Role of Viruses Infecting Bacteria in Subsurface Sediments and Groundwater

Microorganisms play a fundamental role driving geochemical cycles. Viruses are the most abundant biological entity on Earth and often exceed cells. While microbiota influence geochemical cycling in the subsurface, the role of subsurface viruses is poorly understood. Viruses were investigated in relationship to carbon biogeochemistry within two aquifers. In the first study, subsurface sediment slurries collected in Alda, NE were amended with 13C-labeled organic carbon (OC) as acetate and nitrate. Biostimulation resulted in viral production concurrent with OC mineralization and nitrate reduction. Change in viral abundance was positively correlated to OC consumption (r 2=0.63) and 13CO2 production ( r2=0.66), whereas change in cell abundance was not, indicating that viruses lyse active cells. Change in viral abundance also correlated to changes in community structure (Gammaproteobacteria and Betaproteobacteria). In the second study, viral production was demonstrated in response to geochemical changes resulting from in-situ biostimulation (O2 injection) of an aquifer in Rifle, CO. Oxygenated groundwater injected into a previously bioreduced zone resulted in a decrease in reduction potential from -146– -132mV to -317– -304mV. Virus abundance increased 1.1x10 6–2.1x106 viruses/mL to 2.3x106–4.6x10 6 while cell abundance did not change. Virus-to-cell ratio increased 1.8-3.4 fold from 3.9-10.1 to 11.0-17.9, demonstrating stimulation of viral production. This supports the findings from the first study which showed that viruses are produced by stimulation of microbial activity. This occurred in conjunction with fluctuations in dissolved organic carbon (DOC) and reduction of U and Fe(III). When injection paused, U was oxidized. But at a higher rate (2.5x), U oxidation occurred, indicating that NRZs maintain a redox buffer which can be overcome when oxidants increase above a tipping point. After mineral precipitation occurred, viruses decreased from 2.3x106-4.6x10 6 viruses/mL to 3.2x105-2.4x106 , suggesting that viruses were removed from solution by adsorption or mineral precipitation. Lastly, the entire floodplain was examined under natural conditions. Viral and cell abundances were correlated (r s=0.73) to each other and to DOC (rs=0.46,0.53; respectively). Thus, viruses play a role in carbon biogeochemistry and indicate microbial activity. Viruses influence subsurface carbon cycling by infecting and lysing cells, liberating OC, thereby influencing the structure and function of microbial communities. Prokaryotes cannot be considered as the sole biological force in the subsurface. Viruses will influence carbon bioavailability and biogeochemical cycling