SECT. 2] OKGANIC KEGULATION OF PHYTOPLANKTON FERTILITY 205 



L. leichmannii) and many unicellular algae utilize only true B12 (Table XI). 

 But the assays for Long Island Sound (Vishniac and Riley, 1959) were done with 

 a true Bi2-specific organism. The quantities found in Long Island Sound do not 

 differ substantially from the data of Droop and Lewin for other coastal waters ; 

 all of them show ample B12. It is probable, then, that in inshore waters B12 is 

 rarely limiting and not a constraint on fertility. But when the quantities of 

 Bi2-like cobalamins are far lower, as in the open sea, the specificity of the 

 various algae toward the different cobalamins may be decisive. For this 

 reason much work has been done to determine the specificity of the algal 

 species (Droop et al., 1959). Since specificities vary widely (Table XI) both 

 among the species and the ecologically important algal groups, it becomes 

 necessary to measure the Bi2-like cobalamins with several bioassay organisms 

 and to determine the ratio true Bi2/total cobalamins (as recommended 

 by Cowey, 1956; Droop, 1957, and Provasoli, 1958a). Specificity alone is not 

 enough ! 



Sensitivity of the various algal species toward true B12 (Provasoli, 1958a) 

 and probably to the other cobalamins varies also (Ford, 1953). While there are 

 no reasons to doubt that the specificity data based on in vitro studies can be 

 transferred to the ecological situations, sensitivity data are biased. Sensitivity, 

 i.e. the dose-growth response, does not depend solely on the variable to be 

 measured but is obviously influenced by all other cultural conditions, especially 

 the composition of the medium and temperature. After all, the improvement of 

 media and of cultural conditions and the standardization of the inoculum are 

 the routine procedure employed to develop a bioassay, speed of growth and 

 sensitivity being the desiderata of a workable assay (Hutner, Cury and Baker, 

 1959; Hutner, Provasoli and Baker, 1961). Laboratory assay basal media are, 

 from an ecological standpoint, grotesque in the way all constraints on growth 

 have been removed save for the vitamin being assayed : this permits maximum 

 sensitivity in the laboratory. The routine artificial marine media, even when not 

 tailored for a specific organism, also lack many constraints : they generally 

 allow a growth equal or superior to the one recorded in natural blooms, even 

 though light conditions are in general not optimal (200-400 ft-candles of 

 fluorescent light). 



Artificial media are extremely useful in detecting the needed and the utilized 

 metabolites ; this knowledge is of ecological importance but the data on the 

 sensitivity to metabolites, acquired in artificial media, cannot be extrapolated 

 directly to the natural environment because each sample of sea-water is in 

 effect a different basal medium (Johnston, in press). Samples of sea-water en- 

 riched with one-fifth volume of S36 medium (which contains vitamins, N, P, Si 

 and trace metals) inoculated with bacteria-free Skeletonema costatum support, 

 depending upon the sample of sea-water, more or less growth than occurs in 

 the undiluted S36 medium taken as a yardstick. Obviously other properties of 

 these different sea-waters affect the growth response of equal levels of nutrients, 

 including vitamins. Conversely, a level of vitamins and nutrients which in 

 artificial media supports a defined number of cells may support more or less 



