Enhancing ex situ cultivation of Mediterranean Fucales: species-specific responses of Gongolaria barbata and Ericaria crinita seedlings to algal extracts

Mediterranean brown algal forests, dominated by Cystoseira sensu lato species, are undergoing widespread decline due to the cumulative effects of anthropogenic pressure and climate-related stressors. Restoration efforts increasingly rely on ex situ cultivation and outplanting of seedlings, yet early developmental stages often suffer from low survival and growth rates. This study investigated the potential of algal extracts to enhance the seedling development and survival in two canopy-forming species, Gongolaria barbata and Ericaria crinita. We tested extracts from a cyanobacterium (Trichormus variabilis), two microalgae (Desmodesmus sp. and Cylindrotheca closterium), and a commercial macroalgal formulation (AlgatronCifo®) at varying concentrations under controlled mesocosm conditions. Seedling performance was significantly influenced by extract type, and target species identity. Notably, a low-concentration Desmodesmus sp. extract (0.07 mg mL−1) improved survival and growth, whereas T. variabilis exerted an inhibitory effect on G. barbata. AlgatronCifo® did not outperform Desmodesmus sp. extract in promoting seedling development. These findings suggest that specific extracts from green microalgae could improve protocols for the early stages of restoration, offering a scalable tool for rehabilitating degraded marine forests. However, the results underscore the importance of species-specific optimization and the need for in situ validation of biostimulant-based restoration approaches.