In a community, responses to an environmental factor (e.g. ocean acidification) can play out as species composition changes (an ecological process) or changes in genotypic composition (an evolutionary process). Both occur on similar time scales and can thus interact with one another. Marine phytoplankton species as major players in the marine carbon cycle are prime examples for understanding processes that affect community composition in a changing environment. For the first time we present here experiments on single and co-cultures of two different species representing two major phytoplankton groups, the coccolithophore Emiliania huxleyi and the diatom Chaetoceros affinis. In order to understand and disentangle eco-evolutionary dynamics on similar time scales we investigated the growth response of a nine genotype/species population in single and co-culture to increased seawater CO2 concentration. We compared our findings to a simple Lotka-Volterra growth model to test and quantify effects of competition. Due to species-specific different nutrient requirements and responses to CO2 we expected C. affinis to be the stronger competitor in our experiments. This should be pronounced in the high CO2 treatment since alleviation of potential CO2 limitation is favoured by diatom species.
In line with our expectations we could show that (i) ecological and evolutionary sorting varied only slightly between the different CO2 conditions and (ii) that both ecological and evolutionary sorting happened on a similar time scale within about 20 to 40 generations. Furthermore the experimental community after ca. 20 generations was dominated by C. affinis and rapid genotypic selection caused only a few genotypes to remain in the population of each species. The simple Lotka-Volterra model was also constructive to show how intraspecific competition mainly determined genotype sorting while the factor CO2 was of less importance. In contrast to current coexistence theories, intraspecific competition was lower than interspecific competition even though we did have a stable coexistence of the two species. Interestingly, competition was generally higher in the new CO2 environment. This study provides new insight into the processes underlying community response to future environments in a controlled and simple experimental system and highlights the importance of understanding and investigating competition in a system prone to environmental change.