Evolving smaller body sizes improves the ability to persist when resources are limited, but at a cost

We know that the growth and reproduction of an organism are dependent on both energy acquisition and energy use – net energy flux –  but few studies look at both these simultaneously.  Recent reductions in body size across a range of taxa worldwide, has focused attention on increasing our understanding about the role size plays in determining net energy flux.

Martino Malerba and co-authors Dustin Marshall and Craig White used a technique called artificial selection to genetically evolve small and large populations of a single-celled marine alga Dunaliella tertriolecta that differed in size by 500%. They then assessed some physiological and ecological consequences of this size shift.

The research team found that under low energy conditions (ie low light intensities or short light durations) the smaller cells showed faster growth rates than control and larger cells and conversely under high energy conditions larger cells displayed faster growth rates. Surprisingly though, the smaller cells reached lower total biovolumes overall regardless of the light regime.

Scanning electron micrographs of artificially selected algae. “If the environment allows you to “acquire much”, be big! Otherwise, better to “desire little” and be little.

Other traits such as swimming speed and distance travelled showed the highest performance in the control cells; perhaps because they maintained an optimal ratio of cell size to length of swimming flagella.

These results emphasise that the costs and benefits of different cells sizes depend on the context.  In low resource environments smaller cells will have a greater ability to persist but will be less productive than larger cells, while in high resource environments larger cells will perform better.

This research, published in the highly regarded journal Ecology Letters, will inform the debate on how natural ecosystems will respond to human impacts. Open oceans are the most productive systems in the world and single celled algal species dominate this production. These results show that reductions in cell size as a result of human activities such as fishing and climate change can severely reduce this rate of carbon fixation by as much as 40%.