogenic life strategy, suggesting that horizontal gene transfer by viruses among bacteria is likely more prevelant in hadal zone than in the overlying waters. Conclusions: This study demonstrates that viruses dominating the hadal zone of the Mariana Trench are unlike those associated with other marine viral assemblages sampled to date, representing a unique ecological assemblage characterized by a lysogenic life style and horizontal gene transfer events. These results establish a foundation against which viral assemblages from other deep trenches can be compared. Keywords: Challenger Deep, Mariana Trench, ocean viromes, viral metagenomics, lysogenic life strategy, horizontal gene transfer