Barium uptake and isotope fractionation by a marine diatom: Implications for oceanic barium cycle

Barium (Ba) is a nutrient-type element in the ocean and is commonly used as a tracer for reconstructing marine productivity. Despite recent developments in Ba stable isotope analysis and growing research interest, the controls on biological Ba uptake and isotope fractionation remain largely unknown. This study presents a series of culture experiments using the model marine diatom Thalassiosira weissflogii to explore biological Ba uptake contributing to pelagic barite (BaSO₄) formation and its associated isotope fractionation for the first time. The results show that Ba cell quotas (Ba/P) are positively correlated with Ba concentrations in the culture medium, with slopes influenced by diatom-specific growth rates under high and low light levels. Similar trends in Ba, Ca, and Sr uptake suggest that Ba is likely taken up passively through Ca transporters, as a leakage of seawater Ba into the cells. This study also investigates Ba/C ratios in Thalassiosira weissflogii for the first time, revealing significantly lower ratios (down to 43500-fold) compared to those observed in marine field particles. This finding suggests that additional Ba sources are required to sustain particulate Ba flux associated with export production in marine water columns. The Ba isotope compositions of the cultured species indicate preferential uptake of isotopically lighter Ba from seawater, with isotope fractionation Δ¹³⁸Ba_bio-sw values ranging from −0.47 ‰ to −0.14 ‰ as Ba concentrations in the medium increase from 90 to 200 nmol/kg. The fractionation pattern is independent of the growth rates. The Ba isotope results from cultured diatoms provide the first evidence explaining the mismatch between Ba isotope fractionation factors in pelagic and laboratory-precipitated barite, suggesting that initial isotope fractionation from seawater through biological uptake can lead to a more negative fractionation factor in pelagic barite than that observed in laboratory-precipitated barite. Considering the Ba/C ratios between cultured diatoms and sediment traps, biological uptake is unlikely to be the sole or primary source of Ba for pelagic barite formation. This study provides the first constraint on marine diatom Ba cell quotas and their isotope fractionation factors, emphasizing the need to investigate the contribution of Ba from other sources (e.g., microbial processes) and the associated isotope fractionation during pelagic barite formation.