142 C. E. Lucas 



I'eau ou dans certains solvants organiques. . . . D'autre part, il n'est pas impossible 

 que les substances actives produites par des Aigues se developpant massivement dans 

 une collection d'eau aient une influence directe sur la multiplication et le developpe- 

 ment des animaux aquatiques: Entomostraces, Insectes, Mollusques, Vermes et peut- 

 etre meme Poissons " (Lefevre, Jakob and Nisbet, 1952). 



The limitations implied by the title of their paper and the wider relevance of the 

 text (summarized on their pages 173-181) are not without interest, for there seems to 

 have been a strong tendency on the part of various workers to anticipate unfavourable 

 reactions rather than favourable ones in given circumstances; e.g. the review by 

 McCoMBiE, 1953, mentions only the possibility of harmful effects of free metabolites,* 

 while one or two workers, in referring to Lucas' papers, have noticed only his ref- 

 erences to harmful effects. The fact is that one organism's " meat " may be another's 

 " poison " in the ecological nexus, and terms such as " harmful " and " beneficial " can 

 only be used in an immediate and limited sense. In this sense, much of the evidence de- 

 monstrates the development of " favourable " relationships, although admittedly some 

 of the most striking are unfavourable. One of the latter is instanced by the phenomenon 

 of" red tide ", with its harmful effects on marine animals and unfortunate repercussions 

 on man. Lucas instanced such effects of the secretions of plants or animals, and much 

 more evidence has been accumulated since (e.g. Br0ngersma-Saunders, 1948). 

 There is now no doubt that, even though in their more striking forms such phenomena 

 can be regarded as abnormal, they are, in fact, far from unusual in a lesser degree 

 and, further, they are mediated by the release of " toxic " substances, frequently by 

 flagellates. Their nature and the more precise conditions which lead to their production 

 in a mild or extreme form, are being intensively investigated in several laboratories. 



The major task now is to determine the nature and effects of some of the more 

 significant metabolites in aquatic ecology, to trace their probably variable distribution 

 in some natural waters, and to determine the conditions leading to their production. 

 Several lines of work have recently been developed. In the United States, Pratt 

 (1943) has produced clear evidence that Chlorella cells in culture release a growth 

 inhibiting substance, whilst Rice (1954) has grown Chlorella vulgaris and Nitzschia 

 frustulum (both fresh water algae) together, and demonstrated clearly that neither 

 grows so satisfactorily in the company of the other as it does in pure culture (depending 

 upon the size of the populations used). Each was similarly inhibited when grown in 

 the culture medium of the other, after its cells had been removed by filtration, and 

 both were also inhibited when grown in a culture medium prepared with pond water 

 which had supported a dense growth of Pandorina before filtration. Again, the 

 metabolites in solution could be absorbed in charcoal and removed by autoclaving, 

 suggesting volatile substances. 



Harvey, at Plymouth, following on his pioneer culture experiments with growth 

 substances, is now attempting to review all the evidence available so as to define more 



* Lucas drew attention to this tendency (1944 and later) in respect of the term " antibiotic ". He 

 pointed out that not only were antibiotics necessarily favourable to those organisms, such as man, 

 which are preyed upon by the object of antibiosis, but that the antibiotic might well prove beneficial 

 to those organisms succeeding its producer in the ecological chain (just as Akehurst, 1931, had 

 suggested the autotoxic secretions of one algae may be beneficial for its successors). Indeed, the 

 ecological successors of the producers of antibiotics can only succeed by virtue of being adapted to 

 the presence of the antibiotics or their degradation products, and there is already some evidence that 

 this may be true even for a potent substance such as penicillin. Here is a possible theoretical basis for 

 ecological succession. 



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