SECT. 2] ORGANIC REGULATION OF PHYTOPLANKTON FERTILITY 181 



vitamins with all the other organisms. Since their metabolism and rate of 

 division are higher than other consumers, they may be quantitatively the most 

 important consumers. Table V points to vitamin exchanges between some 



Table V 

 Marine Bacteria (Burkholder, in press) 



marine bacteria (Burkholder, in press). Here nutritional interdependency is 

 made dramatically manifest by vitamin producers being themselves dependent 

 on other vitamins. This chemical symbiosis supports Lucas's postulates on the 

 importance of external metabolites in the sea. 



Apparently bacteria are not the only producers of vitamins. Several uni- 

 cellular algae contain appreciable amounts of vitamins (Table VI) ; these data 

 refer to the content in vitamins of the cells and not of the supernatants. There- 

 fore, we do not know whether these algae excrete the vitamins ; if they do not 

 the vitamins could become available for the consumers only after death of the 

 cells. The vitamins are synthesized by the algae since several analyses were 

 done on algae grown in bacteria-free culture and in media lacking the vitamin 

 tested (see Brown et al., 1955; Robbins et al, 1951). Furthermore, the B i2 

 content of the various Chlorella species is of the same order for species grown 

 under impure or aseptic conditions. However, the species of Table VI are fresh- 

 water algae and belong to the green and blue-green algae. Species belonging to 

 these algal groups are not very abundant in the sea, except for some blue-green 

 algae, like Trichodesmiwm erythraeum, which form blooms in warm seas. 



The content in cobalamins of many seaweeds has been analyzed by Robbins 

 et al. (1951), Ericson and Lewis (1953), Southcott and Tarr (1953) and Hashi- 

 moto (1954). These data and their ecological significance are discussed by 

 Provasoli (1958a). In brief, the red seaweeds are richer in cobalamins than the 

 brown seaweeds, and more species of reds than browns contain cobalamins. 

 Since the algae were collected from nature, we do not know whether the 



