Local adaptation of microscopic stages of North Atlantic Laminaria digitata to high and low temperatures along latitudes: survival, reproduction and growth

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. 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. In experiment 2, samples were subjected to (sub-) optimal low temperatures (0-15°C) for 21 days, to assess gametophyte survival, sporophyte formation and growth. During the experiments, samples were kept in 15 µmol photons/m²/s white light under a 16:8h light:dark cycle. 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.