Structural complexity and carbon storage capacity in Mediterranean rhodolith beds

Rhodolith beds are globally distributed, biodiversity-rich habitats formed by free-living coralline algae that function as ecosystem engineers and major carbonate reservoirs. Despite their ecological and biogeochemical importance, these systems are increasingly vulnerable to climate change and ocean acidification. This study integrates morphological and geochemical analyses to evaluate structural characteristics and carbon storage in two Mediterranean rhodolith beds located at contrasting depths (2 m and 51 m). Living rhodoliths represented 83% and 71% of the collected material in the shallow and deep beds, respectively, indicating generally healthy conditions. Inorganic and skeletal CO₃²⁻ fractions were more consistent among praline morphotypes than organic content, and no significant differences were detected at the genus level. Overall, both organic and inorganic fractions appear to be more strongly influenced by environmental factors, such as depth, hydrodynamics, and local physicochemical conditions, than by taxonomic differences among genera. Comparisons with rhodolith beds from other regions revealed contrasting organic contents but broadly similar inorganic fractions, further supporting the dominant role of environmental controls in carbonate accumulation.