A closer look at benthic foraminifera shells – implications for biomineralization mechanisms

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The detailed biomineralization mechanisms of planktic and benthic foraminifera is still enigmaticand a topic of active research. Much progress has been achieved in developing biomineralizationmodels for some benthic hyaline species (e.g. Erez, 2009; de Nooijer, 2014) and in discovering thatthe original phase composition of planktic Orbulina universa (d’Orbigny, 1839) is metastablevaterite and not calcite, supporting a non-classical crystallization pathway for this importantspecies (Jacob et al. 2017). It is, however, less clear to date whether these results can be replicatedin other foraminifera species and models for their formation can be generalized.To extend our earlier studies on planktic species, we studied four species of benthic foraminiferafrom the Australian Great Barrier Reef, namely Amphistegina lobifera (Larsen, 1976),Baculogypsina sphaerulata (Parker and Jones, 1860), Calcarina capricornia (Mamo, 2016) andMarginopora vertebralis (Quoy and Gaimard, 1830). Samples were collected alive and pulsechase labelled with Sr in aquaculture before carrying out a detailed, multi-scale study of theirarchitecture. We used Electron Backscatter Diffraction, Nano-SIMS, Focussed Ion Beam assistedTransmission Electron Microscopy, Micro-Raman Spectroscopy and Photo-induced ForceMicroscopy (Otter et al. 2021) to elucidate and compare phase compositions, micro-architectureand organic chemistry of the shells. Our results contribute to understand the details offoraminiferal biomineralization and to develop a general model for shell formation across allforaminifera species.de Nooijer , L.J. et al. (2014). Biomineralization in perforate foraminifera. Earth-Science Reviews135, 48-58.Erez, J. (2003). The source of ions for biomineralization in foraminifera and their implications forpaleoceanographic proxies. Reviews in Mineralogy and Geochemistry 54, 115-149.Jacob, D.E. et al. (2017). Planktic foraminifera form their shells via metastable carbonate phases.Nature Communications, 8, 1265Otter, L.M. et al. (2021) Nanoscale Chemical Imaging by Photo‐Induced Force Microscopy:Technical Aspects and Application to the Geosciences. Geostandards and Geoanalytical Research45, 5-27.

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