External metabolites in the sea 141 



and it is particularly appropriate that they were made by one who has worked with 

 both Hardy and Lucas. Lucas concluded that his very preliminary experiments 

 were not inconsistent with Hardy's theory and, indeed, appeared to offer support for 

 it. The work had to cease and they went no further than that. Bainbridge's work 

 was not only more detailed but much more precisely executed, and it represents a 

 real contribution to our knowledge of the prey-predator relationship. His criticism 

 of Lucas's " light and dark " experiments is that they simply reflected the tropism 

 of the animals — a possibility discussed by Lucas. It is and always was relevant. For 

 all that, the very weaknesses in Lucas's work and the greatly improved techniques in 

 Bainbridge's are very relevant, and they may still mean that Bainbridge's results 

 are not necessarily so critical of the theory of animal exclusion as they seemed at first 

 sight. 



Bainbridge was careful to use only algal cultures aged not more than " a week 

 or so " (his p. 391) after inoculation, whereas Lucas probably seldom used such fresh 

 cultures in his work. Indeed, Lucas's cultures may, in the light of modern ideas 

 (many of his experiments were made in 1934), have been approaching senescence at 

 times and, whatever may be their potency during the early phase of a culture, we can 

 now see that metabolites released during the later stages (e.g. Pratt, 1943) may well 

 have been harmful and have acted more as deterrents than as attractions. This is, 

 of course, far from certain and exploratory experiments on these lines would be 

 useful. In any event, it is also relevant that, whilst Bainbridge found that most of 

 his plankton animals were attracted by, or at least did not appear to avoid, the 

 majority of the denser phytoplankton cultures, several were either neutral or were 

 markedly avoided or even lethal. Two of the more " harmful " were flagellates. It 

 is also significant that not only did most of his cultures influence the animals but, in 

 several experiments, so did the culture fluids in the absence of the plant cells. The main 

 conclusion is that his experiments demonstrated the release by plant cells of substances 

 which were frequently attractive and occasionally repellent to many of his animals 

 (Bainbridge, 1952, p. 429). Like Lucas, also, he found evidence of an optimum 

 density of plant cells below and above which the animals were presumably either 



starved or poisoned. 



However, other evidence was accumulating. In the first place, there was the work 

 of Lwoff's school (1943), which demonstrated convincingly the release of vitamins " 

 into their environment by some micro-organisms, and the vital need for such vitamins 

 by some other forms which are unable to synthesize them. Lwoff saw this as evidence 

 of a progressive loss of physiological function during evolution, but it provided also a 

 mass of evidence of mainly beneficial inter-relationships, mediated by the release ot 

 metabolites potentially of communal significance. 



Next there is the work of Lefevre and his school (1952) which, while collectively 

 reviewed under the title of " Auto, et heteroantagonisme chez les algues d'eau douce ", 

 demonstrates also quite clearly the very real influence, both favourable as well as 

 unfavourable, which one micro-organism may exert upon others through the mediation 

 of its secretions or excretions. In brief, these workers concluded that the three groups 

 of bacteria, algae and fungi, all have " Faculte d'elaborer des substances actives 

 autoantagonistes, heteroantagonistes ou favorisantes. Specific.te des substances 

 actives produites . . . Decharge rapide des substances accumulees par les cellules 

 quand on les replace dans un milieu neuf Solubilite des substances actives dans 



