The pink Amazon?’ Metabarcoding reveals Rhodolith beds as hidden hotspots of global marine biodiversity

Abstract

Rhodoliths are key foundation species in marine ecosystems, enhancing benthic structural complexity and supporting high biodiversity. However, much of the diversity associated with rhodolith beds remains cryptic, difficult to access, and poorly documented. Here, we assess the potential of COI-based metabarcoding to evaluate invertebrate and macroalgal diversity associated with rhodolith habitats. Samples were collected at two localities along the Brazilian coast: Abrolhos Bank (17°S) and Queimada Grande Island (24°S). Metabarcoding recovered over 1800 exact sequence variants (ESVs) among macroalgae and invertebrates, including 21 potential new records for the Southwestern Atlantic and candidate new species, particularly for macroalgae. Integration of molecular and morphological approaches resulted in the identification of more than 450 putative species. Despite these high values, local biodiversity is likely underestimated due to substantial gaps in reference barcode databases for Brazilian marine taxa. Nevertheless, our results indicate that approximately 0.2–1% of all currently known marine species occur within these relatively small rhodolith-covered areas in the Southwestern Atlantic. Unlike other megadiverse ecosystems, such as tropical rainforests or coral reefs, which are geographically restricted, rhodolith beds are globally distributed across all ocean basins, from polar to tropical regions, and extend from intertidal to depths of up to 200 m. This combination of high local diversity and broad global extent positions rhodolith beds as overlooked but potentially major reservoirs of marine biodiversity. Our findings therefore add to growing evidence that rhodolith beds rank among the most diverse marine habitats and warrant increased attention to safeguard their ecological functions, associated resources, and ecosystem services. We call for coordinated international efforts to improve biodiversity assessments and accurately quantify the global contribution of rhodolith habitats.