The mineral content has been determined in a total of 16 species from four genera of calcareous algae all belonging to the family Udoteaceae. The most heavily calcified species are those belonging to the genus Halimeda. Udotea is the least calcified genus. Rhipocephalus
and Penicillus take intermediate positions. The maximum mineral content expressed in percentage of the algal dry weight is as follows: Halimeda, 97 per cent; Penicillus and Rhipocephalus, approximately 60 per cent; and Udotea, 37 per cent. Within the genus
Halimeda, the maximum mineral content varies with species. The approximate figures are: Halimeda gracilis and H. copiosa, 97 per cent; H. opuntia and H. discoidea, 90 per cent; H. tuna, 80 per cent; H. goreauii, 50 per cent; and H. cuneata,
33 per cent. In some species of Halimeda, there is a tendency for the mineral content to increase with depth. The MgCO3 concentration varies with genus and species, and within different parts of the same organism. In Halimeda, Penicillus, and Rhipocephalus,
MgCO3 is in the region of 0.2-3.0 mol %. In Udotea, MgCO3 tends to be higher and in the range 3-4 mol %. In most algae investigated, the MgCO3 concentration was found to be higher in the younger, scarcely calcified segments in the apical region than
in the fully developed, more heavily calcified segments farther below. Juvenile individuals of Udotea and Penicillus also contain more MgCO3 than adult individuals. Halimeda cuneata is unusual, in that the apical segments are more heavily calcified than the
fully developed segments near the branching points. A comprehensive analysis of all major cations and anions is given for the alga Halimeda opuntia.
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