Unraveling the Hidden World: Variability and Complexity of Holopelagic Sargassum Biofilms

Holopelagic Sargassum spp. —including S. natans var. natans (SNN), S. natans var. wingei (SNW), and S. fluitan s var. fluitans (SFF)—has become a major ecological and socio-economic concern due to recurrent mass strandings in the Caribbean. This study aims to complement the growing knowledge on the holopelagic Sargassum holobiont by incorporating the three-dimensional organization of its biofilms. Here, we provide the first detailed characterization of holopelagic Sargassum biofilms using scanning electron microscopy (SEM) combined with bacterial abundance measurements. Samples were collected from nearshore waters and stranded mats in Guadeloupe. We assessed elemental composition, biofilm maturity and coverage, and documented the presence of diatoms, superficial filamentous bacteria coverage (SFBC), biofilm-coated diatom units (BCDU) with previously undescribed protrusions, and Ca–P–O crystals reported here for the first time in macroalgal biofilms. Biofilm structure and composition varied among taxa, thallus compartments, and environmental contexts. SNN exhibited the lowest surface roughness, the weakest biofilm development in nearshore conditions, and the lowest abundances of Ca–P–O crystals. In contrast, SFF consistently displayed high densities of crystals, diatoms, and BCDU with protrusions, indicating greater microbial and morphological complexity, while SNW maintained a stable biofilm architecture largely independent of thallus part and environment. Stranding increased bacterial abundance across all taxa and enhanced diatom colonization in SNW and SFF. Across taxa, axes were least affected, whereas fronds, particularly in SNN,were most strongly impacted. SFBC, highest in SNN, was negatively correlated with crystal abundance, which was highest in SFF and SNW. Overall, these findings provide a three-dimensional perspective on intra-thallus and interspecific variation in Sargassum biofilms and demonstrate how interactions among host surface traits, microbial components, and environmental context shape biofilm structure, stability, and potential host resilience. • Biofilm traits vary across Sargassum thallus parts and morphotypes, potentially affecting resilience. • Surface roughness differs within and between morphotypes, affecting biofilm development. • Ca-P-O crystals identified for the first time in macroalgal biofilms, were negatively correlated with filamentous bacteria presence. • Fronds are most prone to change upon stranding; bacterial density rises, and filamentous bacteria coverage differ by taxa. • Biofilm-coated diatoms with protrusions seen in SFF and SNN but with an unclear ecological role.