Consistent genetic divergence observed among pelagic Sargassum morphotypes in the western North Atlantic

Abstract

Sargassum natans and Sargassum fluitans are uniquely holopelagic macroalgae, providing open ocean nursery and foraging habitat for commercially and ecologically important species. Recent basin‐wide changes in pelagic Sargassum diversity and distribution have manifested in proliferation of a previously rare morphotype, Sargassum natans VIII, to rival biomass levels of historically dominant S. natans I and S. fluitans III. Precise genetic identification of these morphotypes can improve accuracy and interpretation of ecological studies as well as clarify evolutionary history and population connectivity. For 139 field samples collected from the subtropical and tropical North Atlantic, three mitochondrial genes (cox3, nad6, and mt16S rRNA) were used to examine genetic divergence among the three common pelagic Sargassum morphotypes. These gene sequences successfully differentiated among morphotypes regardless of geographic origin, confirming in situ morphology‐based identifications. Sargassum natans I and S. natans VIII exhibited divergence consistent with that between the S. natans‐complex and S. fluitans III. Phylogenetic analysis of these samples also indicated evolutionary divergence between Sargassum morphologies. The genetic divergence among morphotypes, compared with benthic Sargassum species, suggested that taxonomic reclassification of the three most common pelagic morphotypes may be warranted.

Comprehensive Analysis of Biomass, Nutrient, and Heavy Metal Contributions of Pelagic Sargassum Species (Phaeophyceae) Inundations in South Florida

“…Carbon, nitrogen, and phosphorus content did not differ across morphotypes or spatially but showed temporal variations at Dania Beach (Figure 4). Despite distinct morphological, physiological, and genetic differences across sargasso morphotypes [39,53,54], the lack of differences in nutrient content across morphotypes in our study is similar to the lack of distinctions found between S. fluitans and S. natans (without morphotype distinction) in studies conducted by Lapointe et al (2021) [21] and McGillicuddy et al (2023) [26]. However, Changeux et al (2023) [55] reported local variations of %N across morphotypes sampled in the Mexican Caribbean region.…”

In situ observation of holopelagic Sargassum distribution and aggregation state across the entire North Atlantic from 2011 to 2020

“…Holopelagic Sargassum , a brown macroalgae, drifts at the water’s surface creating a unique and ecologically-significant floating marine ecosystem in the oligotrophic open ocean ( Haney, 1986 ; Wells & Rooker, 2004 ). Presently, two species and multiple morphological forms of holopelagic Sargassum are recognized and distributed throughout the equatorial and western North Atlantic Ocean ( Parr, 1939 ; Butler et al, 1983 ; Goodwin, Siuda & Schell, 2020 ; Dibner et al, 2021 ). At the <1 to 100s of meters spatial scale, holopelagic Sargassum aggregation patterns underpin its ecological value, the dynamic availability of highly localized prey biomass within and beneath the algae, and remote sensing detection and prediction capabilities ( Moser & Lee, 2012 ; Wang & Hu, 2017 ; Wang et al, 2019 ).…”

Revisited Estimation of Moderate Resolution Sargassum Fractional Coverage Using Decametric Satellite Data (S2-MSI)

“…Other invasive species than Sargassum could have been considered. However, in situ sampling carried out across the Atlantic Ocean in previous studies did not reveal evidence of the occurrence of any other invasive species competitive with Sargassum [11,[20][21][22]. Thus, although it might theoretically be possible, it should not be the case here.…”