Calcite crystal orientation patterns in the bilayers of laminated shells of benthic rotaliid foraminifera

• Published in: Journal of structural biology 2021-06, Vol.213 (2), p.107707-107707, Article 107707
• Main Authors: Yin, X., Griesshaber, E., Checa, A., Nindiyasari-Behal, F., Sánchez-Almazo, I., Ziegler, A., Schmahl, W.W.
• Format: Article
• Language: English
• Subjects: Animal Shells - chemistry; Animal Shells - diagnostic imaging; Animal Shells - ultrastructure; Animals; Aquatic Organisms - chemistry; Benthic foraminifera; Calcification, Physiologic; Calcite twins; Calcium Carbonate - chemistry; Crystal organization in foraminiferal tests; Crystallization; Electron backscatter diffrection (EBSD); Foraminifera - chemistry; Foraminifera - ultrastructure; Microscopy, Electron, Scanning
• ISSN: 1047-8477; 1095-8657
• DOI: 10.1016/j.jsb.2021.107707

[Display omitted] •Skeletal elements of the investigated tests are formed of mesocrystals.•Large dendritic mesocrystals are frequently twinned.•The shell wall calcite twinned according to two twin laws: [001] and [6−61]•Small mesocrystals are usually untwinned.•Two calcitic layers within tests can characterized by crystal size and twinning. Shells of calcifying foraminifera play a major role in marine biogeochemical cycles; fossil shells form important archives for paleoenvironment reconstruction. Despite their importance in many Earth science disciplines, there is still little consensus on foraminiferal shell mineralization. Geochemical, biochemical, and physiological studies showed that foraminiferal shell formation might take place through various and diverse mineralization mechanisms. In this study, we contribute to benthic foraminiferal shell calcification through deciphering crystallite organization within the shells. We base our conclusions on results gained from electron backscattered diffraction (EBSD) measurements and describe microstructure/texture characteristics within the laminated shell walls of the benthic, symbiontic foraminifera: Ammonia tepida, Amphistegina lobifera, Amphistegina lessonii. We highlight crystallite assembly patterns obtained on differently oriented cuts and discuss crystallite sizes, morphologies, interlinkages, orientations, and co-orientation strengths. We show that: (i) crystals within benthic foraminiferal shells are mesocrystals, (ii) have dendritic-fractal morphologies and (iii) interdigitate strongly. Based on crystal size, we (iv) differentiate between the two layers that comprise the shells and demonstrate that (v) crystals in the septa have different assemblies relative to those in the shell walls. We highlight that (vi) at junctions of different shell elements the axis of crystal orientation jumps abruptly such that their assembly in EBSD maps has a bimodal distribution. We prove (vii) extensive twin-formation within foraminiferal calcite; we demonstrate (viii) the presence of two twin modes: 60°/[001] and 77°/~[6 –6 1] and visualize their distributions within the shells. In a broader perspective, we draw conclusions on processes that lead to the observed microstructure/texture patterns.