We conducted incubation experiments with the diatom Thalassiosira hyalina under present-day and future temperature and pCO2 treatments. Six fresh isolates from the same Svalbard population were incubated as mono- and multi-strain cultures. We were able to closely follow intraspecific selection within an artificial population in a 2-week experiment using microsatellites and allele-specific quantitative PCR. Our results show that there is substantial variation in how strains of the same species cope physiologically with the tested environments. Although highly reproducible within treatments, changes in genotype composition, production rates and cellular quotas in the multi-strain cultures differed from monoculture performance. Interestingly, we only detected significant strain sorting in those populations exposed to the future treatment. We show that individuals adjust their phenotype not only in response to their physico-chemical, but also to their biological surroundings.

Supplement to: Wolf, Klara K E; Hoppe, Clara Jule Marie; Rost, Björn; John, Uwe; Collins, Sinéad; Romanelli, Elisa; Weigand, Hannah (2019): Company matters: The presence of other genotypes alters traits and intraspecific selection in an Arctic diatom under climate change. Global Change Biology, 25(9), 2869-2884