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Assessing thermal tolerance of marine organisms is a prerequisite to predict their responses to upcoming warming scenarios, but thermal sensitivities broadly vary among life stages and seasons due to thermal acclimation. This study aimed to identify changes in photosynthetic performance of recruits of the intertidal canopy-forming seaweed  Fucus guiryi  in relation to seasonality during early ontogeny. Zygotes were isolated from late summer (LS) and late winter (LW) thalli and were exposed to an ecologically relevant thermal range (15, 20, 23 and 25°C) under laboratory-controlled conditions for 3 weeks. Photosynthetic responses were assessed by  in vivo  chlorophyll  a  fluorescence measurement. Seasonality had a significant influence on initial zygote morphology, with a decrease in area of 17% from LS to LW. Zygotes from LS and LW exhibited the typical photosynthetic responses of sun and shade plants, respectively, with a greater capacity to dissipate heat via non-photochemical quenching in LW. Late winter embryos showed a wider optimal thermal range for photosynthesis (15–23°C) than late summer ones (20–23°C). Embryos attained maximum photosynthetic responses 1 week after fertilization when they developed the apical meristem. Seasonal differences were more evident during early ontogenesis (0–14 d) than once their photosynthetic apparatus become like that of adult thalli. This study evidenced that zygote traits can vary across seasons, but zygotes are able to acclimate their photosynthetic performance up to 25°C within 3 weeks. Recruits of  F. guiryi  possessed a wide photosynthetic plasticity and are probably able to persist in populations close to their thermal limits if thermal thresholds are not exceeded.

Seasonal and ontogenetic variability in the photosynthetic thermal tolerance of early life-stages of Fucus guiryi (Phaeophyceae, Fucales)