Molecular Diversity Among Holopelagic Sargassum Morphotypes in the North Atlantic

A critical review of pelagic Sargassum taxonomy is long overdue. Hundreds of species of Sargassum, a brown macroalgae, are distributed globally in shallow tropical and temperate waters. S. natans and S. fluitans are uniquely pelagic, providing an open ocean nursery and foraging habitat for commercially and ecologically important species. Historically, pelagic Sargassum concentrations were greatest in the Sargasso Sea, with lower concentrations in the Gulf of Mexico and Caribbean Sea. Since 2011, annual episodic inundations of pelagic Sargassum have plagued coastal communities across the tropical Atlantic, causing negative impacts to local tourism and fishing sectors. Initial assumptions based on oversimplified identification were that the inundations originated from the Sargasso Sea. However, shipboard observations indicated that the invading Sargassum was distinct from common morphologies of the Sargasso Sea, highlighting the need for more nuanced identification to facilitate understanding of potential differences in ecological function. Recent examinations of benthic Sargassum molecular phylogeny in the Pacific and Indian Oceans as well as the Mediterranean Sea resulted in a reorganization of the genus and addition to the recognition of multiple new species. The last major pelagic Sargassum classification was in 1939 (designating six morphologically distinct forms), and the only previous attempt at molecular differentiation of pelagic forms was sample and genetic marker limited. Using 111 field samples of Sargassum collected from the tropical Atlantic, Caribbean Sea, Gulf of Mexico and the Sargasso Sea, we examined genetic divergence at four mitochondrial genes (Cox3, Nad6, 16S, and mt23S) that had not been previously applied in this manner. These genes successfully differentiate among both common and rare morphological variants, elucidate evolutionary history, and illustrate biogeographic patterns of pelagic Sargassum.