Sargassum seaweed has inundated Caribbean coastlines, disrupting ecosystems while presenting opportunities for sustainable products like liquid biofertilizers. This study evaluated the microbiological and biosecurity profiles of two liquid biofertilizers produced in the Dominican Republic: Sargassum-based (SBLB-INTEC) and conventional (LB-BANELINO). Both exhibited essential minerals (K, Ca, Mg) and low heavy metal concentrations, meeting agricultural safety standards. Metagenomic analysis (16S rRNA) identified diverse bacterial and archaeal communities, mainly consisting of beneficial taxa such as Firmicutes, Proteobacteria, and Methanobacteria, with no detectable pathogens. Ecotoxicological assays with Lactuca sativa showed phytotoxicity at high concentrations, mitigated by dilution (≤6.25%) and pH adjustment (7.0), achieving germination indexes above 80%. These results suggest that Sargassum-based biofertilizers could be a viable alternative for enhancing soil fertility and promoting sustainable crop production, particularly in coastal agricultural systems where conventional fertilizers may be limited or environmentally harmful. Moreover, this study highlights the critical role of ecotoxicological testing in ensuring biofertilizer safety and underscores Sargassum’s potential as a renewable resource for sustainable farming.

Sargassum seaweed is increasingly abundant in the Caribbean, creating ecological disruption but also providing biomass for agricultural inputs. This study compares the microbial diversity and safety of a Sargassum ‐based liquid biofertilizer (SBLB‐INTEC) with those of a conventional product (LB‐BANELINO) using 16S rRNA amplicon sequencing, rather than culture‐dependent methods. Both formulations contained key nutrients (K, Ca, and Mg) and low levels of heavy metals. They harbored dense but relatively simple bacterial communities dominated by Firmicutes, particularly Bacilli, with Proteobacteria and other phyla at lower abundances. Staphylococcus (Staphylococcaceae) was highly abundant in both products, while SBLB‐INTEC showed a somewhat more balanced community, including Delftia and other Comamonadaceae. Shannon diversity tended to be higher in SBLB‐INTEC, but differences in alpha‐ and beta‐diversity between formulations were not statistically significant. Because 16S data cannot distinguish viable from nonviable cells or resolve strain‐level pathogenicity, these results do not prove the absence of pathogens; instead, they provide a genus‐level baseline to guide targeted culture, qPCR, and functional assays. Overall, the combination of a favorable chemical profile and microbial groups commonly associated with nutrient cycling and plant‐associated functions suggests that SBLB‐INTEC could become a valuable component of integrated nutrient management in tropical agriculture, offering hope for a more sustainable future pending confirmatory plant‐response and biosafety studies. We recommend integrating these microbial data into a national biofertilizer monitoring framework, combining metagenomic surveys with targeted qPCR and resistance gene screening.