Lytic to temperate switching of viral communities

An analysis of 24 coral reef viromes challenges the view that lytic phage are believed to predominate when the density of their hosts increase and shows instead that lysogeny is more important at high host densities; the authors also show that this model is consistent with predator–prey dynamics in a range of other ecosystems, such as animal-associated, sediment and soil systems. Microbial viruses can control host abundances via density-dependent lytic predator–prey dynamics. Less clear is how temperate viruses, which coexist and replicate with their host, influence microbial communities. Here we show that virus-like particles are relatively less abundant at high host densities. This suggests suppressed lysis where established models predict lytic dynamics are favoured. Meta-analysis of published viral and microbial densities showed that this trend was widespread in diverse ecosystems ranging from soil to freshwater to human lungs. Experimental manipulations showed viral densities more consistent with temperate than lytic life cycles at increasing microbial abundance. An analysis of 24 coral reef viromes showed a relative increase in the abundance of hallmark genes encoded by temperate viruses with increased microbial abundance. Based on these four lines of evidence, we propose the Piggyback-the-Winner model wherein temperate dynamics become increasingly important in ecosystems with high microbial densities; thus ‘more microbes, fewer viruses’. Lytic phage can control the abundance of their microbial hosts in a density-dependent manner with 'kill-the-winner' predation dynamics. It was widely assumed that lytic phages would dominate in nutrient-rich conditions favouring high host density, and that lysogenic phage, which integrate into their hosts instead of lysing them, tend to dominate when host numbers are low. This meta-analysis of 24 coral reef viromes challenges that view. Ben Knowles et al. find that lysogeny is more important than lysis at high, rather than low host densities. The authors term this the 'Piggyback-the-Winner' model, and show that it is consistent with predator–prey dynamics in a range of other ecosystems, including animal-associated, sediment, and soil systems.