A Comparison of Viral Populations Inhabiting Atlantic and Pacific Oceanic Crustal Fluids

Fluids within the basalt-hosted deep subsurface of the world's oceans represent one of the most inaccessible and understudied biospheres on Earth. Supported by over 15 years of developing technologies in concert with sampling deep subsurface crustal fluids via Circulation Obviation Retrofit Kits (CORKs) assisted by ROV Jason and HOV Alvin, we have gained an understanding of deep subseafloor viral populations for both the Pacific Ocean along the Juan de Fuca Ridge (JdFR) flank and the Atlantic Ocean at North Pond (NP). At the point of sampling, the fluids circulating through both the sites reside within relatively young crust (3.5 and 8 million years old, respectively). However, JdFR fluids are highly altered and are ca. 65 C when sampled, while fluids from NP are cooler (<20 C) and show less alteration. Viruses are present in both environments. Enumeration via epifluorescence microscopy revealed abundances of 10⁵ ml^-1 at JdFR and 10⁴ ml^-1 at NP. In both cases, viruses were less abundant in the crustal fluids than in overlying bottom seawater. Metagenomic analysis of microbial and viral metagenomes from JdFR revealed a high abundance of archaeal virus-like sequences. Analysis of the CRISPR and prophages of these genomes showed viruses infecting several strains of deeply branching Archaeoglobi, sequenced from JdFR. Preliminary analysis of microbial metagenomes from NP using the program VirSorter revealed both prophages and potential free-living viral sequences. Annotated viral genomes were most similar to head-tail caudo-type viruses, with less similarity to the archaeal-like viruses seen at JdFR. The cumulative data from both sites suggests that viruses contribute to the mortality of deep subsurface prokaryotes through cell lysis analogous to processes in the water column. Additionally, these viruses likely affect the standing stock of genomic diversity in oceanic basement environemnts, altering host capabilities through the processes such as encoding auxiliary metabolic genes and lysogenic conversion.