Temperature‐driven dynamics of epiphytic bacterial communities in <scp> <i>Caulerpa prolifera</i> </scp> and <scp> <i>Caulerpa taxifolia</i> </scp> (Chlorophyta; Ulvophyceae): Implications for algae adaptation

Summary This study examines the response of the Mediterranean native macroalgae Caulerpa prolifera and of its allochthonous invasive congeneric C. taxifolia to temperature and explores the structure of their associated epiphytic bacterial community, aiming to understand the role of bacteria on algal thermal stress tolerance and adaptability. Two experiments were performed in controlled conditions. In the first, the algae were exposed to three different temperatures (20, 24, and 28°C), and the main morpho‐functional traits, the abundance, and the composition of their bacterial communities were assessed. In the second, algae exposed to their intermediate and most stressful temperature were treated with antibiotics. Once again, the main morpho‐functional traits were determined for the different treatments. The results revealed opposite responses of C. prolifera (CP) and C. taxifolia (CT) to temperature. At 28 and 20°C, respectively, for C. prolifera and C. taxifolia , higher mortality rates (CP: 33% and CT: 18%) and a reduction of all the traits were observed. The health condition of both species worsened when antibiotics were used, especially in stressful conditions, where the highest mortality rates occurred (CP: 75% and CT: 81%). Moreover, the epiphytic bacterial communities of both species significantly changed in response to temperature without antibiotics, showing greater bacterial abundance at 28 and 20°C (CP28: 25 × 10 6 cells/cm; CT20: 30 × 10 6 cells/cm). Under suboptimal temperatures, algae were associated with the genera Suttonella and Bacillus, known for their role in enhancing algal tolerance. Overall, these findings prove the role of the epiphytic microbiota in supporting the temperature adaptability of these two seaweeds, suggesting its contribution to the invasiveness of C. taxifolia in the Mediterranean.