can alter subsequent development of the other 

 species. Such effects are difficult to separate 

 from those resulting directly from the presence of 

 metabolites. Various modifications of these basic 

 procedures have been made, including the tech- 

 nique used by McVeigh and Brown (1954) in which 

 two species were grown together in a flask but kept 

 separate by a dialyzlng membrane . 



The effect most frequently observed in cul- 

 ture studies has been the modification of the rate 

 of cell division which may result in reduction or, 

 more rarely, in stimulation of growth of the popula- 

 tion. Morphological modifications, particularly 

 changes in cell size, have been noted along with 

 increased accumulation of storage products within 

 the cell. There appears to be such variability in 

 the nature of the physiological action induced, and 

 in the degree of response shown by various species 

 belonging to the same taxonomic group that is often 

 difficult to generalize from reported results . It is 

 true of course that some of the variability reflects 

 differences in experimental procedures. Diversity 

 in culture media used, variations in temperature 

 and light conditions under which the organisms 

 were grown, age of the culture at the time of ex- 

 traction of active substances, methods of extrac- 

 tion or concentration of the active materials, and 

 physiological conditions of the test inoculum, all 

 may influence the results and make comparison of 

 the findings of several workers very difficult, even 

 when the same algal species are being considered. 



In an attempt to examine the results obtained 

 with a number of different algal organisms in lab- 

 oratory culture, observations reported by a number 

 of different workers have been tabulated in Table I. 

 (Table at end of paper) The most commonly ob- 

 served effect of algal metabolites has been their 

 influence on the rate of multiplication of cells . 

 Relatively few authors (Pratt, 1942; J;5rgensen, 

 1956; Lefevre and Jakob, 1949; McVeigh and Brown, 

 1954; Lefevre and Nisbet, 1948; Lefevre, Jakob, 

 and Nisbet, 1952) have reported an increase in the 

 rate of this process, most attention having been 

 given to those interactions in which a reduction in 

 the rate of cell division appeared. Reports of in- 

 duced morphological changes have likewise been 

 relatively few . 



A certain amount of ambiguity exists in the 

 terms used to describe the reactions of algae to 

 the physiologically active substances. Lefevre 

 and co-workers plainly distinguish in most cases 

 between "toxic" substances which bring about the 

 death of affected cells and "algostatic" substances 

 which retard cell division but do not cause death 

 of the organism. Cells affected by "algostatic" 

 substances resume multiplication if transferred to 

 fresh media . Other workers are not always so def- 

 inite in their descriptive terminology and it is not 

 always clear whether cells were actually killed or 

 merely prevented from undergoing cell division. 



It is apparent from Table I that the algal 

 species which have been demonstrated to be pro- 

 ducers of inhibitory or stimulatory substances rep- 

 resent a relatively few taxonomic groups . The di- 

 vision Chlorophyta is represented by several gen- 

 era in the orders Chlorococcales , Volvocales, and 

 Zygnematales . Chrysophyta is represented by 

 diatom genera of the order Pennales and the Cya- 

 nophyta by various genera in Oscillatoriales and 

 Chroococcales . The nature of the problem of 

 studying growth modifying substances has de- 

 manded eliminating any possible effects of other 

 organisms from the observed reactions. For this 

 reason most of the studies have been carried out 

 with bacterially-free algal cultures. The list of 

 active organisms, therefore, reflects largely those 

 species which may be easily cultivated in pure 

 culture . 



A few investigators (Lefevre , et al, 1950, 

 1951; Rice, 1954; Johnston, 1955; Proctor, 1957) 

 have used natural waters containing high popula- 

 tions of particular species in their studies and 

 have thus extended the laboratory observations to 

 field conditions . Results from studies of this type 

 carried out in fresh-water are grouped in Table II. 

 (Table at end of paper) 



To generalize as to the mode of action or the 

 nature of the physiological processes concerned 

 in the observed response is difficult from present 

 data . The effect produced by one filtrate , for ex- 

 ample, may vary from one test species to another. 

 It may produce stimulating effects on some species, 

 inhibitory effects on others, and have no observ- 

 able effects on still others (Lefevre and Jakob, 

 1949; J/Jrgensen, 1956). The composition of the 

 media may affect the interaction between species 

 in mixed culture as shown by McVeigh and Brown 

 (1954). 



It is the opinion of many investigators 

 (Denffer, 1948; Lefevre, and Nisbet, 1948; Lefevre, 

 Jakob, and Nisbet, 1952) that inhibition of growth 

 induced by algal metabolites results from inter- 

 ference with the cell division process and not, in 

 most cases, with nutritional processes. The few 

 morphological or cytological modifications which 

 have been reported are in many cases apparently 

 the result of the accumulation of abnormal amounts 

 of storage products within cells which failed to 

 divide . 



NATURE OF THE INHIBITING SUBSTANCES 



It seems extremely unlikely that the variety 

 of effects attributed by various investigators to 

 extracellular algal metabolites are the result of a 

 single chemical substance. Such widely differing, 

 specific physiological effects as interference with 

 the dark reaction in photosynthesis (Pratt, 1943), 

 reduction of oxygen-consumption of bacteria 



40 



