Seasonal resource oscillations maintain diversity in bacterial microcosms

Hypothesis: Seasonal variation in availability of resources maintains co-existence between different ecological types due to frequency-dependent interactions. Organism: Escherichia coli. Methods: When populations containing two ecotypes of E. coli that evolved from a common ancestor under sympatric conditions grow in batch culture on a mixture of glucose and acetate, they first use up the available glucose and then switch to acetate consumption, thus creating a fluctuating environment with two different 'seasons'. We removed the alternation between the seasons so that polymorphic populations only experienced the first (glucose) season or the second (acetate) season and monitored the frequency of the ecotypes. We subsequently removed the stationary phase of the batch culture to determine its contribution to competitive interactions between the two ecotypes. (During the stationary phase, a population stops growing because it has depleted its resources and produced too high a level of toxins.) Results: We show that for two stable, heritable ecotypes of the bacterium E. coli co-existing on two seasonally available resources, the removal of either resource season erodes the stable co-existence equilibrium and results in dominance by one or the other ecotype, depending on which season is removed. We observed similar shifts from co-existence to dominance in three evolutionarily independent populations. In two of these, the stationary phase had no effect on competition between the types, but in the third population, stationary phase favoured one type. Because resources are depleted in each batch, in each season the growth rate of one ecotype depends on the growth rate of the other ecotype. Our results thus strongly support the claim that co-existence between the two ecotypes is maintained by frequency-dependent competition in a seasonal environment.