Abstract

Sargassum biomass has attracted increasing attention due to its massive accumulation along the Mexican Caribbean coast (Riviera Maya) and its potential role in carbon cycling. Although previous studies have reported calcium carbonate formation associated with Sargassum, the crystallographic nature of these biomineralized phases and the possible incorporation of Mg into the carbonate lattice remain poorly understood. In this study, carbonate phases associated with Sargassum collected from the Mexican Caribbean were investigated using X-ray diffraction (XRD), Rietveld refinement, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and transmission electron microscopy (TEM). Structural, morphological, and compositional analyses consistently revealed calcite as the dominant carbonate phase, exhibiting lattice modifications associated with Mg incorporation. Rietveld refinement identified crystallographic changes consistent with Mg substitution within the calcite lattice, while complementary characterization confirmed Mg-bearing carbonate domains and local structural distortions characteristic of high-Mg calcite (HMC). The combined results provide strong evidence for the formation of HMC associated with Sargassum, demonstrating that Mg incorporation occurs within the carbonate structures of a non-calcifying brown macroalga, a process previously reported predominantly in calcifying organisms and calcareous algae. These findings expand the current understanding of biomineralization pathways in marine ecosystems and suggest that Sargassum can promote the transformation of dissolved inorganic carbon into carbonate minerals. The occurrence of HMC highlights the potential role of Sargassum as a natural bioremediator and a contributor to transient carbon fixation through carbonate formation, providing new insights into the role of brown macroalgae in carbonate production and carbon cycling.

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