Abstract
ABSTRACTMicrobial life has been detected well into the igneous crust of the seafloor (i.e., the oceanic basement), but there have been no reports confirming the presence of viruses in this habitat. To detect and characterize an ocean basement virome, geothermally heated fluid samples (ca. 60 to 65°C) were collected from 117 to 292 m deep into the ocean basement using seafloor observatories installed in two boreholes (Integrated Ocean Drilling Program [IODP] U1362A and U1362B) drilled in the eastern sediment-covered flank of the Juan de Fuca Ridge. Concentrations of virus-like particles in the fluid samples were on the order of 0.2 × 105 to 2 × 105 ml−1 (n = 8), higher than prokaryote-like cells in the same samples by a factor of 9 on average (range, 1.5 to 27). Electron microscopy revealed diverse viral morphotypes similar to those of viruses known to infect bacteria and thermophilic archaea. An analysis of virus-like sequences in basement microbial metagenomes suggests that those from archaeon-infecting viruses were the most common (63 to 80%). Complete genomes of a putative archaeon-infecting virus and a prophage within an archaeal scaffold were identified among the assembled sequences, and sequence analysis suggests that they represent lineages divergent from known thermophilic viruses. Of the clustered regularly interspaced short palindromic repeat (CRISPR)-containing scaffolds in the metagenomes for which a taxonomy could be inferred (163 out of 737), 51 to 55% appeared to be archaeal and 45 to 49% appeared to be bacterial. These results imply that the warmed, highly altered fluids in deeply buried ocean basement harbor a distinct assemblage of novel viruses, including many that infect archaea, and that these viruses are active participants in the ecology of the basement microbiome.
Highlights
Microbial life has been detected well into the igneous crust of the seafloor, but there have been no reports confirming the presence of viruses in this habitat
Hydrothermal vents have been described as a window into the conditions and processes occurring deeper in the basement [11], and it is possible that some of the viruses previously observed in vent fluid samples originated from deeper in the igneous crust
Chemical analysis of bag sample fluids indicated that the purity of the samples collected for epifluorescence microscopy (EfM) ranged from 99 to 100% (Table 1)
Summary
Microbial life has been detected well into the igneous crust of the seafloor (i.e., the oceanic basement), but there have been no reports confirming the presence of viruses in this habitat. This report provides the first estimate of free virus particles (virions) within fluids circulating through the extrusive basalt of the seafloor and describes the morphological and genetic signatures of basement viruses These data push the known geographical limits of the virosphere deep into the ocean basement and point to a wealth of novel viral diversity, exploration of which could shed light on the early evolution of viruses. Viruses infect every known type of organism, and they appear to be a ubiquitous feature of all biological communities They have been documented in nearly every habitat where life has been found [2,3,4], including deeply buried marine sediments [5,6,7] and in fluids emanating from submarine hydrothermal vents [8,9,10]. With recent improvements in the CORK design [16] and with CORK-compatible in situ sampling equipment [17], it is possible to sample up to hundreds of liters of pristine basement fluids for microbiological analysis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
