Calcium exchange and the measurement of calcification rates in the calcareous coralline red alga Amphiroa foliacea

45Ca washout data of living thalli of Amphiroa foliacea Lamouroux in the light and dark show that there are three kinetically distinct Ca2+-exchanging compartments with approximate half-times (t 1/2) of 300, 20 and 3 min. The two slower compartments appear to be exchange from organic Ca2+-binding components of the cell wall, while the fast compartment probably represents exchange on the CaCO3 crystal surface. Killed and decalcified thalli have a fourth compartment, with a t 1/2 of 20 to 35 min (other compartment half-times are ≈ 300, 40, 3 min), which has been identified as the greatly increased intercellular space produced during drying and decalcification. The 45Ca and 14C uptake data show that a large proportion of the label initially taken up is into compartments other than the CaCO3. As a result of this uptake, binding, and exchange of radioisotope, significant errors occur during the measurement of calcification rates, unless a kinetic analysis is carried out. Using such a technique, CaCO3 calcification rates of A. folicea were measured with 45Ca or 14C as tracers. Light stimulates calcification by up to 2.6 times, depending upon the age of the plant. Young segments have a markedly higher rate of calcification and photosythesis than do the older segments