By John L. Wray (Eds.)
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Extra resources for Calcareous Algae
Intracellular Cell wall Extracellular Surficial Fig. 10. Diagrammatic sketch of calcium carbonate deposition sites in algae. (After Arnott and Pautard, 1970). In general, internal calcification is related to cellular morphology, metabolism, and the selective absorption or assimilation of specific carbonate salts. Little-understood, complex reactions of these factors result in carbonate precipitation and distribution within the thallus. The calcium carbonate is a consequence of the interaction of carbonate, or biocarbonate, with calcium under alkaline conditions, but the process by which carbon dioxide is released or absorbed to achieve this result is not clear.
The skeletal internal structure found in segmented erect plants belonging to the dasycladacean green algae represent a separate category not comparable genetically to the two previous types, because calcification occurs only on external surfaces of stems and branches rather than on filaments within the thallus. These skeletal algae appear typically as perforated cylinders and spheres whose internal organization consists of calcareous molds of an axial stem. and several generations of radiating branches (Fig.
Most forms are characterized by rather complex thalli which are distinctively colored by red pigments. Biologically, the group shows considerable uniformity and may represent a long evolutionary history, possibly having evolved from the blue-green algae during a very early time. The Rhodophyta seem to be intermediate between the Cyanophyta and Chlorophyta, having plastid characteristics similar to the blue-green algae, and a eukaryotic organization, a nuclear division (mitosis), and wall characters similar to the green algae.