Assessing the molecular diversity of Sargassum (Phaeophyceae) in the NE Atlantic Ocean

Resum

Sargassum plays a key role as habitat-forming species in benthic and pelagic ecosystems in tropical andsubtropical regions, and has recently become a global environmental issue due to the societal impact ofits massive blooms. With about 1000 taxa described and ca. 350 taxonomically accepted species,Sargassum represents almost one-sixth of Phaeophyceae and a prime example of a troublesome taxonomy. Species identification has traditionally been hampered by the phenological plasticityexhibited in response to environmental conditions and life history. The advent of DNA phylogenies haschallenged the taxa circumscription of this genus and have proven critical in delimiting the species.However, the molecular diversity of the Atlantic species of Sargassum remains poorly understood, andrecently a few studies have shown a low genetic diversity between the species in the NW Atlantic. Here,we aim to assess the molecular diversity of Sargassum in the NE Atlantic Ocean using a multigeneapproach, combining markers commonly used in delineating species of this genus (rbcLS, ITS2, cox3,mtsp) and exploring new markers (cox2, nad6, psbC, clpC, atpB) to determine the phylogeneticrelationships between the different morphospecies. Both single-gene and multigene phylogeniesconfirmed the low genetic variability and did not support distinct species in Sargassum sect. Sargassumin the N Atlantic benthic species, probably due to a recent colonization and radiation of this basin. Thesefindings contrast with the morphological and ecological diversity of Sargassum in the Atlantic, andshowcase the need for further analyses, including population genetics and the use of high-throughputDNA technologies.