Can nuisance species profit from new materials for marine artificial structures? A pilot study on settlement of Chrysaora hysoscella on biochar-based concrete

Growing ocean-based human activities are causing the expansion of concrete structures in the marine environment, stressing the need for new solutions to minimize their ecological impact and carbon footprint. A promising approach in this perspective involves the use of biochar, a carbon-rich byproduct from thermochemical treatment of waste biomass, as an additive to concrete. However, the introduction of novel substrata could pose unintended threats to marine ecosystems, including the potential diffusion of nuisance species. Here, through a set of laboratory experiments, we provided preliminary evidence on the effects of biochar addition to concrete on settlement and survival of the bloom-forming jellyfish Chrysaora hysoscella. The acute 24/48-h mortality test demonstrated a very low sensitivity of planulae to biochar. At both 7 and 14 days since the exposure to colonization of planula larvae, the number of settled polyps did not differ between concrete containing biochar (5-10 % wt.) and conventional concrete. Once settled, polyps exhibited comparable survival regardless of the substratum. Our findings support the hypothesis that biochar-based concrete does not favor the invasiveness of this nuisance species with respect to conventional concrete, paving the way for future research on a wider range of (bio)char types and potentially harmful species, enabling broader implementations of these materials. The inherent advantages of biochar addition to concrete for marine uses, such as waste recycling and carbon storage potential, combined with its neutral effect on the spread of nuisance species, could represent a practicable and biocompatible solution to enhance the sustainability of marine artificial structures.