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.