User:Zeen Aleaf/Kill the Winner hypothesis

The competition specialist, or “winner”, is often described as having the most biomass, but this is somewhat of a misunderstanding. More appropriately, the "winner" is the fastest growing. Their abundance and activity increase when the population competes for a shared limiting resource (e.g. phosphate) and wins. The resource can exist as a free form or something that needs to be sequestered from biomass. Competition specialists (predators, grazers, parasites) are expected to dominate in oligotrophic environments, where competition is largely an ecological constraint. The increased abundance and activity of the “winner” also increases viral predation.

Defense specialists, tend to invest resources in avoidance strategies that may result in reduced growth and reproduction of the population; hence, the “loser” does not increase viral predation. Defense specialists are expected to dominate in eutrophic environments where competition is less of a selective pressure.

Thus, when competition specialists are found in what, at least theoretically, appear to be uncharacteristically low abundances in oligotrophic environments, the principles of KtW apply.

Competition and Defense Specialists

In the context of the KtW model, competition specialists are "winners". Theoretically, these fast-growing competition specialists can use up entire pools of resources if they go uncontrolled. The mechanism behind KtW describes how these populations of competition specialists are controlled by predation from viral phages, granting more slowly-growing defense specialists access to resources. In other words, these viral phages kill the "winner", and it is through this system that competition specialists and defense specialists are able to co-exist through resource limitations.

Roseobacter strain belonging to family Rhodobacteraceae

Rhodobacteraceae are a family of bacteria that possess many of the aforementioned qualities of competition specialists. They are fast-growing and tend to exist at low abundances in oligotrophic environments because they are primary targets of viral phages and grazers. These population dynamics support the principles of KtW through which these organisms act as "winners" and are targeted for it.

Pelagibacter, a SAR11 bacterium.

Pelagibacterales, or the SAR11 clade, include bacteria that dominate the world's oceans. These organisms are slow-growing. Their ability, thus, to maintain such high abundances in the marine environment suggests they are less vulnerable to viral predation. Accordingly, SAR11 is categorized as a defense-specialist.

Respiration Rate and Size

The differences in metabolic activity between competition and defense specialists can partially be explained by size. 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.

Smaller cells sizes are generally less metabolically active and more slow-growing. The way such size differences contribute to bacterial growth rate have been the subject of numerous debates arguing whether or not the bulk of bacterial communities in marine ecosystems are metabolically active and thus "alive". Arguments that most bacteria in the ocean are "dead" come from observations that few bacteria are nucleoid-containing, meaning most are inactive due to the absence of DNA. Others argue that most bacteria in the ocean are not "dead", but rather sleeping, as cells without nucleoids need not stay that way, and the amounts of DNA these cells possess may simply be hard to trace.

In 2022, contemporary research suggests that staining methods may not be sensitive enough to measure the low respiration rates of the very abundant, but very slow-growing, smaller bacterial cells such as those from the SAR11 clade, giving the impression that only a small fraction of bacteria are "alive". With respect to KtW, the controlled abundance of fast-growing competition specialists by viruses thus has an effect on interpretations of the metabolic activity of bacterial communities at large.

Viruses

Marine viruses have different lifestyles which dictate how they interact with their hosts. When lytic viruses infect their hosts, they cause lysis or the bursting of cells; alternatively, lysogenic viruses are able to integrate into the host genome. Different environmental conditions such as light, temperature, and nutrients may favour one virus lifestyle over the other. In general, there tends to be less infections by lysogenic viruses in more productive waters where viruses can easily reproduce.

A myovirus which infects cyanobacterium Prochlorococcus.

Predictions under KtW suggest that competition specialists and defense specialists are infected by viruses with different lifestyles. Fast-growing competition specialists are more likely to be infected by lytic viruses. Following infection, these viruses replicate quickly, rapidly killing their hosts. It is in this way that populations of competition specialists are so tightly controlled by viral predation.

Slow-growing defense specialists are more likely to be infected by lysogenic viruses. Rather than quickly replicating and killing their hosts, these viruses assimilate into the host cells and can remain there for some time. As a result, populations of defense specialists remain stable amidst the threat of viral predation.

Observations of KtW in the World

Primarily, KtW is observed in aquatic ecosystems. In these environments, the population dynamics of bacteria and their host-specific viruses reveal that these systems are indeed coupled. For example, data from Lake Constance in Germany shows that spikes in bacterial abundance generally lead to spikes in viral infection. Additionally, mortality caused by viruses makes up a significant portion of total mortality in bacterial communities, reinforcing how tightly viruses control bacterial abundance.

In 2023, ribosomal sequencing seeking to quantify taxon-specific viral lysis revealed that abundant slow-growing taxa could remain abundant because they were subject to low viral lysis. Contrastingly, fast-growing taxa were rare as a result of high viral lysis. These observations support the principles of KtW in coastal seawater.

Interestingly, the principles of KtW are seldom observed outside of water. The microbial community of the human gut, for example, does not appear to follow KtW as these bacteria are less susceptible to virus-caused mortality. It is suspected that the gut protects bacteria from infection by providing physical barriers as this environment is only partially liquid.