Figure 4.

Ultrastructure of the microfossil Polysphaeroides filiformis and of a modern cyanobacterium and a modern red alga

SEM (A–C) images of Polysphaeroides filiformis and TEM (D–G) images of P. filiformis, Stigonema robustum CBFS-A027 (H–J) and Bangiopsis franklynottii CCMP3416 (K and L).

(A) P. filiformis showing a thickened median part surrounding the filaments inside the sheath and laterally thinner flattened parts of the sheath.

(B) Granular appearance of the sheath.

(C) Taphonomic folds and holes in the sheath. Ultrastructure of Polysphaeroides filiformis (D–G), Stigonema robustum CBFS-A027 (H–J) and Bangiopsis franklynottii CCMP3416 (K and L).

(E) Red arrow shows the intracellular space as a black line between the two sides of the sheath that have merged.

(F) The red curly bracket shows a cell within the two sides of the sheath in P. filiformis.

(G) The variable thickness of the two sides of the sheath of P. filiformis is visible, due to variable compression or tangential sectioning.

(H) Stigonema robustum CBFS-A027 has a thin sheath enveloping a thick gel-like matrix in which a cell is embedded (I). The cell has its own cell wall (J).

(K and L) multilayered laminated cell wall with an outer mucilage (polysaccharidic) layer of Bangiopsis franklynottii CCMP3416. Letters correspond to: Resin (R), Sheath (Sh), Cell (C), Cell envelope (CE), Cell wall (CW), Gel-like matrix (G-LM), Laminated cell wall (LCW), Mucilage (M). (Scale bars: A = 20 μm; B = 1 μm, C = 10 μm, D–G = 200 nm; H and I = 2 μm; J–L = 500 nm).