Abstract

The genus Sargassum includes over 350 primarily benthic species with only two recognized holopelagic representa-tives. Massive accumulations and strandings of holopelagic species of Sargassum on Caribbean, western African and Bra-zilian shores have reawakened interest in this important yet understudied brown macroalga that has been dubbed "the gold-en floating rainforest of the Atlantic Ocean". A rare form of S. natans referred to as S. natans VIII after the work of Parr in the 1930's, has been identified as the form accumulating in unprecedented quantities, but to date, no genomic data are avail-able for any holopelagic Sargassum species. Our laboratory has been applying comparative metagenomics on known forms of the Sargassum holopelagic species: S. fluitans III and S. natans I, alongside the formerly rare, S. natans VIII to eluci-date the relationships between the different forms, with the ultimate goal of producing population genomics markers with which to delineate different populations. In addition, amplicon sequencing strategies targeting the V6-V4 hypervariable region of the small-subunit ribosomal RNA gene of bacteria to better characterize the Sargassum microbiome reveal diverse assemblages of bacteria that include Cyanobacteria, Proteobacteria, Bacteroidetes, Firmicutes, Chloroflexi and Actinobacte-ria. As seen with species of Sargassum from Asia, comparative organelle genomics is useful in differentiating between At-lantic Sargassum species, however the holopelagic forms are very closely related compared to their benthic cousins. Despite gene synteny and high sequence conservation, the holopelagic Sargassum species differ in their ecology and distribution patterns, warranting a more in-depth examination of the holopelagic Sargassum biome as a whole.

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