Influence of ambient temperature on the photosynthetic activity and phenolic content of the intertidal Cystoseira compressa along the Italian coastline

Understanding the physiological responses of intertidal seaweeds to environmental factors is fundamental to characterize their local physiological adaptation and success in the face of climate change. We measured the photosynthetic activity and the total phenolic content of the intertidal alga Cystoseira compressa and explored their relationship with latitude or local ambient air and seawater temperatures. Our results show that, when submerged, the photosynthetic activity of C. compressa showed values typical for non-stressed thalli, and the seawater temperatures found across sites explained the variability of these values. We observed a decrease in the photosynthetic activity of C. compressa when exposed to air, compared to a submerged condition. This activity remained stationary up to 28 °C and then started to decrease with higher air temperatures. The total phenolic content of C. compressa at the end of low tide changed across the study sites from 0.12 to 0.53 % DW. Phenolic variability was explained by the long-term thermal water conditions experienced by the algae, rather than short-term variations encountered during tidal cycles. Overall, our results suggest a crucial role played by temperature in driving the physiological traits of the intertidal C. compressa.