* To whom correspondence should be addressed. Tel: Department für Geo- und

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1 The effect of hydrogel matrices on calcite crystal growth morphology, aggregate formation, and co-orientation in biomimetic experiments and biomineralization environments Fitriana Nindiyasari a *, Andreas Ziegler b, Erika Griesshaber a, Lurdes Fernández- Díaz c,d *, Julia Huber b, Paul Walther b Wolfgang W. Schmahl a a Department für Geo- und Umweltwissenschaften and GeoBioCenter, Ludwig-Maximilians- Universität, Munich, Germany. b Central Facility for Electron Microscopy, University of Ulm, Ulm, Germany. c Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid, Madrid, Spain. d Instituto de Geociencias (UCM, CSIC). C/ José Antonio Novais 2, Madrid, Spain. Supporting Information. Cryo-FE-SEM images of high-pressure frozen gel samples showing the fabric of silica, agarose, gelatin. Cryo-FE-SEM images of fracture surfaces showing the fabrics of high-pressure frozen Mgfree hydrogels (gelatin, agarose and silica) that were not treated with isopropanol. The assemblage of silica nanospheres to a very loose 3D hydrogel network. The fabric of agarose and gelatin hydrogels after complete decalcification of the hydrogel - calcite aggregate. FE-SEM images showing magnifications of subunit III (a) and subunit I (b) presented in Figure 4b (a Mg-free gelatin - calcite aggregate). FE-SEM image of microtome cut, polished, etched and critical point dried surfaces of Mg-free and Mg-beaing hydrogel-calcite aggregates (silica, agarose and gelatin hydrogel). EBSD band contrast images that highlight internal features of the investigated single crystals and aggregates obtained from the three different hydrogels, in the presence and absence of Mg in the growth medium. This material is available free of charge via the Internet at * To whom correspondence should be addressed. Tel: Department für Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität, Munich, Germany. f.nindiyasari@lmu.de or lfdiaz@geo.ucm.es.

2 Figure S1. Cryo-FE-SEM images of high-pressure frozen gel samples showing the fabric of: silica (a, b), agarose (c, d), gelatin (e, f). Distinct fabrics and pore dimensions are observed for the different hydrogels: silica hydrogel is an assemblage of interconnected nanometric silica spheres, agarose hydrogel exhibits a network of fibers with large pore size variations, the fabric of gelatin hydrogel is, in comparison to agarose gel, slightly denser with pore sizes being more uniform. 2

3 Figure S2. Cryo-FE-SEM images of fracture surfaces showing the fabrics of high-pressure frozen Mg-free hydrogels that were not treated with isopropanol: (a, c) Gelatin, (b, d) Agarose. The fabrics of the isopropanol treated hydrogel samples are shown in Figures 1a to 1c. We observe a good correspondence between the hydrogel microstructures of gels treated with and without isopropanol. Thus, the gel fabric is not distorted by ice crystal formation. 3

4 Figure S3. The assemblage of silica nanospheres to a very loose 3D hydrogel network. Figure S4. The fabric of agarose (a) and gelatin (b) hydrogels after complete decalcification of the hydrogel - calcite aggregate. Aggregations of hydrogel fibers are observable in both gels, most probably due to the crystallization pressure of the mineral. 4

5 Figure S5. FE-SEM images showing magnifications of subunit III (a) and subunit I (b) presented in Figure 4b. Well visible in a (see yellow arrows) is the dispersed occlusion of gelatin gel into calcite. In (b) we observe patches of mineral (yellow stars), thick gel networks (yellow arrows) and differently sized membranes (white stars). 5

6 Figure S6. FE-SEM image of microtome cut, polished, etched and critical point dried surfaces of Mgfree and Mg-beaing hydrogel-calcite aggregates: Silica hydrogel (a, d), Agarose hydrogel (b, e), Gelatin hydrogel (c, f). As the morphologies of the aggregates show, the effect of Mg in the growth medium on aggregate formation is immense. 6

7 Figure S7. EBSD band contrast images that highlight internal features of the investigated single crystals and aggregates obtained from the three different hydrogels, in the presence and absence of Mg in the growth medium. We see the effect of the hydrogel on single crystal (a) and aggregate (c to f) formation and its occlusion within the aggregates as dark lines (see arrows in b, e) surrounding subunits and their entities of the major units of the aggregate (c, d, e, f). 7