Higher-order interaction inhibits bacterial invasion of a phototroph-predator microbial community

The composition of an ecosystem is thought to be important for determining its resistance to invasion. Studies of natural ecosystems, from plant to microbial communities, have found that more diverse communities are more resistant to invasion. It is thought that more diverse communities resist invasion by more completely consuming the resources necessary for invaders. Here we show that Escherichia coli can successfully invade cultures of the alga Chlamydomonas reinhardtii (phototroph) or the ciliate Tetrahymena thermophila (predator), but cannot invade a community where both are present. The invasion resistance of the algae-ciliate community arises from a higher-order (3-way) interaction that is unrelated to resource consumption. We show that the mechanism of this interaction is the algal inhibition of bacterial aggregation which leaves bacteria vulnerable to ciliate predation. This mechanism requires both the algae and the ciliate to be present and provides an example of invasion resistance through a trait-mediated higher-order interaction. In a separate project we explore how the environment determines evolutionary trajectory when there exists a trade-off between beneficial traits. We select Escherichia coli for faster migration through a porous environment, a process which depends on both motility and growth. Evolving faster migration in rich medium results in slow growth and fast swimming, while evolution in minimal medium results in fast growth and slow swimming. Given that both fast growth and fast swimming would enhance migration rate, this result suggests that there is a trade-off between these two phenotypes and that the direction of evolution depends on the environment