Effects of ambient calcium concentration on the deposition of calcium oxalate crystals in Antithamnion (Ceramiales, Rhodophyta)

A survey of 18 species of the Ceramiales grown in culture revealed calcium oxalate crystals in Antithamnion antillanum Borgesen, A. callocladum Itono, and A. sparsum Tokida. The needle-shaped crystals were present within the cytoplasm of cells of the indeterminate axes but not in cells of the determinate lateral branches. No such crystals were present in A. nipponicum Yamada & Inagaki. The four species of Antithamnion and three additional members of the Ceramiales that do not normally deposit calcium oxalate were grown in natural seawater culture medium in which calcium concentration was elevated by addition of 5-20 mM calcium chloride. Elevated calcium supply did not induce the deposition of calcium oxalate crystals in species that did not previously exhibit them, and it did not change the crystal size range or the cellular or subcellular localization patterns in species that normally formed crystals. At high calcium concentrations, all species ceased growth whether or not they were able to sequester calcium as an oxalate salt. In a second experiment, the four species of Antithamnion were grown in artificial seawater with reduced calcium concentrations. All appeared healthy and grew well in artificial seawater containing 2.5-10 mM CaCl2. In 2.5 mM CaCl2, A. antillanum released tetraspores that grew into thalli having abundant calcium oxalate crystals. Even in 1.0 mM CaCl2, A. callocladum and A. sparsum continued to grow and deposit calcium oxalate crystals of the size range typical for the species. Thalli of all four species died within 12 days if artificial seawater containing only 0.1 mM CaCl2 was provided. Calcium oxalate crystals were present in many cells of the dead thalli, indicating that the crystals did not provide a calcium reserve that could be readily mobilized. Culturing the thalli in complete darkness for 14 weeks also did not stimulate mobilization of the crystals. The presence/absence of calcium oxalate crystals over a broad range of calcium concentrations and light intensities suggests that deposition of this mineral is a constitutive feature and thus could serve as a taxonomic character for these species. It also suggests that calcium oxalate deposition in Antithamnion does not have a role in calcium regulation.