Figure 1 Starting material for experiments showing (a) Six high pressure, high temperature (HPHT) experimental conditions investigated and their corresponding metamorphic grades spanning lower and upper blueschist (BS), greenschist (GS) and amphibolite (Amph.). Grey depicts HPHT conditions investigated previously (Schiffbauer et al., 2012
Schiffbauer, J. D., Wallace, A.F., Hunter, J.L., Kowalewski, M., Bodnar, R.J., Xiao, S. (2012) Thermally‐induced structural and chemical alteration of organic‐walled microfossils: an experimental approach to understanding fossil preservation in metasediments. Geobiology 10, 402–423.
; Li et al., 2013b
Li, Y.L., Konhauser, K.O., Kappler, A., Hao, X.L. (2013b) Experimental low-grade alteration of biogenic magnetite indicates microbial involvement in generation of banded iron formations. Earth and Planetary Science Letters, 361, 229–237.
Li, J., Bernard, S., Benzerara, K., Beyssac, O., Allard, T., Cosmidis, J., Moussou, J. (2014) Impact of biomineralization on the preservation of microorganisms during fossilization: An experimental perspective. Earth and Planetary Science Letters 400, 113–122.
; Picard et al., 2015a
Picard, A., Kappler, A., Schmid, G., Quaroni, L., Obst, M. (2015a) Experimental diagenesis of organo-mineral structures formed by microaerophilic Fe(II)-oxidizing bacteria. Nature Communications 6, 6277 (2015), doi: 10.1038/ncomms7277.
Picard, A., Obst, M., Schmid, G., Zeitvogel, F., Kappler, A. (2015b) Limited influence of Si on the preservation of Fe mineral-encrusted microbial cells during experimental diagenesis Geobiology 14, 276–292.
; Alleon et al., 2016
Alleon, J., Bernard, S., Le Guillou, C., Daval, D., Skouri-Panet, F., Pont, S., Delbes, L., Robert, F. (2016) Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis. Chemical Geology 437, 98–108.
Alleon, J., Bernard, S., Le Guillou, C., Daval, D., Skouri-Panet, F., Kuga, M., Robert, F. (2017) Organic molecular heterogeneities can withstand diagenesis. Scientific Reports 7, 1508.
; Miot et al., 2017
Miot, J., Bernard, S., Bourreau, M., Guyot, F., Kish, A. (2017) Experimental maturation of Archaea encrusted by Fe-phosphates. Scientific Reports 7, 16984.
). (b) Si-rich microbial filamentous material and coccoidal cell structures are observed under SEM in the the starting material, shown in (c) by a view ~ 1 cm across; forming Si-rich biofilms around calcite grains (Fig. S-1a,b).