Benthic contributions to the chemical seascape: Insights from Mediterranean underwater caves

Abstract Chemical mediation plays a major role in the functioning of marine ecosystems, yet most of the molecules sustaining species interactions remain largely unknown. As an initial step toward clarifying these processes, this study investigates how benthic biodiversity shapes the chemical composition of seawater, using underwater caves as model systems. Caves are known for the contrasting patterns of biodiversity between the communities at the entrance (SD, Semi‐Dark) and at the back‐end (D, Dark), at either side of a confinement gradient. To capture marine diluted metabolites near benthic communities, we deployed the I‐SMEL (In‐Situ Marine moleculE Logger) device in four Mediterranean underwater caves. Mass spectrometry (MS)‐based analyses, combined with chemometrics and putative molecular identifications, allowed the comparison of seawater chemical diversities. Our results revealed that the level of confinement drives major differences across the four investigated caves. The chemical seascapes at the entrance were enriched in brominated alkaloids and terpenoids, likely deriving from the specialized metabolisms of dominant sponge species. Conversely, high molecular weight fatty acids and polyketides with a high degree of oxidation were more prevalent in the seawater at the back of the caves. The four caves were more differentiated by their chemical seascape around their SD community. The mean species richness characterizing the benthic biodiversity correlates, for two caves, with the total number of chemical families in the surrounding water. Collectively, these findings highlight the contribution of the benthic biodiversity to the local chemical seascape and offer molecular‐level insights into the complex interplay between community composition, metabolite release, and ecosystem dynamics.