ALBERT

Description: Ecological and evolutionary processes both determine how species and communities will react to climate change, but how they interact is poorly understood. We studied adaptation in two bloom forming and globally co-occurring phytoplankton species, Emiliania huxleyi and Chaetoceros affinis, in response to long-term exposure to increased CO2 while competing with one another. Here we show that over approximately 200 generations, the interactions of C. affinis and E. huxleyi changed strongly, converting the initial winner C. affinis to the loser species and vice versa. Surprisingly, these changes were associated with rapid and reproducible genotype sorting that left a single remaining genotype among the initial nine genotypes in both species. Most likely, the experimental conditions with a nutrient limited stationary phase represented a selection regime overriding the intended CO2 and two-species treatments. Paradoxically, the reciprocal emergence of a single genotype as winner for both species across all treatments caused a change at the interspecies level, demonstrating that eco-evolutionary coupling alters the dynamics in the simplest possible two-species phytoplankton "community" Our results call for an inclusion of more realistic experimental evolution conditions including multi-species settings and nutrient limitation in future studies.