Due to the global rise in temperature, recent studies predict species shifts towards higher latitudes. The temperate kelp Laminaria hyperborea (Phaeophyceae) is an abundant European bioengineering species that is widely distributed between northern Portugal and northern Norway, however not yet found in the High Arctic. To investigate its ability of the species to invade the high latitudes under past, present and future temperature scenarios, we conducted a long-term multi-factorial experiment using tissue of adult sporophytes of L. hyperborea collected from Porsangerfjorden, Norway. The samples were exposed to three different photoperiods (PolarNight, PolarDay, LongDay) at 0, 5 and 10°C for three months. The maximum photosynthetic quantum yield of photosystem II (Fv/Fm; Imaging-PAM, Walz GmbH Mess- und Regeltechnik, Effeltrich, Germany) was monitored once a week. For monitoring of potential growth, the size of the algal discs was photographed every two weeks and analyzed with ImageJ (Version 1.52a). Every four weeks, subsamples for monitoring the dry weight and for the biochemical analyses were taken. Laminarin content was determined following via enzymatic digestion. Mannitol concentration was analyzed using a HPLC. Phlorotannins were analyzed following the photometric Folin-Ciocalteu method. Absolut pigment concentrations were also analyzed using a HPLC and pool sizes, the de-epoxidation state of the xanthophyll cycle (DPS), and the ratios calculated afterwards.

Treatment details – multi-factorial experiment--------------------------------------------------------------------------------------------------Laminaria hyperborea samples from the Porsangerfjorden, Finmark, NorwayPolarDay: 24:0 light:dark-cycle; 30–35 μmol photons m-2 s-1 (PAR)LongDay: 18:6 light:dark-cycle; 30–35 μmol photons m-2 s-1 (PAR)PolarNight: 0:24 light:dark-cycle; total darknessSample: These values are used to differentiate multiple similar samples within the same replicate. They do not denote a relationship between weeks. E.g. Sample #3 from week 0 (w0) of Replicate 2, is not necessarily a sample of the same organism for Sample #3 from week 1 (w1) of Replicate 2. Meaning that Samples should not be taken as time series of measures of the same organism through time.w0 = week 0 (start)w4 = week 4w8 = week 8w12 = week 12 (end)set-up:2-L aerated Kautex bottles, 1/40 PES, medium changed 2x/week