Silicon isotopes of deep sea sponges: new insights into biomineralisation and skeletal structure

• Published in: Biogeosciences
• Main Authors: Cassarino, Luca, Coath, Christopher D, Xavier, Joana R, Hendry, Katharine R
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2018
• Subjects: Porifera; silicon isotopic composition; European Union (EU); Horizon 2020; Grant Agreement No 679849; Deep-sea Sponge Grounds Ecosystems of the North Atlantic: an integrated approach towards their preservation and sustainable exploitation; SponGES; Acosta; North Atlantic
• Online Access: https://zenodo.org/record/2645023; https://doi.org/10.5194/bg-15-6959-2018

The silicon isotopic composition (δ30Si) of deep sea sponges’ skeletal element – spicules – reflects the silicic acid (DSi) concentration of their surrounding water and can be used as natural archives of bottom water nutrients. In order to reconstruct the past silica cycle robustly, it is essential to better constrain the mechanisms of biosilicification, which are not yet well understood. Here, we show that the apparent isotopic fractionation (δ30Si) during spicule formation in deep sea sponges from the equatorial Atlantic ranges from −6.74 ‰ to −1.50 ‰ in relatively low DSi concentrations (15 to 35 μM). The wide range in isotopic composition highlights the potential difference in silicification mechanism between the two major classes, Demospongiae and Hexactinellida. We find the anomalies in the isotopic fractionation correlate with skeletal morphology, whereby fused framework structures, characterised by secondary silicification, exhibit extremely light δ30Si signatures compared with previous studies. Our results provide insight into the processes involved during silica deposition and indicate that reliable reconstructions of past DSi can only be obtained using silicon isotope ratios derived from sponges with certain spicule types. ACKNOWLEDGEMENTS. We acknowledge the science team and the crew of JC094 and Laura Robinson for cruise organisation. We would also like to thank Paul Curnow for constructive com- ments, Stuart Kearns for his SEM training and assistance, and Maria López-Acosta for her help. Finally, the funding from the Royal Society (grant code RG130386) and from the European Research Council is acknowledged. Joana R. Xavier received support from the European Union's Horizon 2020 research and innovation program through the SponGES project (grant agreement no. 679849). SAMPLE AVAILABILITY. Samples and sample images are available at the University of Bristol; for further detail contact Katharine R. Hendry, email address: [e-mail]