(deposition of CaCO,) counterbalances this trend by reducing the 

 buffering capacity of the upper oceans, and increasing the partial 

 pressure of dissolved C0 ? . All of the CaC0 3 in the earth's crust 

 has been deposited as a result of biological activity. The CaCO-, 

 reservoir is the largest pool of non-aqueous carbon on earth. 

 Curiously, despite the importance of calcification in the 

 deposition of carbon on geological time scales, almost nothing is 

 known about the molecular basis of the process. 



Three major groups of oceanic planktonic organisms, 

 foraminiferans, coccolithophores, and pteropods, secrete a calcium 

 carbonate skeleton. Among the benthos, certain areas of the oceans 

 are dominated by organisms that deposit calcium carbonate. For 

 example, approximately 15% of the shallow sea floor between and 

 30 m is occupied by coral reef communities whose primary 

 productivity and calcification contribute significantly to global 

 carbon flux. Unexpectedly, coral reefs from different latitudes 

 and with different biological components exhibit considerable 

 physiological uniformity. Their photosynthesis: respiration ratio 

 is about 1.15, and their rate of calcium carbonate deposition is 



.0.1. 



about 4 kg CaCO, my. This apparently "standard rate" provides 

 a quantitative basis for checking the effects of large-scale 

 environmental stress (i.e., at the community level). In fact, 

 nutrient overload (e.g., elevated nitrogen and phosphorus) changes 

 the standard rate of community metabolism which can be measured 

 long before any changes in community composition can be detected 

 visually. 



Organic matrix proteins secreted by calcifying tissues are 

 thought to control both calcium carbonate nucleation and species- 

 specific skeletal architecture. Molecular biological approaches 

 may provide insight into the synthesis of matrix proteins, many of 

 which are beta sheets rich in aspartic acid residues: they may be 

 thought of as calcium carbonate synthetases. 



II-7 



