Award: OCE-1636402

Intellectual Merit: This project provides a first look at the abundance and genetic diversity of viruses present in the seawater that percolates through the rock deep below the seafloor at a location along the Mid-Atlantic Ridge. The water of the subsurface marine aquifer is referred to as ?basement fluid?. We sampled basement fluid from boreholes drilled through the sediments and into the rock at a sediment-filled basin called North Pond along the mid-ocean ridge in the center of the Atlantic Ocean. The results show that the microbes and viruses in the basement fluids at this location, where the seafloor is spreading slowly and the fluids are cold, are present at lower concentrations and are of very different types from those found along a ridge we sampled previously in the Pacific Ocean, where the seafloor spreads more rapidly and the basement fluids are hot. Changes in the viral composition among multiple samplings from the same boreholes at North Pond suggest that either the viruses in the fluids vary significantly over time and space or that flushing of the sampling lines was not always adequate to ensure collection of a stable, representative sample of fluid. In both the cold basement fluid samples in this project and the hot fluids sampled previously, the viruses and microbes in the basement fluids differ from bottom seawater suggesting that selective pressures in the aquifer lead to shifts in microbial community composition and the accompanying viruses. In both locations, we detected viruses that code for molecular machinery (known as diversity-generating retroelements) that allows them to change one of their proteins very quickly. This is believed to help the viruses rapidly adapt to changes in the cells that they infect. This work provides new insights into life in the marine deep subsurface at a location with cold basement fluid that is expected to be more representative of the global ocean basement biome. Broader Impacts: This project supported a female post-doctoral researcher who was hired into a tenure-track faculty position in the second year of the project, who mentored graduate and undergraduate students on the project, and who has now competed successfully for her own NSF funding to work in this research area. This project also partially supported a graduate student working on doctoral dissertation in bioinformatics, provided training, data and material for a master?s student of Native Hawaiian ancestry, and provided financial support and laboratory experience for two female undergraduate students. The project also resulted in the development of a bioinformatics tool that is applicable to many types of microbial and viral metagenomic analyses, regardless of habitat from which the sample derive. The tool is thus of general use for scientists engaged metagenomics in disparate areas ranging from the human microbiome to environmental microbiology. Last Modified: 04/16/2021 Submitted by: Grieg F Steward