Microorganisms engage in a wide variety of social interactions, including cooperation. A cooperative behavior is one that benefits an individual (the recipient) other than the one performing the behavior (the actor). This article outlines the various forms of cooperative interactions (mutualism and altruism) seen in microbial systems, as well as the benefits that might have driven the evolution of these complex behaviors.
Introduction
Microorganisms, or microbes, span all three domains of life – bacteria, archaea, and many unicellular eukaryotes including some fungi and protists. Typically defined as unicellular life forms that can only be observed with a microscope, microorganisms were the first cellular life forms, and were critical for creating the conditions for the evolution of more complex multicellular forms.
Although microbes are too small to see with the naked eye, they represent the overwhelming majority of biological diversity, and thus serve as an excellent system to study evolutionary questions. One such topic that scientists have examined in microbes is the evolution of social behaviors, including cooperation. A cooperative interaction benefits a recipient, and is selected for on that basis. In microbial systems, cells belonging to the same taxa have been documented partaking in cooperative interactions to perform a wide range of complex multicellular behaviors such as dispersal, foraging, construction of biofilms, reproduction, chemical warfare, and signaling. This article will outline the various forms of cooperative interactions seen in microbial systems, as well as the benefits that might have driven the evolution of these complex behaviors.
History
From an evolutionary point of view, a behavior is social if it has fitness consequences for both the individual that performs that behavior (the actor) and another individual (the recipient). Hamilton first categorized social behaviors according to whether the consequences they entail for the actor and recipient are beneficial (increase direct fitness) or costly (decrease direct fitness). Based on Hamilton's definition, there are four unique types of social interactions: mutualism (+/+), selfishness (+/−), altruism (−/+), and spite (−/−) (Table 1). Mutualism and altruism are considered cooperative interactions because they are beneficial to the recipient, and will be the focus of this article.
Explaining cooperation remains one of the greatest challenges for evolutionary biology, regardless of whether the behavior is considered mutually beneficial or altruistic. According to classical evolutionary theory, an organism will only behave in ways that maximize its own fitness. Therefore, the origin of cooperative interactions, or actions by individuals that result in other individuals receiving fitness benefits, seems counterintuitive.
Theoretical explanations for the evolution of cooperation can be broadly classified into two categories: direct fitness benefits or indirect fitness benefits. This follows from Hamilton's 1964 insight that individuals gain inclusive fitness directly through their impact on their own reproduction (direct fitness effects), as well as through their impact on the reproduction of individuals with related genes (indirect fitness effects).
Types of cooperation
Mutualism
Perhaps the most common cooperative interactions seen in microbial systems are mutually beneficial. Mutually beneficial social interactions provide a direct fitness benefit to both individuals involved, while outweighing any cost of performing the behaviour. In an environment with individual microbes, mutualism is most often performed in order to increase individual fitness benefit. However, in a community, microorganisms will interact on a large scale to allow for the persistence of the population, which will thereby increase their own fitness.
The majority of the time, organisms partaking in these behaviours have a shared interest in cooperation. In microbial systems, this is often seen in the production of metabolically expensive molecules, known as public goods. Many microbes, especially bacteria, produce numerous public goods that are released into the extracellular environment. The diffusion that occurs allows for them to be used by neighbouring organisms, despite being produced for the individual.