194 L. HZLLIS-COLINVAUX 



type seawater-polysaccharides-skeleton, with free space and organelles 

 qualifying as additional compartments. The dynamics of the system 

 include the uptake of calcium from the seawater and its release into the 

 seawater. Some calcium is deposited and remobilized in the skeleton, 

 with most if not all of the calcium that is returned to the seawater 

 originating from the carbonate deposits. The water-soluble poly- 

 saccharides of the wall mcorporate calcium 7-14 times more rapidly than 

 the calcium carbonate deposits. Bohm and Goreau did not, however, 

 obtain conclusive evidence for linking calcium binding of the wall and 

 crystallization. 



Many aspects of this work have been continued by Borowitzka and 

 Larkum (1976a, b, c, 1977) working in Australia with the species 

 cylindracea, discoidea, macroloba and tuna from the Great Barrier Reef. 

 Lheir experiments with the living alga utilized branches of seven or more 

 segments rather than entire thalli. Light intensities in photosynthesis 

 experiments, measured at the water surface, were 2800 lux, while 

 Halimedae which were planted but not yet used in experiments at the 

 time received 1200 lux at the water surface over a 16-hour light period. 

 Carbon- 14 techniques were used for much of the photosynthetic work. 



Their study included an investigation of the exchange of calcium 

 between seawater and alga using ^^Ca, of the sources of inorganic carbon 

 for photosynthesis and calcification, and of the effects of metabolic 

 inhibitors on these two processes. Part of the work included calculations 

 of the effects on pH and on the concentration of carbonate ions in a 

 closed seawater system such as the spaces of a Halimeda segment, when 

 carbon dioxide and bicarbonate ions are removed by photosynthesis, 

 and calcium carbonate precipitation occurs. 



From their work and the results of others, they concluded that 

 calcification in Halimeda is primarily a function of the anatomy of the 

 alga and the uptake of carbon dioxide during photosynthesis, and that 

 the calcium-binding polysaccharide of the wall plays little or no role in 

 the process. 



Since the spaces of the segment are enclosed, entry of ions into them 

 must be by movement through the confluent walls of the peripheral 

 utricles or through the filaments. Borowitzka and Larkum considered 

 that the pathway of carbon and calcium to the spaces was mamly by 

 diffusion through the shared peripheral utricle walls. In the light, 

 carbon dioxide uptake for photosynthesis from the seawater of the 

 spaces, which is supersaturated with calcium carbonate, results, 

 according to their calculations, in an increase of pH and in the con- 

 rentration of carbonate ions within the spaces, thereby stimulating the 

 cate of aragonite precipitation. When respiration only takes place, the 



