Viral abundance and activity in the deep sub-seafloor biosphere

Subsurface abundance and distribution of viruses and prokaryotes was determined along a depth profile, down to 96 m below seafloor (96 mbsf), at Challenger Mound from the Porcubine Seabight (IODP Expedition 307). Viral and prokaryotic abundance decreased exponentially with sediment depth from 1.0 10 8 viruses cm -3 and 3.8 10 6 cells cm -3 at 4 mbsf to 4.9 10 6 viruses cm -3 and 9.8 10 5 cells cm -3 at 96 mbsf. The age of the sediment ranges from ca. 0.5 million yr before present (Ma) at 4 mbsf to ca. 2 Ma at 96 mbsf. Assuming that the decline in viral abundance with depth reflects a gradual decay of the viral assemblage over time, the estimated decay rate of the viral community is 1.2 10 -6 0.3 10 -6 (SD) yr -1 , corresponding to a half-life of the viral community of 5.8 10 5 yr. Measurements of viral and prokaryotic change in abundance were performed in incubations of undiluted, but homogenized, sediment samples (13.3 and 79.8 mbsf) in anaerobic bags. Viral abundance decreased rapidly (decay rates of 0.010 0.002 [SD] and 0.022 0.018 [SD] h -1 , respectively) in the incubations, suggesting that homogenization exposed the viruses to degradation processes. We hypothesize that most of the deep subsurface viral communities inhabit a microenvironment where the viruses are protected against decay, and can therefore persist in undisturbed sediments for hundreds of thousands, perhaps even millions, of years.