Narrow thermal range restricts fertilization and early growth in the habitat-forming seaweed Durvillaea potatorum (Phaeophyceae)-Implications for aquaculture and climate resilience.

Abstract

The southern bull kelp Durvillaea potatorum is a key habitat-forming macroalga in southeastern Australia and has been identified as a species of interest for sustainable seaweed aquaculture. However, the species is threatened by rising ocean temperatures and other anthropogenic factors. Assessing the thermal limits across different life stages of D. potatorum is therefore crucial for understanding its response to warming and optimizing future aquaculture practices. Using a full two-factorial design, we tested the effects of a wide range of temperatures (~3.5-30.0°C) and two light regimes (zero/low light: 0/~40, and high light: ~120 μmol photons · m^-2 · s^-1) on the reproductive and early life stages of D. potatorum from Eaglehawk Neck, Tasmania. Gamete release, fertilization, and early growth after 15 min, 24 h, and 7 days, respectively, were assessed. Thermal performance curves revealed that fertilization was the most thermally sensitive stage, exhibiting the lowest thermal optimum (T_opt = ~12.85°C), and narrowest thermal breadth (T_br = ~2.5°C) independent of light conditions. Temperature and light had little to no effects on egg and sperm release, whereas early germling growth exhibited thermal optima of 16.33 and 14.50°C under low and high light treatments, respectively. These results indicate that fertilization conditions need to be closely monitored during the hatchery phase of aquaculture. In addition, natural D. potatorum populations may become increasingly susceptible to ocean warming due to impaired fertilization, potentially leading to demographic shifts and range contractions toward cooler waters.