Abstract

Signs of microbial life have been detected well into the igneous crust of the seafloor (i.e., the oceanic basement), but there have not yet been any reports confirming the presence of viruses in this marine deep subsurface habitat. To detect and characterize an ocean basement virome, we sampled the hot fluids (ca. 60-65° C) from 117 to 292 m deep into the ocean basement using seafloor observatories (CORKs) installed in two drill holes (IODP U1362A and U1362B) on the eastern flank of the Juan de Fuca Ridge. Concentrations of virus-like particles in the fluids were on the order of 0.2 -2 x 10⁵ ml^-1 (n=8), higher than prokaryote-like cells in the same samples by a factor of 9 (range 1.5 to 27). Electron microscopy revealed diverse viral morphotypes (tailed icosahedra, lemon-, spindle- , and rod-shaped, and globular) similar to known bacteria- and thermophilic archaea-infecting viruses. Viral metagenomes were constructed from DNA extracted from each of 17 buoyant density fractions. Analysis of these 17 metaviromes indicated that physical fractionation resulted in enrichment of archaea virus-like sequences in low-density fractions. Assembly of viral metagenomes resulted in 34 closed (circularized) viral genomes, and 110 linear viral scaffolds. Consistent with the morphological evidence, many of the virus-like sequences were most similar to genes from members of the order Caudovirales; and from thermophilic archaeal viruses in the families Bicaudaviridae, Rudiviridae, and Fuselloviridae. These results imply that hot, sediment-covered ocean basement harbors a distinctive assemblage of novel viruses that participate in the ecology of the basement microbiome.