Many of us are familiar with the idea that a bit of healthy competition can improve performance but can it affect metabolic rates, size and growth rates? Giulia Ghedini and Dustin Marshall asked this question for a single-celled alga and found that competition selects for smaller and more energy efficient cells.

Giulia and Dustin wanted to know whether predictions from the Metabolic theory of Ecology would hold true. Metabolic theory looks at the relationship between size and metabolic rate (rate of energy use) of individuals and makes predictions about population growth rates, maximum population size and maximum population biomass.

To test theoretical predictions Giulia and Dustin evolved populations of the green alga Dunaliella tertiolecta for 70 generations in three environments. They grew Dunaliella either on its own (no competition), with more Dunaliella(intraspecific competition) or with three other species of algae (interspecific competition). The focal populations of Dunaliella were inside dialysis bags so that they were physically isolated but still experienced changes to nutrient and light availability brought about by competition with other algae present outside the bags.

At 35 and 70 generations subsamples of the focal populations were moved into the same environment for several generations – a ‘common garden’. This allows researchers to distinguish between short term or ‘plastic’ responses to an environment from persistent ‘evolved’ changes.

Giulia and Dustin found that after 35 generations of evolution, cells that had evolved in the presence of competitors were smaller, reached greater population densities but got there more slowly (population growth rates were slower) than cells evolved without competitors. But after 70 generations, cells evolved in competitive environments had the same rates of population growth as cells evolved on their own and yet still managed to reach the same high maximum population densities.

The evolution of greater metabolic flexibility appeared to be the key to enable cells grown in a competitive environment to reach these high population growth rates and densities. By measuring photosynthesis and respiration at two time points, (when cells were low in number versus when cell numbers were very high), Giulia and Dustin could see that when resources are abundant, competition-evolved cells increase metabolic rates more than cells evolved without competition. And the reverse was true; when resources are scarce, competition-evolved cells are able to downregulate energy-capture and use, better than cells evolved without competition.

The evolution of enhanced metabolic flexibility was not anticipated by any theory and Giulia and Dustin are keen to see further studies testing if competition drives metabolic plasticity in other systems.

But the Metabolic Theory of Ecology did predict most of the other changes that the pair saw and so they likely represent a common response to competition. In other words, theory predicts the evolution of more energy efficient cells and it is the original relationship between size and metabolic rate that dictates whether those cells will be bigger or smaller. See also: Travelling in time: an experimental evolution experiment changes what we thought we know about size and the cost of reproduction.

This research is published in Current Biology.