ECOLOGY AND TAXONOMY OF Halimeda 193 



adhere laterally, the solution fillmg these spaces is not in free com- 

 munication with the surrounding seawater except in the very youngest 

 segments. They considered that a completely enclosed volume within 

 the segment would favour crystallization, but that crystallization might 

 also occur without complete isolation from the seawater outside. 



2. Calcium-binding properties of the filament wall 



With the first aragonite crystals being formed in close proximity 

 to the filament wall, it would seem natural to scrutinize the nature and 

 activities of the wall more closely. Bohm (1972) reported the presence 

 of a calcium-binding polysaccharide fraction m the wall of opuntia. In a 

 more detailed study of the mucilages (water-soluble polysaccharides) 

 the calcium-binding strength was described as low and of the same 

 order of magnitude as succmate (Bohm, 1973b). 



The fibrous matting layer, on the basis of tests with ruthenium red 

 and proteolytic enzymes, appears to be polysaccharide (Borowitzka 

 and Larkum, 1977). 



3. Calcification and metabolic activity 



The enhancement of calcification by light was initially demonstrated 

 by Goreau (1963) working with eight species of Halimeda, and later by 

 Stark et al. (1969), the latter group working with opuntia and discoidea 

 in the reefs of Puerto Rico and the laboratory of the University of 

 Maryland. Goreau's results were, however, somewhat clouded by 

 greater rates being obtamed for deposition of calcium carbonate in the 

 dark with four of the eight species. The overall average for the eight 

 species, however, gave a greater light : dark ratio. The difficulty appears 

 to lie in the *^Ca methodology which included a short labellmg time 

 followed by a long wash in unlabelled medium (Borowitzka and Larkum, 

 1976a). 



Calcium pathways were first worked on by Stark et al. (1969) and 

 Bohm and Goreau (1973). By comparmg the differential washout rates 

 for *^Ca absorbed in the light with those obtamed in the dark, Stark et 

 al. (1969) suggested a two-step mechanism in the calcification process. 

 The ions were first bound to the wall and their ionic concentration 

 increased. Then there was precipitation. 



Bohm and Goreau (1973), from an analysis of theii' ^^Ca activity 

 data, proposed a two-compartment exchange system, the seawater-alga 

 system. They considered that at least two pools existed within the alga, 

 making the whole at least a three-compartment catenary system of the 



