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 prokaryotic abundance decreased exponentially with sediment
depth from 1.0 × 108 viruses cm–3 and 3.8 × 106
cells cm–3 at 4 mbsf to 4.9 × 106 viruses cm–3 and 9.8
× 105 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 × 105 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.
Middelboe, M