Global warming affects the Arctic much faster than the rest of the globe, with the highest amplitude of temperature rise occurring during the Polar Night. To determine the stress resistance of the ecosystem-engineering kelp Laminaria digitata against Arctic winter warming, non-meristematic discs of adult sporophytes from Porsangerfjorden (Finnmark, Norway) were kept in total darkness at 0°C and 5°C over a period of 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 (area) 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. C:N ratio was analyzed with an Elementar Analyzer. Mannitol concentration was analyzed using a HPLC. 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 ----------------------------Laminaria digitata samples from the Porsangerfjorden, Finnmark, Norway0°C and 5°C, total darkness for 3 monthsw0 = week 0 (start)w4 = week 4w8 = week 8w12 = week 12 (end)set-up:2-L aerated Kautex bottles, 1/40 PES, medium changed 2x/weekSample: 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.