Thermal Tolerance Varies Latitudinally and Broadly Mirrors Genetic Structure in the Seaweed <i>Phyllospora comosa</i> Across Its Entire Latitudinal Range

Climate-driven warming is causing rapid changes in marine environments, contracting ranges and reshaping ecosystems. Understanding how genetic structure and phenotypic variation interact to determine populations' ability to tolerate warming aids in predicting biogeographic shifts and informs conservation. We tested whether photosynthetic thermal tolerance in the habitat-forming seaweed <i>Phyllospora comosa</i> reflects underlying genetic differentiation across its full latitudinal range in south-eastern Australia. We sampled 15 male and 15 female individuals at replicate sites representing three previously defined genetic groups. We measured critical temperature (<i>T</i> _crit) and maximum quantum yield of PSII (<i>F</i> _V/<i>F</i> _M) as metrics of thermal tolerance using temperature-dependent chlorophyll fluorescence, and assessed relationships with genetic group, sex, latitude and sea surface temperature (SST). We found that thermal tolerance of <i>Phyllospora</i> decreased by ~1°C per degree latitude, with low <i>F</i> _V/<i>F</i> _M in warmer low-latitude waters, indicating thermal stress. Thermal tolerance patterns loosely mirrored genetic groupings: the warm-edge group showed the greatest tolerance, while the cool-edge group was least tolerant. Considerable variation among sites within genetic groups likely reflected both genetic diversity and environmental factors. Males at the warm edge and in central groups tended to show slightly higher tolerance than females, a pattern reversed at cooler latitudes. Although the warm-edge group showed patterns consistent with local thermal conditions and exhibited higher thermal tolerance, it had poor photosynthetic health when sampled mid-summer. Geographic patterns of thermal tolerance in <i>Phyllospora</i> reflect a combination of genetic differentiation and environmentally driven acclimatory responses, with warm-edge populations showing high tolerance but reduced diversity and photophysiological health. Central populations which exhibited higher diversity and versatile tolerance may act as a reservoir for restoration. Combining warm-edge adaptive alleles with central diversity through controlled genetic mixing and outplanting could help future-proof kelp forest restoration under climate change.