Non-classical crystallisation mechanisms in hyaline foraminifer biomineralisation

Foraminifers are an ubiquitous group of marine unicellular organisms, which form shells (tests) made of CaCO­3­­. Since their tests can withstand dissolution and accumulate in the sedimentary record­­­­­, foraminifers are a critical part of the global carbon cycle and provide one of the most important archives for paleoclimate research. Therefore, a confident link between environmental parameters and proxy signals recorded in their shells is essential, and a detailed mechanistic understanding of their biomineralisation is vital knowledge.

We have synthesised a diverse array of microscopic and diffraction-based methods to investigate hyaline foraminifer tests on a nanometre- to micrometre scale. We show that chamber walls exhibit a nanogranular fracture surfaces together with a crystallite domain size of approximately 100 nm, and that these nanocrystals are organised in crystallographically aligned micrometre sized units oriented towards the chamber surfaces. From this, we conclude that hyaline foraminifer tests are made of mesocrystals, which strongly indicates the presence of non-classical crystallisation mechanisms during foraminiferal biomineralisation (1).  

Non-classical crystallisation pathways likely involve metastable carbonates, phase transitions to stable phases, and organic matter in diverse functions. Hence, they are expected to distinctly influence isotopic fractionation and trace element incorporation into the final biomineral, which are proxies for environmental conditions. This highlights the necessity to closely investigate non-classical processes in the organisms as well as in experimental models, to reduce uncertainties arising from vital effects and enable accurate and precise reconstructions.

(1) A. I. Arns et al., accepted in Geochemistry, Geophys. Geosystems (2022), doi:10.1029/2022GC010445.