- Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms



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Figure 1: Area of study, the frame of the map sheet Orahovac and the outlines of the countries superimposed on NASA World Wind satellite image.  

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Figure 2: A simplified presentation of the geological setting of the Decastronema sampling site above the bauxite palaeokarst horizon of the Metohia, Mirdita zone, internal Dinarides. 1. Karstified Lower Cenomanian limestone: Pseudorhapydionina dubia (De Castro), Rotalia mesogeensis Tronchetti, Nummoloculina sp. and other miliolids; 2. Three types of Santonian or Lowermost Campanian limestones were recognized: a) limestone with numerous Fe ooids and Rotorbinella scarsellai Torre, discorbids, and rare Clypeina dusanbrstinai Radoičić; b) limestone with dispersed fertile ampullae of Neomeris and c) Limestone with foraminifera Accordiella conica Farinacci, Pseudocyclammina sphaeroidea Gendrot, Dicyclina and Nezzazatinella sp.; red - bauxite; black - lentiform level, in the lower part with layers of densely packed and well sorted fragments of Decastronema kotori. 

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Figure 3: Petrographic thin section of Decastronema kotori (De Castro) combinatio nova: (A) A filament in longitudinal section; note the thinning and bending of diverging outer layers of the wall; (B) Fragmented filaments of varied orientation in transverse and oblique sections; note the conspicuously clear lumina; (C-E) Filaments in nearly longitudinal section, becoming tangential at the lower end where the filaments curve out of the plane of the section; (F) Filament with "terminal chamber", representing a combination of longitudinal and transversal section at the position of a false branch (modified from De Castro, 1975). Scale bar in C is 50 µm long for all pictures.

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Figure 4: SEM images of Decastronema kotori on polished slightly etched rock surfaces: (A) View of a filament almost 1 mm long with upward diverging wall layers; (B-C) Oblique, transverse and longitudinal sections through filaments showing a concentric, upward divergent layering of the wall; the wall is comprised of micritic grains, whereas the lumen is filled with microsparite similar to that occupying pore spaces in the surrounding sediment; (D) Two fossil filaments etched from the matrix by prolonged application of acid. The wall appears spongy for it consists of a Fe-enriched non-crystalline material; (E-F) A similar etching of filaments in longitudinal and oblique sections; (G) Enlargement of a negative print of B, as negative, showing grain relationships in the micritically preserved sheath; (H) Detail of E, showing the spongy Fe-enriched texture of the wall; note the fine submicron-size porosity; (I) Detail of D: The upper filament, a nearly transverse section viewed from below, shows the core and two layers of the wall (comp. with Fig. 5B ). The scale bar in A is 50 µm long; the scale bar in F is 50 µm long valid for B-F; the scale bars in G-I are 10 µm long. 

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Figure 5: SEM of critical-point-dried specimens of Recent (modern) Scytonema sp. from the mats covering intertidal carbonate mud flats on the west coast of Andros Island, Bahamas. (A) Intact, upward curving filaments; the surface shows no appreciable carbonate precipitation. (B) View down a fractured filament of Scytonema showing the concentric arrangement of layered sheath enveloping a trichome cf. Fig. 4I . (C) Oblique view of another fractured filament with a still turgescent cell in the center of concentric sheath layers. Note the reticulate texture of the polysaccharide in the upper part of the picture. Scale bars are 10 µm long; the bar in C is valid for B.    


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Figure 6: Filament tips of modern Scytonema sp. under light microscope showing the development of divergent sheath collars: (A) The tip of the trichome excretes layers of EPS that first stretch and then break as the trichome extends. (B) Sheath production is most intense at the tip and less basipetally. The lower parts of the trichome are enveloped by a series of inserted cones. Note the thinning of envelope margines in both pictures. The scale bar in A is 10 µm for A and 30 µm for B.  

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Figure 7: Two species of Scytonema growing on cliffs wetted by freshwater. They show distinction in size, sheath construction and the ability to support carbonate precipitation. The smaller one (left) is Scytonema julianum with sheaths encrusted with calcite crystals, the larger one (right) is S. myochrous producing divergently layered non-calcifying sheaths. Scale bar is 10 µm. 

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Figure 8: Plot of the size relationships between external diameters (x) vs. internal diameters (y) of filaments in fossil populations of Decastronema (A-C) and Recent populations of Scytonema from Andros Island, Bahamas (1-3). Mean values plot at the point of intersection in each of the cross diagrams, with arms extended for one standard deviation on each side of the mean. The total range of data for each population is inside a field of two standard deviations on each side of the mean. 

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Figure 9: Scytonema mats around mangrove bushes on Intertidal flats of Andros Island, Bahamas form a dense brown cover over white carbonate mud. Inset: Core taken from the same area showing dark Scytonema fragments interlayered by carbonate mud.   

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Figure 10: Aerial view of tidal creeks on the NW coast of Andros Island, Bahamas, rimmed by Scytonema cover (reddish brown) around bright carbonate mud in shallow ponds. 


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Figure 11: Westward view of the ponded intertidal mud flats of Andros Island, a possible analogue of the environmental setting of the Cretaceous Decastronema. 



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