Coupled Si-Al Biogeochemistry: Occurrence of Aluminum in Diatom-Derived Biogenic Silica

Diatoms play an important role in the biogeochemical cycling of aluminum (Al) in oceans. This occurs via the uptake of biological Al (Albio), which is incorporated into the structure of diatom-derived biogenic silica (DBSi) and the formation of adsorbed Al (Alads) on the DBSi surface of post-mortem diatoms. Al occurrence influences DBSi dissolution and thus diatom-driven carbon sequestration. However, the mechanism of Al occurrence in DBSi remains unclear. In this study, Albio and Alads of DBSi from various diatom fossils in marine diatomaceous sediments were identified and quantified by combined focused ion beam thinning, elemental analysis, and the Al K-edge X-ray absorption near edge structure. Results showed the coexistence of Albio and Alads in all diatomaceous sediments and Al-bearing DBSi thus constitutes a biological Al pool. Albio and Alads were mainly fourfold- and sixfold-coordinated, respectively and Alads was much more abundant than Albio. Moreover, even at low concentrations (with an Al/Si atomic ratio of 0.0031), Albio can inhibit DBSi dissolution, effectively decreasing the extent of DBSi dissolution by similar to 14%. Albio also significantly increased the mechanical strength of DBSi. The average Young's modulus (a measure of the stiffness of a material) of cribrum layers in Al-incorporated DBSi was similar to 1.4 times higher than that of Al-free DBSi. Our results further demonstrate that diatoms play a dominant role in the biogeochemical cycling of Al in oceans, and the Al of DBSi participates in diatom-driven Si and C coupled cycles in oceans, influencing the effectiveness of diatom-driven carbon export by regulating the dissolution and mechanical strength of DBSi. Diatoms contribute to oceanic carbon sequestration by exporting carbon from the ocean's surface to its depths. Aluminum (Al) is taken up by living diatoms to be incorporated into the structure of diatom-derived biogenic silica (DBSi) (biogenic Al) and adsorbed by DBSi of post-mortem diatoms (adsorbed Al). Al can inhibit the dissolution of DBSi and thus influence carbon sequestration. However, the mechanism of Al occurrence in DBSi remains unclear. Herein, we identified and quantified Al in DBSi from various diatom fossils using nano-scaled microscopic analyses and investigated the influence of Al on DBSi properties. The results showed that biogenic Al and adsorbed Al have different coordination states in DBSi, and adsorbed Al was much more abundant than biogenic Al. The trace presence of biogenic Al effectively decreased the extent of DBSi dissolution by similar to 14%. Moreover, biogenic Al influenced the properties of DBSi-the mechanical strength of Al-incorporated DBSi was much higher than that of Al-free DBSi. These results indicate that Al taken up by living diatoms noticeably influences the chemical and physical properties of diatom shells, which impacts the effectiveness of carbon export by diatoms. Coupled Si-Al biogeochemistry was revealed by investigating the occurrence of Al in diatom-derived biogenic silica (DBSi)Nano-scaled microscopic analyses were performed to identify and quantify Al in DBSi, which constitutes a biological Al poolAl effectively inhibits DBSi dissolution and significantly increases the mechanical strength of DBSi