Addressing challenges in marine algal restoration: Lessons learned from 3D-printed structures on artificial reefs

In the last three decades, an increasing number of man-made marine hard defence structures have been built as a rapid and cost-effective means of coastal protection. The idea of integrating ecological principles into urban infrastructures is relatively new and challenging. Using coastal structures to host endangered species holds a significant promise for biodiversity conservation in coastal cities worldwide. Nevertheless, marine infrastructures provide unconventional substrates for benthic communities due to a lack of surface complexity, orientation, exposure, structure, and texture, affecting the recruitment, survival, and growth of organisms. The main goal of the present study is to identify the best material and shape to enhance the attachment and growth of macroalgal forests ( i.e. Ericaria amentacea ) in such unconventional substrates, in order to investigate the potential of restoration on man-made defence structures applying the ex-situ restoration technique (outplanting of germlings cultured on tiles in the laboratory). The study area is located inside the partial protection zone of the Portofino Marine Protected Area (Liguria, Italy). In 2018, this area was strongly affected by a huge sea storm (Vaia storm) which boosted the construction of a protective breakwater. Specifically designed 3D-printed structures made of different sustainable materials (aragonite and beachsand) were used as supports for E. amentacea growing. Beachsand-based 3D-printed structures outperform aragonite in supporting E. amentacea growth, both in terms of thallus length and percent cover. They also performed better than clay, the material commonly used for macroalgae ex-situ restoration on natural reefs. This material also supported long-term efficient cultures (over 30 % cover after 16 weeks in culture), boosting restoration success. Unfortunately, the deployment in the field was not possible due to fixing inconveniences, thus assessing restoration efficacy in the field remains a challenge for future studies. Aligning with the EU Nature Restoration Law requirements, our study is a step forward to the development of innovative, nature-based solutions for mitigating anthropogenic impacts on marine artificial ecosystems.