Microbial debris in Rhynie chert
deutsche Version

Black dust grains or flakes of no definite shape, and loose aggregates thereof, are not rare in the Rhynie chert. They may be found among tissue or along former gel surfaces formed during silicification of the silica-rich water. Th
e shapeless black debris is seen in Fig.1 below the fungus sphere, in the narrow gap of gel cracks, and along the faces of wide gel cracks filled with bluish chalzedony. Observations on several samples have raised the suspicion that the black matter is possibly the carbonaceous remains of degraded microbial coatings. The present sample offers evidence in favour of this idea.
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Fig.1 (far left): Small amounts of black matter along former silica gel surfaces.

Fig.2 (left): "Hard" black debris below and "soft" brown formations above, suggesting microbial colonies on former silica gel surfaces not seen here.
Height of either picture 1mm.

Fig.3 (below): Hyphae of an aquatic fungus coated with silica gel, overgrown with microbial slime turned into black debris, finally coated with more layers of silica gel.
Width of the picture 1.3mm, same magnification as Figs.1,2.

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The brown formations of fluffy appearance as seen in Fig.2 are seldom preserved in the chert. Probably they degraded and shrunk into black debris soon. This is supported by arrangements like the middle one in Fig.3, where a microbial colony grew on a coated hypha of an aquatic fungus inside hollow Aglaophyton (not shown here), then decayed soon and turned into black debris before further silica was being deposited. Deposition went on for a longer while until a thick multi-layered coating, diameter 0.34mm, had built up. The less compact arrangement of the debris around the hypha on the left in Fig.3 seems to be due to branching of the hypha. It shows that the debris is essentially the same as in Fig.2.

What is seen on the present sample seems to be related to other phenomena in the Rhynie chert. As mentioned earlier, the dark aspect usually observed with hollow straws of Aglaophyton and with the dark rings on cross-sections of Ventarura is probably brought about by microbial deposits on the cell walls, and "the dark deposit is not always strongly bonded to the substrate but may become reduced to detached flakes", as stated in Rhynie Chert News 83 and shown in Rhynie Chert News 60, Fig.7.
In view of the related observations mentioned here it may be justified to say that Fig.2 offers a rare sight strongly suggesting an interpretation of the black dust-like particles as the remains of microbial colonies.

H.-J. Weiss   2015
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