VIRAL INFLUENCE ON TERRESTRIAL SUBSURFACE CARBON BIOGEOCHEMICAL CYCLING

Microorganisms play a fundamental role driving geochemical cycles throughout Earth history. Viruses are the most abundant biological entity on Earth and often exceed cell abundance. While microbiota significantly influence geochemical cycling, viral infection of metabolically active microorganisms is poorly understood, especially in terrestrial subsurface systems. Here, we investigated the production of viruses in relationship to carbon biogeochemistry in two shallow aquifers. Subsurface sediment slurries collected in NE were amended with ¹³C labeled organic carbon (OC) as acetate and nitrate. Biostimulation resulted in viral production concurrent with acetate oxidation, ¹³CO₂ production, and nitrate reduction. Interestingly, change in viral abundance was positively correlated to OC consumption (r²=0.63) and ¹³CO₂ production (r²=0.66) whereas change in cell abundance was not correlated to carbon consumption or ¹³CO₂ production. This result indicates that viruses are lysing metabolically active cells. Change in viral abundance was also correlated to changes in microbial community structure, specifically Gammaproteobacteria and Betaproteobacteria. These results are not necessarily artifacts of bottle experiments. During an in situ biostimulation experiment (O₂ injection) in a shallow aquifer near Rifle, CO, virus to cell ratio increased (from 6.4±3.3 to 15.5±4.0) in association with variation in groundwater redox chemistry and OC. Within unstimulated groundwater, viral abundance is also correlated to dissolved OC (r²=0.30). This further supports our bottle experiments that viruses are produced in response to OC or by stimulation of microbial activity. Together these results demonstrate that viruses play a role in carbon biogeochemistry and can serve as an indicator of microbial metabolic activity. While viruses are incapable of carbon metabolism, viruses influence subsurface carbon cycling by infecting and lysing host cells. This results in release of OC bound in biomass, subsequently influencing the structure and function of microbial communities. Thus, prokaryotes cannot be considered as the sole biological force in the subsurface because viral infection will influence carbon bioavailability and biogeochemical cycling.