SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 203 



almost always produce enough vitamin B12 to satisfy the B12 requirement of 

 the diatom. The addition of vitamins improved only 38% of the sea-water 

 assayed with unialgal Skeletonema (Johnston, in press). This explains why 

 Ryther and Guillard did not report any decrease in growth when the vitamins 

 were omitted from the complete enrichment. These authors enriched sea-waters 

 which contained their natural microflora. It is well known that when sea-water 

 is put in glass containers the added surface of the containers promotes heavy 

 bacterial growth (Zobell and Anderson, 1936; Jones, Thomas and Haxo, 1958) 

 and that many marine bacteria produce vitamins. 



These observations prompted Johnston to enrich sea-water samples (to be 

 assayed with bacteria-free Skeletonema) with a one-fifth volume of medium S36, 

 which contains vitamins along with N, P, Si and trace metals. With this enrich- 

 ment, the growth of bacteria-free Skeletonema in the numerous samples of sea- 

 water varied from substantially inferior to superior to the controls (i.e. growth 

 obtained in undiluted medium S36). The clear need for a vitamin supplement 

 demonstrates that Skeletonema requires higher levels of vitamins than are 

 present in the water tested. This experimental evidence contradicts the asser- 

 tion of Droop (1957a) that the lowest amount of B12 found by Cowey (1956) in 

 the North Sea — 0.1 mjxg/1. — should support a crop of twenty-five million cells 

 of Skeletonema per liter. Johnston employed Droop's bacteria-free culture of 

 Skeletonema for his assay of waters from the North Sea. The assertion of Droop 

 was based on a calculation of the molecules of B12 required to produce 1 [j. 3 of 

 living protoplasm of Bi2-requiring algae. 1 Daisley (1957) had already contested 

 the validity of applying such data to a dynamic ecological situation. The 

 results of Johnston are even more remarkable because Skeletonema can utilize, 

 besides true B12, the widest range of Bi 2 -like cobalamins ; in nature this species 

 should have an advantage over other vitamin B12 requirers with narrow 

 specificity. 



Another important result obtained with the two Skeletonema assays is that 

 the enriched sea-water samples elicit "poor" to "very good growth", indicating 

 that unknown substances, which are none of the nutrients added with the 

 enrichment (vitamins, trace metals, N, P and Si), affect Skeletonema growth, 

 else all waters should have responded alike to the enrichments. For instance, 

 the following behavior, observed in many samples, supports the postulate of 

 unknown beneficial factors : 



Sea-water + one-fifth medium S36 = often far more growth than 100% S36 

 (for bacteria-free and unialgal Skeletonema assay). 



Sea-water + vitamins + trace metals = often far more growth than 100% 

 S3 6 (unialgal Skeletonema assay). 



1 This value was obtained by dividing the volume of a cell of Monochrysis lutheri by the 

 number of cells obtained with given amounts of B12. The value obtained is three molecules 

 of B]2 for 1 (jl 3 protoplasm. The values obtained, with similar calculations, for the existing 

 growth data for Euglena and Stichococcus, being of the same order, made him confident 

 of the validity of this coefficient. Applying this coefficient to the volume of a Skeletonema 

 cell, Droop derived the number of cells of Skeletonema that could be supported by 0.1 m/u.£ 

 of B 12 . See also Droop (1961). 



