ECOLOGY AND TAXONOMY OF Holimeda 



195 



released carbon dioxide lowers the pH and concentration of carbonate 

 ions in the spaces, inhibiting calcification. 



Before light stimulation of calcification can occur the photosyn- 

 thetic rate must exceed a certain threshold value (Borowitzka and 

 Larkum, 1976b), and the rate of carbon dioxide removal by photosyn- 

 thesis from the spaces must exceed, by a certain factor, the supply 

 entering the spaces. A model of this scheme is shown in Fig. 60. 



Fig. 60. A model of calcification in Halimeda, which takes place within the spaces of 

 the segn:ient. These spaces are seiDarated, in most species, from the outer seawater 

 environment by a fusing or adhering of the covering lamellae of adjacent peripheral 

 utricles. The extent of the adhesion depends on the species and somewhat on the 

 age of the material. The postulated movements and fluxes of ions during photo- 

 synthesis, which takes j)lace within the jDerijDheral utricles and to a lesser extent in 

 the remainder of the filament system of the segment, are shown. These include a 

 light-stimulated proton flux. The end result is the precipitation of calcium carbonate 

 in the interutricular and interfilamental spaces of the segments. Black dots at the 

 plasmalemma indicate fluxes postulated to be active. (Modified from Borowitzka, 

 1977.) 



4. Progress in calcification studies: a review 



Calcification, although not widespread among algae, occurs regu- 

 larly in over 100 genera. The calcium carbonate crystals are deposited 



