Viral abundance and activity in the deep sub-seafloor biosphere

Abstract

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 prokary- otic abundance decreased exponentially with sedi- ment 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 undi- luted, but homogenized, sediment samples (13.3 and 79.8 mbsf) in anaerobic bags. Viral abundance de- creased rapidly (decay rates of 0.010 ± 0.002 (SD) and 0.022 ± 0.018 (SD) h -1 , respectively) in the incu- bations, suggesting that homogenization exposed the viruses to degradation processes. We hypothe- size that most of the deep subsurface viral communi- ties inhabit a microenvironment where the viruses are protected against decay, and can therefore per- sist in undisturbed sediments for hundreds of thou- sands, perhaps even millions, of years.