Posidonia oceanica is a foundation species of the Mediterranean coastline, producing approximately 20 liters of O₂ per square meter per day while removing CO₂, stabilizing sediments, mitigating coastal erosion, and providing habitat for roughly a quarter of Mediterranean marine fauna. Despite these critical ecosystem services, P. oceanica meadows are in continuous decline, primarily driven by anthropogenic pressures: harbor and breakwater construction, altered hydrodynamic and sedimentary regimes, eutrophication, rising sea temperatures, turbidity, and the spread of invasive species. Given the extremely slow natural growth rate of this seagrass, active restoration has become essential to counteract habitat loss.

Several experimental and industrial restoration techniques are currently employed, ranging from concrete frames and bio-derived geotextile mats to modular synthetic supports and gravel-filled cages. However, many of these solutions are not fully biodegradable, raising concerns about their long-term environmental compatibility within sensitive marine ecosystems.

To address this limitation, this work proposes a fully bio-based hydrogel material derived from agarose, a polysaccharide extracted from marine red algae, functionalized with lignin and essential plant-relevant salts. The hydrogel was characterized in terms of morphology, water-interaction and stability, mechanical properties, release and antioxidant properties, and biodegradation. Afterward, the hydrogels were tested hosting P. oceanica shoots within a controlled, slow-release nutrient microenvironment, sustaining germination, rooting, and shoot growth at optimized rates. A key advantage of this material is its castability: the gel can be molded into custom shapes and structures, enabling functionalization tailored to the deployment context, whether in an experimental aquarium setting or directly on the seafloor.

This new approach, based on the design of hydrogel materials, can support marine plant growth and provide a sustainable, fully biodegradable, and adaptable tool for P. oceanica meadow recovery.