Abstract
Viruses are ubiquitous and cause significant mortality in marine bacterial and archaeal communities. Little is known about the role of viruses in the sub-seafloor biosphere, which hosts a large fraction of all microbes on Earth. We quantified and characterized viruses in sediments from the Baltic Sea. The results show that the Baltic Sea sub-seafloor biosphere harbors highly abundant viruses with densities up to 1.8 × 1010 viruses cm−3. High potential viral production down to 37 meters below seafloor in ca. 6000-years-old sediments and infected prokaryotic cells visible by transmission electron microscopy demonstrate active viral infection. Morphological and molecular data indicate that the highly diverse community of viruses includes both allochthonous input from the overlying seawater and autochthonous production. The detection of cyanophage-like sequences showed that viruses of phototrophic hosts may persist in marine sediments for thousands of years. Our results imply that viruses influence sub-seafloor microbial community dynamics and thereby affect biogeochemical processes in the sub-seafloor biosphere.
Highlights
Supplementary information The online version of this article contains supplementary material, which is available to authorized users.The deep biosphere extends hundreds to thousands of meters below the seafloor [1,2,3,4]
Viral counts were very similar among four drilled holes from three sites in the Baltic Sea (Fig. 1, Fig. S2, see Supplementary Methods for details)
As a general trend for all four holes, viral abundances increased with depth beneath the surface and peaked at 4–5 mbsf, where a peak in total organic carbon (TOC) occurred, followed by a steady decline with depth
Summary
The deep biosphere extends hundreds to thousands of meters below the seafloor [1,2,3,4] It harbors by far the largest reservoir of organic carbon on Earth and is inhabited by an estimated number of 3 × 1029 prokaryotic cells, or half of all prokaryotes in the ocean [5]. These viruses represent a large proportion of the virus particles estimated to occur on Earth [12]. Indirect evidences such as high virus-to-cell ratios and the expression of viral homologs in metatranscriptomes, suggest ongoing production of viruses in deep sediments with consequent ecological and biological effects [9, 13].
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