Induction of prophages from deep-subseafloor bacteria

The deep-subseafloor biosphere harbours a major part of the total microbial biomass on Earth. However, how life and death in this environment are regulated is not yet understood. While organisms from higher trophic levels appear to be absent, viruses might be a factor for microbial mortality. In this study, we found an increasing ratio between viral and total cell counts with depth in deep-subseafloor sediments recovered during Leg 201 of the Ocean Drilling Program. A phylogenetically diverse culture collection from corresponding sediment layers was tested for the presence of inducible prophages. A treatment by mitomycin C as inducing agent indicated the presence of prophages in 46% of the bacterial isolates. Different morphotypes of myoviruses and siphoviruses were detected by transmission electron microscopy. Pulsed-field gel electrophoresis applied to viral DNA extracts showed their genetic diversity. Three host strains even harboured more than one prophage. Thus, our results prove the existence of functional viruses in the deep-subseafloor biosphere. Bacteriophages might take over the role of the main predators, as anoxia and oligotrophy favour the importance of viruses as mortality factors. Furthermore, the fact that the viral shunt recycles organic compounds might be of special relevance to this severely nutrient-depleted habitat.