Fig. 3. Examples of biotic pathways of nanomaterial (NM) formation.

a Example for pathway IIIa: transmission electron micrographs (TEM) of magnetite (mag) NMs formed via microbial alteration of hematite⁶³. b, c Example for pathways IIIb: (b) scanning TEM image of a focused ion beam section extracted from a soil sample obtained near a smelting center in Ontario, Canada; occurrences of petrified spirilla with the same orientation on the surface of jarosite (jt) are encircled; (c) TEM image of biogenic magnetite (mag) NMs in close proximity to one of the spirilla encircled in (b)³⁵; (d, e) example for pathway IIIc: scanning electron micrographs (SEM) of gold (Au) NMs in biofilms through biological alteration of gold-containing ores⁶⁹; the area shown in (e) is outlined with a white rectangle in (d), (f, g) example for pathway IVa: SEM images of elemental sulfur NMs formed in Sulfurovum-dominated streamer biofilms in the Frasassi Cave, Italy¹⁰⁶; the occurrence of bipyramidal S(0) crystals and S(0) spheroids are indicated with red and white arrows in (f, g), respectively; (h, i) example for pathways IVb: (h) STEM images and (i) STEM-EDS chemical distribution maps for NMs of calcium carbonate (Cc, green) and Polyp granules (PG, red) formed inside cyanobacteria⁶⁸; (j, k) example for pathway IVc: (j) TEM and (k) high-resolution TEM image of metacinnabar (HgS) NMs formed in sulfidic niches of contaminated streambank soil⁷⁴; the HgS NMs are composed of (j) clusters with (k) domains depicting lattice fringes of different orientation (highlighted with white dashed lines); the area shown in (k) is highlighted with a red square in (j); the different types of pathways are indicated with the codes “IIIy” or “IVy” with y = a, b and c; “III” and “IV” indicate main categories and “a”, “b” and “c” subcategories (see text and Table 1 for details); (a, d, e) are reproduced with permission from Elsevier, (b, c) from the Geological Society of America, (f, g) from Frontiers, (h, i) from the Proceedings of the National Academy of Sciences and (j, k) from the Royal Society of Chemistry.