Heat stress responses of the kelp Laminaria digitata (Phaeophyceae) across Northeast Atlantic populations: growth, biochemistry, chlorophyll fluorescence, pigments

To understand the thermal plasticity of a coastal foundation species
across its latitudinal distribution, we assess physiological responses to
high temperature stress in the kelp Laminaria digitata in combination with
population genetic characteristics and relate heat resilience to genetic
features and phylogeography. We hypothesize that populations from Arctic
and cold-temperate locations are less heat resilient than populations from
warm distributional edges. Using meristems of natural L. digitata
populations from six locations ranging between Kongsfjorden, Spitsbergen
(79°N), and Quiberon, France (47°N), we performed a common-garden heat
stress experiment applying 15°C to 23°C over eight days. We assessed
growth, photosynthetic quantum yield, carbon and nitrogen storage, and
xanthophyll pigment contents as response traits. Population connectivity
and genetic diversity were analysed with microsatellite markers. Results
from the heat stress experiment suggest that the upper temperature limit
of L. digitata is nearly identical across its distribution range, but
subtle differences in growth and stress responses were revealed for three
populations from the species’ ecological range margins. Two populations at
the species’ warm distribution limit showed higher temperature tolerance
compared to other populations in growth at 19°C and recovery from 21°C
(Quiberon, France), and photosynthetic quantum yield and xanthophyll
pigment responses at 23°C (Helgoland, Germany). In L. digitata from the
northernmost population (Spitsbergen, Norway), quantum yield indicated the
highest heat sensitivity. Microsatellite genotyping revealed all sampled
populations to be genetically distinct, with a strong hierarchical
structure between southern and northern clades. Genetic diversity was
lowest in the isolated population of the North Sea island of Helgoland and
highest in Roscoff in the English Channel. All together, these results
support the hypothesis of moderate local differentiation across L.
digitata’s European distribution, whereas effects are likely too weak to
ameliorate the species’ capacity to withstand ocean warming and marine
heatwaves at the southern range edge.