Fig. 6: Microtextures of Y3 (Y3 B) mat.

SEM-BSE images of microbial filaments showing an in situ model of increasing enrichment and crystallization of sheaths and stalks with FeOx from the upper (a, b), middle (c–g), to the bottom (h), parts of the Y3 B mat. As the mat aged, the microbial filaments are usually encrusted by poorly crystalline ferrihydrite (Fh)⁵⁷ first act as nucleation sites, followed by gradual transformation into well-crystallized FeOx that cover them with successive steps of FeOx crystallization (b–f), and eventually disappear (g–h). The orange arrow reported in (d) shows the decreasing proportion of filamentous structures with increasing depth.