MULTIMINERAL CALCAREOUS DEPOSITS IN THE MARINE ALGA ACETABULARIA ACETABULUM (CHLOROPHYTA; DASYCLADACEAE)

The ultrastructure and mineralogy of cultured and wild specimens of the green algae Acetabularia acetabulum were studied. Sites of calcification were clarified using TEM and SEM. Minerology was determined via electron diffraction, energy‐dispersive x‐ray microanalysis, and Fourier transform infrared spectral analyses of the calcareous deposits. These deposits were found covering the surface and within the cell wall of the cultured juveniles and adult wild specimens of A. acetabulum. Deposits within and directly over the wall were amorphous calcium carbonate (ACC) in granular form. In‐wall and outer cell wall ACC appear to develop independently from each other. The formation of the in‐wall ACC may be mediated by the wall structure and/or chemistry, whereas mucin may be the factor mediating formation of outer cell wall ACC. In contrast to the stable in‐wall ACC, outer cell wall ACC could be transitory, transforming into aragonite. In cultured cells, a small amount of monohydrocalcite crystals and calcite were found. It is possible that these were transformation products of ACC. In wild specimens, whewellite (calcium oxalate monohydrate) was present inside and outside of the cap wall. It is unclear if whewellite is present within the cytoplasm. Local and temporal differences in the microenvironments for the multiphase mineralization are suggested.