Identification of Li as a reference element in Sargassum bioaccumulation of conservative-type elements (Mg, Mo, Sb, Cs and U)

Abstract

We reported the accumulation of conservative-type elements (lithium (Li), magnesium (Mg), molybdenum (Mo), antimony (Sb), cesium (Cs), and uranium (U) in pelagic species of Sargassum brown alga (S. fluitans and S. natans). We also included individuals of benthic Sargassum species, inhabiting a reef ecosystem influenced by urban developments and tourist activities. By assuming a set of premises for a normalizing element, Li was used to calculate the bioaccumulation factor (BAF) of the bio-unlimited elements in pelagic and benthic Sargassum relative to the seawater levels. Both pelagic species displayed levels in the same order of abundance as in seawater from the central Atlantic Ocean and the Caribbean Sea (Mg> > Li ~ Mo > U > Sb > Cs), except Sb. Bio-unlimited element levels and BAFs were comparable in S. fluitans and S. natans but lower than benthic Sargassum, except for U. Temporal variations were observed in the metal contents, except Li, with higher values from July to October regarding the February–May period. Those variations were unexpected considering their conservative-type behavior in oceanic waters. Metabolic changes related to the life cycle explain variations for Mg and Mo. Anthropogenic Sb inputs, via atmospheric dust, could constitute a significant metal source to surface seawater and the pelagic Sargassum. S. fluitans and S. natans arriving at the Caribbean coasts are excellent biomonitors to study trace metals scarcely studied in oceans, such as bio-unlimited elements. Lithium met the premises to be used as a reference element to determine their bioaccumulation of metals (uptake and retention). So, estimations of BAFs by normalizing to Li constitute a useful tool for metal assessment using macroalgae as an environmental biomonitor.