Kill the Winner hypothesis
The "Kill the Winner" hypothesis (KtW) is an ecological model of population growth involving prokaryotes, viruses and protozoans that links trophic interactions to biogeochemistry.
It assumes that prokaryotes adopt one of two strategies when competing for limited resources: priority is either given to population growth ("winners") or survival ("defenders").
[1] As "winners" become more abundant and active in their environment, their contact with host-specific viruses (also known as phages) increases, making them more susceptible to viral infection and lysis.
Some investigations into virus-bacteria interactions in laboratory settings have suggested viruses play a major role in maintaining microbial diversity and provided more evidence in support of KtW.
[1][2] The KtW model represents an idealized microbial food web with mathematical parameters that only account for viral predation studied in vitro.
[1][3] Because it assumes environmental conditions are stable, it can only predict population dynamics over a small time frame relative to a microbial community's history.
[7] In a well-mixed pelagic environment, with conditions being roughly constant, prior biological theories (e.g. the competitive exclusion principle) suggested one species should eventually dominate.
[13] Selective predation, symbiotic interactions, and variations in environmental conditions over space and time were initially proposed as solutions to the paradox.
[7] Early modelling of viral infections in bacterial populations assumed a predator-prey relationship between viruses and bacteria following the Lotka-Volterra equations.
Thingstad later found that bacteria with varying growth rates could coexist stably, with faster-growing bacterial species maintaining a higher abundance of viruses.
[16] With respect to KtW, the moderating effect of viruses on fast-growing competition specialists impacts interpretations of the metabolic activity of bacterial communities at large.
For example, studies of respiration rate among the marine microbe communities of the Gulf of Maine show that Rhodobacteraceae are typically larger than members of the SAR11 clade.
[17] In 2022, researchers suggested that staining methods may not be sensitive enough to measure the low respiration rates of very abundant, but slow-growing and small bacteria, such as those from the SAR11 clade.
[17] While the respiration rates of larger competition specialists are easier to detect, these taxa tend to be rare as per the conditions of KtW, giving the same impression.
Size effects and KtW make it difficult to quantify the number of aquatic bacteria that are actively growing in the world's oceans.
Unlike KtW, the Piggyback-the-Winner (PtW) hypothesis makes a key distinction between lysogenic and lytic viruses and considers viral life cycles.
[11] Introducing stochasticity into the KtW model resulted in successive viral and prokaryotic extinctions, showing that species coexistence could not be maintained.
