resulted in stimulation of growth of certain species 

 and in complete inhibition of others. Decomposi- 

 tion products of algae and of fish gave comparable 

 results but affected cultures in different ways . 

 Each had a specific effect and each demonstrated 

 individual thresholds of activity for particular algae. 

 The action was essentially unchanged whether the 

 decomposition products were added in combination 

 to distilled water or were added separately to min- 

 eral media. As noted previously in studies of sub- 

 stances interfering with cell multiplication, dis- 

 tinct cellular anomalies were observed in inhibited 

 cells . 



Preliminary studies of a somewhat similar 

 nature are under way in our laboratories (Hartman 

 and Demoise, unpublished). In these investiga- 

 tions, extracts have been prepared from large quan- 

 tities of algal material collected from Pymatuning 

 Reservoir at the end of the summer bloom. The 

 species present in the material tested were mostly 

 Microcystis aeruginosa and Coelosphaerium kuetz - 

 inganum . When water extracts, prepared by freez- 

 ing, repeated centrifugation , and millipore filtra- 

 tion were added to cultures of Scenedesmus obli - 

 quus and Dictyosphaerum ehrenbergianum the growth 

 rate of these organisms was approximately double 

 that of control cultures, as measured by gravimetric 

 or colorimetric techniques . 



Extrapolation of generalizations based on re- 

 sults from laboratory culture studies to conditions 

 existing in natural waters must always be made 

 with an appreciation of the many difficulties in- 

 volved . Excreted substances which remain in 

 fairly concentrated form in confined culture media 

 may become diluted below their limit of physio- 

 logical activity in the great unconfined volume of 

 the natural situation. Activities of associated 

 fresh-water bacteria may result in rapid decompo- 

 sition of substances which remain active for long 

 periods in bacterially-free cultures. Active ma- 

 terials may also be rendered inactive in natural 

 waters by formations of complexes with other sub- 

 stances . Finally, some consideration must be 

 given to the fact that modifications of algal strains 

 under conditions of the laboratory may have given 

 rise to alterations in enzyme systems or to changes 

 in metabolic pathways which differ from those in 

 naturally occurring algal species . 



The significance of these physiological active 

 substances in the ecology of the algae of natural 

 waters is difficult to appraise. Many workers since 

 the time of Akehurst have leaned heavily on exter- 

 nal metabolites in their attempts to explain such 

 seasonal succession of species and the often-ob- 

 served situation where a single species completely 

 dominates a phytoplankton bloom community. Much 

 of the efforts of Lefevre and his group have been 

 attempts to explain these phenomenon by the action 

 of algal inhibitors (Lefevre and Nisbet, 1948; 

 Lefevre, Jakob and Nisbet, 1950, 1951; Lefevre and 



Farrugia, 1958). Lucas (1947, 1949, 1955) has 

 brought together much of the findings from other 

 areas of aquatic biology in support of the "exclu- 

 sion theory" as applied to plant and animal rela- 

 tionships. From his interpretation of the available 

 evidence he has suggested that many organisms 

 have adapted themselves to tolerate or take advan- 

 tage of the external metabolites of their neighbors 

 and has postulated further that many "stimulating" 

 and "inhibiting" ecological relationships may have 

 arisen in this way . 



The work of Proctor (1957) represents one of 

 the few attempts to follow the presence of inhib- 

 iting substances in natural waters over several 

 seasons and to relate the occurrence of these sub- 

 stances to community structure. Work of this type 

 carried the criticism that the measurement of in- 

 hibition or stimulation is based on the reactions in 

 the laboratory of a single organism, in this case 

 Haematococcus pluvialis . The method is also open 

 to criticism on the grounds that there is not direct 

 evidence that phytoplankton populations are the 

 source of the active substances being studied . In 

 spite of such objections this type of approach com- 

 bining seasonal studies on natural waters with lab- 

 oratory culture studies of bloom-producing species, 

 promises to be the most rewarding in terms of under- 

 standing the role of algal metabolites in influencing 

 the structure and occurrence of many phytoplankton 

 communities . 



In summary, it would appear that ample evi- 

 dence now exists to indicate that many species of 

 fresh-water algae are capable of producing physio- 

 logically active metabolites which may function as 

 toxins, growth inhibitors, or growth stimulators to 

 themselves or to associated algae. Natural waters 

 supporting bloom concentrations of algae have been 

 shown to contain substances inhibitory to various 

 species although evidence is not absolute that the 

 active substances are derived directly from algae . 

 Chemical and physical studies of the active sub- 

 stances have indicated that in a number of cases 

 these materials appear to be related to fatty acids . 



There remain many unsolved problems and un- 

 answered questions: One is concerned with the 

 chemical nature of the inhibiting or stimulating sub- 

 stance and the mechanism of physiological action 

 on affected organisms . A second is concerned with 

 the nature of the production of the active materials 

 and its release from the source organism. Is it a 

 metabolite produced and excreted during normal 

 growth of the organism or is it produced only on 

 death and subsequent breakdown of cells? Finally 

 there remains the question most important perhaps to 

 to the algal ecologist - Are these substances pro- 

 duced in physiologically effective concentrations in 

 nature and do they remain at these concentrations 

 for sufficient time to affect the growth of other 

 algae? 



42 



