Laminaria digitata gametophyte survival and sporophyte formation during exposure to increased temperatures

To assess the thermal adaptation of microscopic stages of the kelp Laminaria digitata along latitudes, we conducted laboratory experiments on samples from six locations in the NE Atlantic (Spitsbergen (SPT), Tromsø (TRM), Bodø (BOD; all Norway), Helgoland (HLG; Germany), Roscoff (ROS) and Quiberon (QUI; both France)), spanning the species' entire distribution range. Gametophyte stock cultures from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research were used. Prior to the experiments, cultures were stored at 15°C in iron-free ½ Provasoli enriched seawater in 3-4 µmol photons/m²/s red light. In experiment 1, we exposed gametophytes to (sub-) lethal high priming temperatures (20-25°C) for two weeks, followed by two weeks of recovery at 15°C, to observe gametophyte survival and sporophyte formation. During the experiments, samples were kept in 15 µmol photons/m²/s white light under a 16:8h light:dark cycle.