Sailing the Sargasso Sea for a floating golden forest: the application of genetic markers to investigating Sargassum diversity and distribution in the field
Biodiversity and life in the oligotrophic Sargasso Sea are sustained by the free-floating macroalgae, Sargassum. Consisting of two species, S. fluitans and S. natans, pelagic Sargassum provides food, nutrients, and habitat to a diverse array of marine organisms. These two species are classified further into genetically distinct morphological forms, with S. fluitans III, S. natans I, and S. natans VIII being the most common in the Western North Atlantic. Since 2011, pelagic Sargassum populations have been shifting, delivering the previously rare morphotype, S. natans VIII, to Caribbean beaches in unprecedented quantities. These inundation events are detrimental to local fisheries, tourism, and coastal ecosystems. Developing efficient and cost-effective genetic tools to differentiate the three forms is imperative for studying morphotype distribution and the forces driving Caribbean inundation events. In our field-based study, we designed novel genetic primers to target two regions of the Sargassum mitochondrial genome, the cytochrome oxidase subunit 3 (Cox3) and NADH dehydrogenase subunit 6 (Nad6) to assess their efficacy to function as genetic markers. Both primers successfully differentiated the three morphotypes with varying accuracies (~89% for Cox3 and ~69% for Nad6) but have limitations in their applications. We also examined morphotype distribution patterns and found the highest concentrations of Sargassum in the Antilles Current and South Sargasso Sea, and overall density in both regions was comprised primarily of S. natans I and S. fluitans III. An examination of algal growth data indicated that S. natans VIII was older than other forms, which supports the hypothesis of a new Sargassum growth region in the tropics fueling inundation events.