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Calcification in the green Alga Halimeda. I. An ultrastructure study of thallus development

Borowitzka, M.A.ORCID: 0000-0001-6504-4563 and Larkum, A.W.D. (1977) Calcification in the green Alga Halimeda. I. An ultrastructure study of thallus development. Journal of Phycology, 13 (1). pp. 6-16.

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The ultrastructure of 4 species of the calcareous, siphonaceous alga Halimeda (H. cylindracea Decaisne, H. discoidea Decaisne, H. macroloba Decaisne and H. tuna (Ellis & Solander) Lamour) has been studied, and the observed changes during growth and development are related to changes in the degree of calcification. A distinct gradient in the types and quantities of cell organelles exists in a growing apical filament. As these filaments grow, branch, and eventually develop into a mature segment, changes in the organization of organelles such as mitochondria and chloroplasts are observed. Calcification begins when the chloroplasts reach structural maturity and when the peripheral utricles adhere (fuse). This adhesion of the peripheral utricles isolates the intercellular space (ICS) in which calcification occurs from the external seawater. Calcification begins in the outermost (pilose) cell wall layer of the walls facing into the ICS. The cell walls at the thallus exterior undergo extensive changes after utricular fusion; the pilose layer is lost, the cuticles of adjacent utricles fuse forming a ridge at their junction, and multiple cuticles are formed. The aragonite (CaCO3) crystals which are initially precipitated within the pilose wall layer, rapidly increase in size and number, eventually filling much of the ICS. Only the initial nucleation of aragonite is associated with the pilose wall layer, the later precipitation of aragonite is totally independent of the pilose layer. In older segments secondary deposition of CaCO3 also occurs around existing aragonite needles.

Item Type: Journal Article
Publisher: Blackwell Publishing
Copyright: © 1977 Wiley-Blackwell
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