Article: Calcified Plectonema (blue-green algae), a Recent example of Girvanella from Aldabra Atoll

Girvanella, not previously reported from rocks younger than Cretaceous, is described from the Recent. It occurs as the calcified sheath of the filamentous blue-green alga Plectonema gloeophilum Borzi in small freshwater pools on Aldabra Atoll in the Indian Ocean. The sheaths are heavily impregnated during the life of the alga by micrite grade crystals of magnesian calcite which make them hard and potentially fossilizable and which preserve the filaments as small calcareous tubes 4-10 ftm in diameter. This confirms the view that Girvanella represents a filamentous blue-green algae preserved by the calcification of its sheath, and it extends the range of the genus from Cambrian to Recent. The consistent relatively thin-walled morphology of Girvanella-type impregnated sheaths contrasts with the thickly microsparite encrusted sheaths of blue-green algae in environments of CaCO3 cementation. This suggests a greater degree of control of the calcification process in Girvanella and supports the concept of specificity for calcification in some blue-greens. P. gloeophilum is believed to be only one of a number of extant filamentous blue-greens whose calcified sheaths are referable to Girvanella. At present Girvanella is still most conveniently placed in the Porostromata, which should be regarded as a group of tubiform calcareous algae.