Calcium oxalate crystals in the marine red alga Spyridia filamentosa (Ceramiales; Rhodophyta)

Birefringent polyhedral crystals up to 8 pm on a side were found both within cells and on the thallus surfaces of several genetically distinct isolates of the red alga Spyridia filamentosa (Wulfen) Harvey. Diagnostic chemical solubility tests and silver nitrate-dithiooxamide staining identified the crystals as calcium oxalate. Calcium oxalate crystals were observed in all cell types of the vegetative erect axes: axial cells of indeterminate filaments, corticating cells of the main axes, and axial and corticating cells of the determinate lateral filaments (brachyblasts). Thalli cultured in natural seawater medium had few crystals close to the growing apices, whereas older portions of the thalli developed a surface coating of fine crystals and scattered larger extracellular and intracellular crystals. Thalli cultured in natural seawater medium whose calcium concentration was supplemented with 20 mM calcium chloride had an abundance of intracellular and extracellular calcium oxalate crystals. Thalli grown in artificial seawater medium having low calcium concentrations (1.0 and 2.5 mM calcium chloride) deposited few calcium oxalate crystals, but when these thalli were transferred to calcium-rich medium and stressed, crystals formed rapidly. The nonconstitutive nature of calcium oxalate deposition in Spyridia may explain conflicting previous accounts regarding presence of calcium oxalate crystals in this genus.