(Steeman Nielson, 1955, 1955a), blockage of mi- 

 tosis (Denffer, 1948), interference with the filter- 

 ing action of E)aphnia (Ryther, 195 4) , as well as 

 such ambiguous and all inclusive terms as "stimu- 

 lation" and "inhibition of growth", have all been 

 laid to algal metabolites . 



A growth- modifying substance may, of course, 

 be stimulating to a specific reaction in low concen- 

 trations and inhibitory in higher concentrations . 

 The action of auxins is illustrative of this effect. 

 Bentley (1958) has demonstrated that auxin-like 

 substances are produced by a variety of algae and 

 that these substances are excreted into the media . 

 While 3-indoleacetic acid was shown to have a 

 stimulating effect on algal growth, Bentley con- 

 cludes that it is not the major natural auxin pro- 

 duced by algae. It should be noted that in the ex- 

 traction and study of these materials the petroleum 

 ether soluble fraction was extracted from the algae 

 or the media and discarded in order to forstall any 

 inhibitory effects by fatty substances. This latter 

 group is perhaps most suspect as the agents active 

 in inhibitory or antibiotic reactions among the 

 algae. The work of Spoehr, et al (1949) , for ex- 

 ample, indicated that the antibacterial properties 

 of chlorellin resulted largely from photooxidizable 

 unsaturated fatty acids. Proctor (1957a) has shown 

 that the inhibition of Haematococcus pluvlalis by 

 Chlamydomonas reinhardi resulted from fat- like 

 extracellular substances. He suggested that the 

 inhibitor produced by Chlamydomonas was probably 

 a long-chain fatty acid or a mixture of such acids . 

 Eight long-chain fatty acids were tested against 

 six algae in culture, all of them showing varying 

 degrees of inhibition. Considerable differences in 

 sensitivity could be observed with Haematococcus 

 being most sensitive, among the algae tested, and 

 with Chlorella vulgaris and Scenedesmus quadri - 

 cauda the least. Proctor is of the opinion that the 

 release of the toxic substance is dependent largely 

 on the death of the cell. Although there has not 

 been sufficient evidence to establish this with 

 other species, it is obvious that age of the culture 

 is important in determining the activity of a fil- 

 trate . Phormidium uncinatum, for example, was 

 found by Lefevre and Jakob (1949) to be stimulated 

 by a filtrate from a young culture of Scenedesmus 

 quadricauda but to be inhibited by a filtrate from an 

 old culture. This does not appear to be a consist- 

 ent reaction, however, since the filtrate of the 

 young culture of this species brought about com- 

 plete inhibition of Scenedesmus oahuensis . Proc- 

 tor (1957a) has observed stimulatory effects of fil- 

 trates from young cultures of Chlamydomonas rein- 

 hardi and inhibitory effects from filtrates of older 

 cultures when tested against Haematococcus 

 pluvialis . This early stimulation, he feels, re- 

 sults from a conditioning of the medium, pre- 

 sumably by removal of heavy metals. 



In the greater number of cases studied 



(Table I), strongest Inhibition has been obtained in 

 filtrates from old cultures. In only a few cases, 

 however, has the production of inhibitor been fol- 

 lowed through the growth period of a culture by 

 quantitative procedures. In studies on Chlorella 

 vulgaris , Pratt, Oneto, and Pratt (1945) have shown 

 that almost as much of the inhibiting substance, 

 chlorellin, could be extracted from two day old cul- 

 tures as could be extracted from cultures that had 

 attained their full growth. During the period of 

 rapid growth, however, the chlorellin content de- 

 creased rapidly, increasing again as the rate of 

 growth of the colony decreased . Whether similar 

 conditions prevail in the case of inhibiting sub- 

 stances from other algae has not been adequately 

 determined . 



The higher concentration of inhibiting sub- 

 stances observed in old cultures may of course re- 

 sult from an increased rate of production by these 

 cells; it may represent merely the accumulation of 

 large quantities of the material from prolonged 

 growth of the algae; or it may result from release 

 of larger quantities of cellular substances through 

 changes in permeability of the cell membrane or 

 from actual autolysis of old cells. 



EFFECT OF DECOMPOSITION PRODUCTS 



While attention has been directed thus far 

 largely to the action of substances released during 

 growth and development of algal organisms and pre- 

 sumably formed and excreted as a part of normal 

 metabolic processes, another source of soluble or- 

 ganic substances must be considered. The vast 

 quantity of material, both plant and animal, re- 

 leased in natural waters from organisms which 

 undergo a seasonal decomposition by aquatic bac- 

 teria and fungi give rise to a multitude of degrada- 

 tion compounds which makes identification or 

 quantitative estimation of active substances ex- 

 tremely difficult. Nevertheless it must be recog- 

 nized that such substances, particularly when they 

 result from the decomposition of highly concentrated 

 bloom communities represent a potentially important 

 source of soluble organic materials. Critical 

 studies of the effect of such degradation products 

 on algae in culture have been few in number. 



Lefevre and Farrugia (1958) have recently re- 

 ported the results of studies of the effect of de- 

 composition products from the alga Cladophora 

 glomerata and the gudgeon (Gobio fluviatilis) on the 

 growth of seven algae in culture. Their experiments 

 were carried out by adding various concentrations of 

 the decomposition products from these organisms to 

 distilled water and inoculating with pure cultures of 

 algae. Results indicated that concentrations of the 

 decomposition products as low as 1 mg./l. per- 

 mitted growth of some algal species without the ad- 

 dition of mineral salts . Higher concentrations 



41 



