Skeletal ultrastructure of the calcified red alga Galaxaura oblongata, Hainan Island, China

Aragonite calcification in the red alga Galaxaura oblongata (Ellis et Solan der) Lamouroux (Chaetangiaceae, Nemaliales) from southern China, occurs at three sites in the cortex: (1) intercellular spaces (ICS); (2) the cell wal l, and (3) the inner surface of the cortex. The ICS is the principal site o f calcification and contains aragonite mainly in the form of equidimensiona l granules similar to 1-2 mu m in size, although needle-like crystals up to 3 mu m in length are also present locally Granules range from densely to l oosely packed. A dense and even layer peripheral to the ICS is interpreted to represent calcification of the cell wall. Coarser radial clumps of elong ate crystals, forming a thin discontinuous layer on the inner surface of th e cortex, resemble a cement fringe facing into the medulla, but form during the life of the alga. Calcification in these specimens shows similarities, and differences, to both coralline red algae, in which calcification is li mited to the cell wall, and halimedacean green algae in which it is limited to the ICS. Some members of the Palaeozoic family Moniliporellaceae show s imilarities in their skeletal organization to Galaxaura. (C) 1999 Elsevier Science B.V. All rights reserved.