AQUATIC ORGANISMS AND EXTERNAL METABOLITES 509 



6. Nor can such processes be neglected in attempts to increase 

 production by artificial increase of nutrients. At the lowest level, 

 free metabolites may be concerned in securing the maximum 

 sustainable basic production. As in (5), however, they are even 

 more likely to be relevant to any specific objectives for which 

 fertility is being increased, directly or indirectly. By contrast, as 

 has already been found in the carp culture of Palestine, free 

 metabolites (e.g., of Prymnesium parvum) can also be extremely 

 harmful, and the simple destruction of the "pest" may not 

 necessarily be the most efficient cure, ecologically speaking. 



7. Perhaps to a lesser degree, ectocrine processes may also be 

 relevant in any attempts which may yet be made to culture "food" 

 algae on a commercial scale (e.g., Burlew, 1953). However this 

 may be, and there are serious problems to be faced before it 

 becomes a reality, there still remain possibilities of algae being 

 cultured for the extraction of their metabolites, if one can judge 

 by the interest some of those few ectocrines already isolated have 

 attracted in the medical world (e.g., Schwimmer and Schwimmer, 

 1955; Hutnere/a/., 1958). 



8. These, then, are a few of the reasons why intensive but 

 selective work in this field is likely to be reasonably profitable. In 

 these hunches, however, I have been concerned chiefly with those 

 metabolites which perhaps best deserve the term "ectocrines," 

 and which are effective at concentrations of the order of 10~^ to 

 10~^. Yet they are only a minute fraction of the metabolites 

 released by aquatic organisms. We now know that, apart from 

 oxygen, normal algal metabolism can involve the release of 

 relatively very large amounts of carbohydrates, etc., even up to 

 50% of production (Allen, 1956; Lewin, 1956; Fogg, 1958; Guillard 

 and Wangersky, 1958). These amounts, along with proteins, etc., 

 may be increased when an algal flowering decays and when 

 animals excrete and die. Krause (1959) has shown how rapid may 

 be the breakdown of dead zooplankton, up to one-third of the 

 whole being lost to the enviionment within a tew hours and one- 

 half within a day. It is not, therefore, surprising that the total 

 quantities of organic matter in solution in natural waters may 

 frequently be very large, often exceeding several-fold the total 



