Calcification in the green alga Halimeda: II. THE exchange OF ca2+ AND the OCCURRENCE of AGE gradients IN calcification AND photosynthesis

In Halimeda cylindracea and H. tuna segments, the concentration of CaCO3, MgCO3, protein, and chlorophyll, as well as segment volume and wet and dry weight, increase with 'age' i.e. from the apex of a branch downwards. Photosynthetic and calcification rates decrease with age as does the degree of light stimulation of calcification.Studies of the exchange of 45Ca between the Halimeda thallus and the sea water under various conditions showed that most of the Ca exchange is between the cell walls, the aragonite crystals, and the intercellular space. The cell wall has two distinguishable phases with half-times (t0·5) of 200 and 35 min while the CaCO3 has a rapidly exchanging phase with a t0·5 of approximately 6 min. The t0·5 of the exchange of Ca between the intercellular space and the external medium is estimated at about 6 min, on the basis of uptake studies. If the integrity of the barrier between the intercellular space and the external sea water, created by the adpressed peripheral utricles is destroyed the t0·5 is smaller (≪3 min).These kinetic studies as well as comparative measurements of calcification rates by both isotopic and chemical methods show that the 45Ca method for measuring calcification rates overestimates the calcification rate, due to binding of 45Ca in the cell walls and retention of 45Ca in the intercellular space. The 14C method gives more accurate results and has the further advantage of allowing simultaneous measurement of the photosynthetic and calcification rate on the same segment