ECOLOGY AND TAXONOMY OF Holimeda 189 



1967) have shown that only aragonite is deposited in the genus, all the 

 then known species being tested. 



The crystals are generally needle-shaped, reaching about 10 ]xm. in 

 length, 0-08-0-60 [iva. in width and 0-01 \iva and less in thickness 

 (Wilbur et al., 1969; Marszalek, 1971; Borowitzka et al., 1974). Wilbur 

 et al. (1969) observed granular and polygonal crystals, up to 0-6 \xvii in 

 diameter, in some material of incrassata. The size and numbers of 

 crystals vary with the age of segment, with the species and to some 

 extent from specimen to specimen of the same species, particularly 

 specimens from different sites. 



Carbonate deposition seems to be an important function in the 

 metabolism of Holimeda, and needs to be understood both as a physio- 

 logical process, and as an adaptation to life in a reef. 



In its simplest form the reaction of calcification is the following : 



Ca2+ + 2HC03- , CaCOgj + HaO-fCOa 



That more than a physical precipitation from a supersaturated solution 

 is involved for many organisms is shown by the isotopic composition of 

 the algal carbonate. That of Halimeda is enriched in ^^C and poor in 

 1^0 as compared to natural limestone (Fig. 57), and the organic matter 

 of this alga is enriched in ^^C (Milliman, 1974). These differences indicate 

 metabolic involvement of the plant in calcification. 



1. Aragonite deposition: a process working outside filament walls 



Askenasy (1888) in an early microscopical study of the calcium 

 carbonate deposits in Halimeda observed that deposits were present 

 soon after the segment was completed, that they increased with age, 

 and that deposition began on the outer surface of the lateral walls of the 

 peripheral utricles and soon spread over the entire space between them. 



Few further studies were made of the calcification of any alga until 

 the 1960s (Lewin, 1962), by which time new techniques and equipment, 

 particularly radiocarbon isotopes and the electron microscope, had 

 become available. 



These new tools, however, had already been applied, in the years 

 before Lewin's review of the subject, to studies of animal calcification, 

 and research by two workers into calcification included algae as well. 

 Goreau, working on the reefs of Jamaica and calcification in corals, 

 included the calcareous red and green algae, and Wilbur at Duke 

 University, working with molluscs, included coccolithophorids. 



It was known from the work of Wilbur and others that calcium 

 carbonate deposition in molluscs was associated with an external 



