Valorization of Posidonia oceanica biomass through the optimization and characterization of biostimulant extracts for horticultural crops

Posidonia oceanica is an endemic Mediterranean seagrass that plays a key role in coastal ecosystems. However, its natural life cycle leads to the accumulation of beached biomass on shorelines which, although contributing to coastal stability, entails significant management costs associated with its removal during the tourist season. This marine biomass represents a natural source rich in nutrients and secondary metabolites with potential applications in sustainable agriculture. In this context, the aim of this study was to contribute to the valorization of P. oceanica biomass through the production of aqueous extracts, optimizing extraction conditions and characterizing their biostimulant properties for horticultural crops. To optimize the extraction process, aqueous extractions were performed at 50 °C using four extraction times (1, 2, 4, and 6 hours), with water as the sole solvent under constant stirring. The obtained extracts were subsequently characterized by quantifying agronomically relevant bioactive compounds to identify the extraction conditions that maximized the recovery of metabolites with biostimulant potential. The results showed a significant influence of extraction time, with the 6 h treatment at 50 °C exhibiting the most favorable phytochemical profile. This extract reached a total phenolic content of 73.9 ± 1.6 mg GAE L⁻¹ and an antioxidant capacity of 121 ± 6 mg TE L⁻¹ (ABTS), supporting its potential as a plant-derived bioactive fraction for the development of plant biostimulants. These findings support the valorization of P. oceanica biomass as a strategy to reduce the accumulation of coastal waste and promote its use as a resource for agricultural applications. Furthermore, they provide a scientific basis for the development of marine-derived biostimulants for horticultural crops, in accordance with circular economy principles.