‘Kill the Killer of the Winner’ hypothesis- intraguild predation and its effect of bacterial species richness

Kill the Winner (KW) hypothesis is an important mechanism in microbial food webs. It maintains a high bacterial diversity within the food webs by viruses (the Killer) destroying cells of the most successful bacterial species with the ecosystem (the Winner) prevent one species competitively dominating. It also frees up nutrients allowing it to stay in the microbial loop and diminishes flow from bacteria to higher trophic levels. However, there has been the suggestion of intraguild predation occurring between viruses and protozoa, both indirectly (grazing on infected bacterial cells) and directly (grazing of free-living viral cells). This has been coined ‘Kill the Killer of the Winner’ (KKW). Due to the protozoa grazing on viruses, their number diminishes and therefore lysis of competitively dominant bacterial cells is reduced, causing a reduction bacterial species richness (BSR).

In the investigation, they focused on how KKW may affect the bacterial community composition in an ecosystem. An idealized food web model was used. They also considered whether increasing the latent period of the viruses’ effects intraguild predation, and thereby affect BSR.  It was found that although direct grazing on viruses occurred, indirect grazing was more frequent. Indirect grazing caused a reduction in the BSR, whereas direct grazing was found to have little impact on BSR. The latent period of virus was shown to be a key parameter in determining the intensity of indirect grazing by protozoa on viruses. If the latent period was short the KKW process has little effect. However, as the latent period of the viruses increases, the KKW process has more of an impact. This leads to a reduction in virus abundance and this leads to a reduction of BSR.  The results also showed that if the viruses’ latent period is long, increasing the nutrients supply on in the system will lead to in the short term, an increase in both protozoa and viruses, due to the higher bacterial production. Although, as an increase in protozoa has a negative effect on viruses through intraguild predation, the long term effect of eutrophication events is negative on viral cell abundance and hence decrease the effect of Kill the Winner processes.

Miki, T., Yamamura, N. (2005).’Intraguild predation reduces bacterial species richness and loosens the viral loop in aquatic systems: ’Kill the killer of the winner’ hypothesis’. Aquatic Microbial Ecology, 40(1), 1-12.