Phytoplankton is estimated to account for 50 % of oxygen production on the planet. It drives the marine food web and major marine biochemical cycles, acting as an important carbon sink. The Arctic is affected by global warming four times faster than the average. This includes changes in sea ice cover and the influx of warm, nutrient rich water from the Atlantic Ocean, a process called Atlantification.

In light of this, we aim to determine which mechanisms allow arctic algae, adapted to cold temperatures and long dark winter periods, to persevere in their environment in comparison to incoming temperate ecotypes. By understanding whether they use similar adaptations to persist, we will be able to better understand their ability to contribute to ecosystem reassembly in the warming Arctic.

The experimental setup included six arctic and five temperate microalgae belonging to four phyla. To understand whether temperate ecotypes can replace their arctic counterparts, we tested for differences in their biotic interactions with one another. The algae were grown in single batch culture or in co-culture with the other species at 1°C and a 16:8 h light:dark cycle. To test for an effect of polar daylengths, we recorded growth rates of the single strains at 1°C with a 2:22 h, 16:8 h or 24:0 h light:dark cycle. To survey adaptive responses to changing light and temperature conditions, we used long-read isoform sequencing to obtain extensive transcriptomic profiles for temperate and arctic phytoplankton species.

First results show, that not all tested temperate strains are able to survive at 1°C and a 2:22 h light:dark cycle expected for winter and early spring in the arctic. This indicates that less robust algae need to be re-introduced to the arctic by the currents. Temperate algae that are able to persist at low light and temperature conditions grow at reduced rates, which are comparable to the growth rates of their arctic counterparts. This applies to both single and co-cultures, thus, showing their ability to compete in the arctic system.

This dataset serves as a foundation for further transcriptomic exploration of an arctic winter/summer scenario. Leveraging the data, we will search for common patterns of adaptation and metabolic plasticity among arctic survivors versus less robust taxa. The resulting patterns will then be compared to field-transcriptomes, to determine whether they can be detected consistently in the environment.