DISSOLVED SALTS AS LIMITING FACTORS 



199 



large amount used annually— now estimated 

 at 73± 18 X 10" tons by land plants alone 

 (G. A. Riley, 1944)— indicated the proba- 

 bility that carbon dioxide vvas, in fact, a 

 limiting factor for photosynthesis, an indi- 

 cation that has proved to be true (E. C. 

 Miller, 1938). In this case the amount of 

 the limiting substance regularly present 

 remains remarkably constant in the atmos- 

 phere, and, as frequently happens, no good 

 estimates exist of the fate of accretion of 

 useable carbon dioxide other than those ob- 

 tained by considering the rate of consump- 

 tion. 



Unlike the illustration just given, quanti- 

 tative estimates of the available amount of 

 possibly limiting substances may give little 

 information about their relative importance. 

 This point can be made by considering the 

 role of nitrates, nitrites, and ammonia as 

 nitrogen sources for organisms. These forms 

 of fixed nitrogen, considered as such, occur 

 in sea water in the following ratios. 



6ONO3 : 4NO2 : 5NH3 



Generahzing from what is known about the 

 use of nitrogen by land plants (cf. Miller, 

 1938), and without knowledge about such 

 factors as the kind and age of plant, pH of 

 the medium, the other ions present, and 

 light, one would be unable to judge with 

 certainty concerning the relative value of 

 the nitrates and ammonia salts, but might 

 hazard the guess that the nitrate nitrogen, 

 being twelve to fifteen times more concen- 

 trated, would usually prove to be the most 

 useful. Sverdrup, Johnson, and Fleming 

 (1942, p. 225) cite evidence that "supports 

 the theory that marine plants can use any 

 of these inorganic forms of niti-ogen equally 

 well." If so, the nitrates should still be the 

 most important because of the greater 

 quantity present. 



Such a conclusion does not necessarily 

 follow. Dr. Garrett Hardin" states the case 

 thus: 



"The usual quantities of nitrate, nitrite and 

 ammonia nitrogen might lead one to suppose 

 that nitrogen is supplied to the system mostly 

 as nitrate. There are reasons for supposing that 

 this is not the case. In the first place, ammonia 

 nitrogen is a universal product of cell metab- 

 olism and organic decomposition, whereas the 

 other two forms are uncommon products of 

 either normal metabolism or decomposition. 



' Personal communication. 



Most of the nitrite and nitrate production re- 

 sults from secondary decomposition of the 

 ammonia by such bacteria as Nitrosonionas and 

 Nitrobacter, and will be dependent on the 

 supply of ammonia-N. In tlie meantime, much 

 of tlie ammonia-N may be removed and used 

 for cell synthesis by micro-organisms of all 

 sorts— bacteria, molds and algae. The nitrate 

 content of the environment will show a relative 

 rise simply because it is a less useful material 

 than is ammonia. One can safely generalize and 

 say that all micro-organisms can use ammonia 

 nitrogen. But a great many cannot use nitrates 

 (true of many bacteria and molds; but ap- 

 parently of no algae); and those that can use 

 both, utilize ammonia-N more readily. Con- 

 sequently nitrates may pile up. . . . Chlorella 

 vulgaris furnished with both nitrate-N and 

 ammonia-N will use up all the ammonia before 

 beginning on the nitrate (Pratt and Fong, 

 1940). In this way one may be completely mis- 

 led as to the relative importance of ammonia 

 and nitrates in nature, simply because data 

 concerning the amounts present only are avail- 

 able. One needs to know the rate of input." 



FIXED NITROGEN 



Nitrates, nitrites, and ammonia are car- 

 ried to the sea in substantial quantities by 

 rivers and are washed directly into the 

 ocean from the atmosphere by precipitation. 

 The Mississippi River alone carries some 

 361,000 metric tons (2204.6 pounds per 

 ton) of nitrate (NO3) nitrogen annually 

 into the Gulf of Mexico.* In regions with a 

 precipitation of about 23 to 33 inches an- 

 nually, one or two metric tons of ammoni- 

 acal or nitrate nitrogen are annually swept 

 down from the atmosphere for each square 

 mile of land or ocean. f 



Such sources are the more important, 

 since they bring usable nitrogen into sur- 

 face waters, where it is available for pro- 

 tein-building by plant plankters These or 

 the animals higher in the food pvramid 

 eventually die, and their protoplasm disinte- 

 grates as the dead bodies sink; only the 

 larger carcasses or the more resistant parts 

 reach bottom in deeper water. The rather 

 wide depth zone in which the nitrogen is 

 returned to solution in the water as am- 

 monia, nitrite, or nitrate is called the zone 

 of nitrogen regeneration, in contrast with 

 the zone of nitrogen utilization, which oc- 

 curs in the upper, better-lighted waters. The 



" Calculated from data given by Clarke 

 (1924). 



-}• Calculated from data given by Schreiner 

 and Brown (1938). 



