What Does It Mean to Be(Come) Arctic? Functional and Genetic Traits of Arctic- and Temperate-Adapted Diatoms

Climate change-induced warming is expected to drive phytoplankton poleward as they track suitable thermal conditions. However, successful establishment in new environments requires adaptation to multiple abiotic factors beyond temperature alone. As little is known about how polar species differ in key functional and genetic traits, simple predictions of poleward movement rely on large assumptions about performance in other relevant dimensions other than thermal responses (e.g., light regime, nutrient uptake). To identify evolutionary bottlenecks of poleward range shifts, we assessed a range of thermal, resource acquisition, and genetic traits for multiple strains of the diatom Thalassiosira rotula from the temperate North Sea, as well as multiple strains of the closely related Arctic Thalassiosira gravida. We found a broader thermal range for the temperate diatoms and a mean optimum temperature of 10.3°C ± 0.8°C and 18.4°C ± 2.4°C for the Arctic and temperate diatoms, respectively, despite similar maximum growth rates. Photoperiod reaction norms had an optimum photoperiod of approximately 17 h for temperate diatoms, whereas the Arctic diatoms exhibited their highest growth performance at a photoperiod of 24 h. Nitrate uptake kinetics showed high intraspecific variation without a habitat-specific signal. The screening for convergent amino acid substitutions (CAAS) of the studied diatom strains and other publicly available transcriptomes revealed 26 candidate genes in which potential habitat-specific genetic adaptation occurred. The identified genes include subunits of the DNA polymerase and multiple transcription factors (zinc-finger proteins). Our findings suggest that the thermal range of the temperate diatom would enable poleward migration, while the extreme polar photoperiods might pose a barrier to the Arctic. Additionally, the identified genetic adaptations are particularly abundant in Arctic diatoms as they may contribute to competitive advantages in polar habitats beyond those detected with our physiological assays, hampering the establishment of temperate diatoms in Arctic habitats.