Sample collection
The experiment was conducted with individuals from two native and two non-native populations. These populations were also visited as part of the global field survey conducted in Bonthond et al. (2020) and included Futatsuiwa (Japan; collected on September 14th 2016), Akkeshi (Japan; September 15th 2016), Nordstrand (Germany; September 20th 2016) and Kiel (Germany; September 21th 2016, see Table S1 for details). Individuals were sampled with gloves and with at least a meter distance in between (Krueger‐Hadfield et al., 2017), stored in separate plastic bags and transported to the lab in coolers. Apart from Akkeshi, where single algae were too small to obtain enough tissue for all field and experiment samplings, 12 individuals were collected from each site. In Akkeshi, pseudo-individuals were created by pooling a number of individuals from within 20 cm2 into a single bag and from this point, these algae were treated in the same manner as the separate individuals from other populations. Within 12 hours of collection, the first sampling of epiphytic communities was conducted. This was done in the same manner as in Bonthond et al. (2020). In brief, epiphytic communities were sampled by taking an apical fragment of approximately 1 g from each alga with sterilized forceps. The sample was transferred to 50 mL tubes containing 15±1 glass beads (4 mm) and 7.5 mL autoclaved seawater made from standard sea salt and distilled water using the salinities measured at the collection site. After vigorous vortexing for 3 min the supernatant was filtered through 0.2 µm PCTA filters and the procedure was repeated one time, using the same algal fragment. The filters were stored in 2 mL tubes at -20 °C immediately and those taken in Japan were transported on dry ice to Germany. The remaining thallus was used for the experiment and thalli collected in Japan were stored at 4 °C until they were transported in separate plastic bags to Germany. The transportation time (from refrigerator to aquaria) was less than 24 hours.