234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



the element. In an aqueous solution phosphates would have main- 

 tained a simple chemical form much longer than phosphates fused 

 in rock composed largely of other complex-forming minerals. It is 

 possible, therefore, that the first unicellular living organisms of the 

 sea were able to obtain their phosphorus requirements from this orig- 

 inal solar supply. Even so, it would have been a limited source of 

 phosphorus, in time becoming inadequate for the demands of expand- 

 ing marine life. The principal store of phosphorus was in the land's 

 igneous rock surface. Although apatite is insoluble in water, it is 

 slowly and slightly dissolved by carbonic acid (formed when carbon 

 dioxide dissolves in water). Eain water, thus acidified, steadily ex- 

 tracted minute amounts of phosphate from the apatite. There were 

 then neither soils nor land plants to utilize any of this dilute solution 

 of phosphate as it ran off into streams and sea-bound rivers. But in 

 the seas the phosphate could be promptly utilized by marine plants. 



Higher forms of sea life evolved. Then, as now, one form of life 

 in the seas fed upon another. The same "quota" of phosphorus could 

 support a succession of lives and thus be indefinitely retained within 

 the biological sequences of the marine cycle. The initial assimilation 

 of phosphate is largely made by algae, minute diatoms which multiply 

 by self-division like yeasts ; but their synthesis of phosphate and other 

 simple nutrients in the sea's solution is dependent upon a supply of 

 energy from sunlight (i. e., they are photosynthetic), and their float- 

 ing existence is confined to the upper levels of the sea. In the mass 

 these algae are known as plankton. Plankton is the food for zooplank- 

 ton, larger and longer-living forms of very simple sea life. Young 

 fish when hatched feed first upon the plankton and then upon the 

 zooplankton; later, however, most sea fish feed carnivorously, upon 

 other species and even upon their own species. 



However, all sea life does not die by becoming the diet of other 

 organisms; were this so, there would be no losses of biologically or- 

 ganized phosphorus from the marine cycle. A large proportion of 

 sea plants and sea animals die naturally. Their remains sink to lower 

 depths. The eventual decomposition of this organic matter returns 

 the phosphorus (and other nutrients) to the sea. But most of this 

 liberation takes place at depths beyond the sun's penetration; the 

 simple nutrient-assimilating organisms are not present to remobilize 

 this phosphate. There is, therefore, in the lower water levels of the 

 seas a steady building up of phosphates and other nutrients. How- 

 ever, this phosphate is not accumulatively retained in the sea solution. 

 Some of it is assimilated by crustacean and other bottom-living or- 

 ganisms ; much of it is steadily precipitated to the sea floor ; and in 

 temperate regions there are regular inversions of the upper and lower 

 layers of sea water which result in the further utilization of deep-sea 

 phosphate for plankton growth. 





