10.1: Microbial interactions

Interactions appear to be a fundamental characteristic of life and can take many forms (Moënne-Loccoz et al., 2014). Organisms can interact without any positive or negative response to those interactions. This type of interaction is known as neutralism (Fig. $10.1$). However, most types of interactions do involve a positive or negative response. The paragraphs below consider some interactions defined by conflict, in which at least one of the microorganisms involved is negatively affected by the interactions. Also considered are beneficial interactions, in which neither microorganism is negatively affected and at least one has a positive response to the interaction. As you read about each interaction, it is important to note that interactions between organisms are not necessarily fixed but can change with circumstances, including the conditions of the environment and the development of the organisms.

Figure $10.1$: Types of interactions between two hypothetical species (1 and 2). This figure is a modified version of figure 1 in Johnson et al. (1997). Along each axis, species responses are characterized as positive (+), negative (-), or neutral (0).
https://commons.wikimedia.org/wiki/File:Biological_interactions.png

Predation is characterized as a short-term interaction in which one organism, the predator, kills and eats another organism, the prey. Microbial predators impact microbial communities and their ecosystems in diverse ways, including by helping turn over resources, influencing the abundance and diversity of prey species, and helping induce the evolution of community traits (Nair et al., 2019; Velicer and Mendes-Soares, 2009).

Parasitism occurs when one organism, the parasite, lives on or in another organism, the host, and causes it harm (e.g., feeding on the host). Viruses can be parasites or predators (Chen and Williams, 2012). The thing that differs between the two interactions is the time scale of interaction. Parasites feed off of the host for extended periods of time whereas predators rapidly kill their prey (Chen and Williams, 2012). Viral parasitism has similar impacts to the ecosystem and community as predation (Chen and Williams, 2012; Fernandez et al., 2018).

Competition occurs when multiple organisms require the same resource and the use of that resource lowers growth rates of one or both populations involved (Fredrickson and Stephanopoulos, 1981). Competition can be considered as a natural selection towards the most efficient microbial strategies (González-Cabaleiro et al., 2015). Scramble competition occurs when competitors take up the limiting resource as quickly as they can without directly interacting, whereas contest competition (i.e., interference competition) involves direct antagonistic interactions between competitors, with the winner getting the rewards (Hibbing et al., 2010). For example, a species may restrict their competitor’s access to a resource or excrete substances that inhibit their competitor (Hibbing et al., 2010). Competition can ultimately result in the loss of a species or functional group from a habitat, a phenomenon known as competitive exclusion (Hibbing et al., 2010). However, competitors often coexist (Fredrickson and Stephanopoulos, 1981).

Mutualism occurs when two different organisms interact and both benefit from the interaction (Orphan, 2009). Syntrophy is a form of mutualism involving nutrition in which one microorganism consumes the metabolic products of another (Morris et al., 2013). In a syntrophic interaction, one microorganism benefits by obtaining energy resources whereas the other benefits from limited product accumulation. Kehe et al. (2021) note that positive interactions have often been described as rare but may be much more common within microbial communities than previously recognized. Some members of the community play key roles and if they are lost, the function of the community can be significantly altered (Garcia et al., 2015). Thus, positive interactions have the potential to shape community composition as well as function.

Commensalism is similar to mutualism except one organism benefits while the other is unaffected. For example, the activity of one microorganism may alter the composition of an environment in such a way that growth of another microorganism becomes more favorable. In this case, the microorganism altering the environment has no clear gain from the interaction, but the other microorganism gains a better habitat.

In contrast to commensalism, amensalism is an interaction that negatively affects one of the organisms involved while the other organism is unaffected (Moënne-Loccoz et al., 2014). Thus, we can think of amensalism as the opposite of commensalism. Microorganisms interacting through amensalism are not competing with one another for a resource. Instead, the negative affect of one microorganism on the other occurs because one of the microbes alters the chemical and/or physical properties in the environment in such a way that it inhibits the ability of the other microorganism to grow. Often, this negative affect occurs through the release of toxic compounds (Moënne-Loccoz et al., 2014).