May 30, 2013
Tim Engelhardt, University of Oldenburg
Viruses in Marine Subsurface Sediments
The marine subsurface comprises nutrient-depleted environments as sources of organic carbon in deeply buried sediments are vanishingly small. While community sizes of living microorganisms are apparently controlled by energy availability, the role of bacteriophages as a factor for prokaryotic mortality and as potential drivers of microbial life in the deep subsurface has been barely addressed so far. The general ecological impact of phages on microbial communities for pelagic environments is well described and basically covered by three mechanisms: (1) The top-down regulation of microbial communities by viral predation is described by the “killing the winner”-theory. (2) The viral shunt causes a bottom-up regulation by providing nutrients to the remaining microbial populations with the release of cell debris after viral lysis. (3) In their function as mobile genetic elements, viruses mediate horizontal gene transfer via transduction.
As many phages integrate their DNA into the host genome, culture collections from a particular habitat represent an archive of its viral diversity. To access phages originating from the marine subsurface, we analysed bacterial isolates obtained from deep subsurface sediments (ODP Leg 201) for the presence of inducible prophages. In particular, a highly abundant population of Rhizobium radiobacter (36 strains) turned out to be commonly infected by prophages. To further address the relevance of lysogeny as viral proliferation mode in marine subsurface environments, the in situ abundance of R. radiobacter and associated temperate rhizobiophages were quantified in the sediments.
We analysed various subsurface sediments for viral and prokaryotic abundance, comprising globally distributed sampling sites of high and extremely low microbial activity. The number of viruses always exceeded that of prokaryotic cells, but generally decreased with sediment depth. However, the virus-to-cell ratio increases in deeper and more oligotrophic sediments, exhibiting values of up to 225 in the deep subsurface of the South Pacific Gyre. The presence of phages in enormous numbers suggests them as a controlling factor for cell abundance and diversity. Furthermore, life of prokaryotic communities might be influenced by viral contribution to the carbon budget in the marine deep biosphere.
Tim Engelhardt is a graduate student at Oldenburg University’s Institute for Chemistry and Biology of the Marine Environment (ICBM). His current research is focused on phage inventory of the deep biosphere by ecophysiological studies on ODP/IODP isolates in order to answer the following questions: to which extent are deep-biosphere populations controlled by viral infections? What is the inter- and intraspecific diversity and host-specific viral biogeography? Can viral infections tell us something about the physiological state of indigenous microorganisms? Finally, Engelhardt seeks to obtain estimates for the viral shunt as an important factor for sustaining the deep biosphere.