Chapter XI IT 



THE PHOSPHORUS CYCLE 



The phosphorus cycle is relatively simple compared with cycles of 

 nitrogen, sulfur, or carbon. Plants utilize phosphate phosphorus which 

 they convert into organic phosphorus compounds. Animals depend upon 

 plants for their phosphorus requirements. Bacteria fulfill a most impor- 

 tant function in the regeneration of phosphate from the remains of ani- 

 mals and plants. The storage of phosphorus in bacterial cells and the 

 effect of bacterial activity on the solubihty of phosphate are of secondary 

 importance in the phosphorus cycle in the sea. 



Assimilation of phosphate by microorganisms: — Phosphate is es- 

 sential for the growth of phytoplankton. At times a lack of phosphate 

 may limit the primary productivity of the sea (Redfield, 1934). Atkins 

 (1926) discusses certain hydrographic factors which influence the replen- 

 ishment and utilization of phosphate in the English Channel, where the 

 vigorous growth of phytoplankton often leads to a depletion of phosphate. 

 In such an event, the area has yielded its maximum production until phos- 

 phates are regenerated by bacterial activity or replaced by the influx of 

 water from nutrient-rich areas. 



The utilization of phosphate and nitrate in the synthesis of organic sub- 

 stances proceeds at approximately parallel rates, according to Sverdrup 

 et al. (1942). Any deviation from the nitrate-phosphate ratio is believed 

 to be dependent primarily upon factors which influence the rate of nitrifi- 

 cation or the bacterial regeneration of phosphate. The ratio of nitrate-N 

 to phosphate-P in offshore waters is approximately the same as the ratio 

 of organic-N to organic-P in marine plankton, namely 7:1. This agree- 

 ment suggests that the composition of the composite animal and plant 

 population of the sea is determined by the rate of available nitrogen to 

 phosphate-P in sea water. 



The optimum concentration of phosphate-P for the development of 

 diatoms is about 50 mgm./M.', according to Harvey (1933). Water 

 throughout the euphotic zone rarely contains more than 5 mgm./M.^ 

 during the photosynthetic season, and generally less than i mgm./M.^ 

 At such times a majority of the total phosphorus content of the water oc- 

 curs in plant and animal tissues. Upon the death of the organisms, phos- 

 phate is rapidly regenerated. 



As pointed out by Renn (1937a), bacterial cell substance is notoriously 

 rich in phosphorus. How^ever, it is apparent from their minute size and 

 small numbers that bacteria do not compete effectively for the sea's lim- 

 ited store of this element. There is no evidence that phosphate limits 

 bacterial multiplication in sea water unless the latter is experimentally 

 enriched with organic matter. Renn noted a slight decrease in the phos- 

 phate content of sea water incubated in the dark, followed by an increase. 

 He attributed the initial decrease to the assimilation of phosphate by 

 bacteria, and the increase to the liberation of phosphate from organic com- 

 pounds including autolyzing bacterial cell substance. 



