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

The detailed biomineralization mechanisms of planktic and benthic foraminifera is still enigmatic
and a topic of active research. Much progress has been achieved in developing biomineralization
models for some benthic hyaline species (e.g. Erez, 2009; de Nooijer, 2014) and in discovering that
the original phase composition of planktic Orbulina universa (d’Orbigny, 1839) is metastable
vaterite and not calcite, supporting a non-classical crystallization pathway for this important
species (Jacob et al. 2017). It is, however, less clear to date whether these results can be replicated
in 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 foraminifera
from the Australian Great Barrier Reef, namely Amphistegina lobifera (Larsen, 1976),
Baculogypsina sphaerulata (Parker and Jones, 1860), Calcarina capricornia (Mamo, 2016) and
Marginopora vertebralis (Quoy and Gaimard, 1830). Samples were collected alive and pulse
chase labelled with Sr in aquaculture before carrying out a detailed, multi-scale study of their
architecture. We used Electron Backscatter Diffraction, Nano-SIMS, Focussed Ion Beam assisted
Transmission Electron Microscopy, Micro-Raman Spectroscopy and Photo-induced Force
Microscopy (Otter et al. 2021) to elucidate and compare phase compositions, micro-architecture
and organic chemistry of the shells. Our results contribute to understand the details of
foraminiferal biomineralization and to develop a general model for shell formation across all
foraminifera species.

de Nooijer , L.J. et al. (2014). Biomineralization in perforate foraminifera. Earth-Science Reviews
135, 48-58.

Erez, J. (2003). The source of ions for biomineralization in foraminifera and their implications for
paleoceanographic 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, 1265

Otter, L.M. et al. (2021) Nanoscale Chemical Imaging by Photo‐Induced Force Microscopy:
Technical Aspects and Application to the Geosciences. Geostandards and Geoanalytical Research
45, 5-27.