Intraguild predation reduces bacterial species richness and loosens the viral loop in aquatic systems: ‘kill the killer of the winner’ hypothesis

Abstract

Viruses perform an ecological function (diversity regulation) and a biogeochemical function (viral loop) in microbial food webs. We used an idealized food web model to evaluate the impact of viral loss by protozoan grazing on these 2 functions. Viral loss by protozoan grazing is an example of intraguild predation (IGP); protozoan predators kill viruses, which in turn kill the winner of the nutrient competition among bacteria. The model considers the interactions among uninfected and infected host cells, free-living viruses, protozoa, and nutrients. Calculations showed that this 'kill the killer of the winner' (KKW) process has a negative effect on bacterial species richness (BSR). This is because the KKW process reduces the virus to bacteria ratio, which undermines the 'kill the winner' (KW) process. In particular, the intensity of the KKW process is influenced by the latent period of viruses; a longer latent period leads to a reduction in BSR by causing the viruses in host cells to be more at risk of being killed by grazing. In addition, under high IGP with a long latent period, eutrophication leads to a decrease in the virus to protozoa ratio, resulting in a lower BSR and a smaller contribution of the virus loop to nutrient cycling, whereas the changes are minimal under low IGP with a short latent period.