ALGAE AND METABOLITES OF NATURAL WATERS 



Richard T. Hartman 

 University of Pittsburgh 



Increased attention has been given in recent 

 years by investigators working in various aspects 

 of algal ecology to the quantity, chemical nature, 

 and source of the organic constituents of natural 

 waters. This study of a long-ignored portion of the 

 aquatic environment has resulted from a growing 

 awareness of the role of organic substances in het- 

 erotrophic algal nutrition and from an appreciation 

 of the possible effect of these substances in influ- 

 encing the size and species composition of algal 

 communities by stimulating or inhibiting the growth 

 of particular species . Two important and compre- 

 hensive reviews have recently appeared on the sub- 

 ject of organic matter in natural waters. Vallen- 

 tyne (1957) has brought together the available in- 

 formation on the organic compounds Isolated or 

 identified from water, seston, and sediments of 

 lakes and oceans. Saunders (1957) has reviewed 

 studies on the interrelationships between dissolved 

 organic matter and phytoplankton pointing out four 

 functions which dissolved organic substances may 

 have for algae: (1) direct nutritional value, 

 (2) source of accessory growth factors, (3) toxins 

 or growth inhibitors , and (4) chelation of trace 

 minerals. 



In the present paper it is my intent to review 

 primarily the evidence in support of the existence 

 of organic metabolites produced by algae , to ex- 

 amine the properties of these substances , and to 

 consider the possible role of such substances as 

 inhibitors, stimulators, or regulators of growth in 

 natural algal populations . The role of organic 

 metabolites in direct nutrition of the algae and the 

 production of substances toxic to fish and other 

 vertebrates will not be discussed since these 

 phrases have been or will be treated by other pa- 

 pers in this symposium. 



Biological literature is replete with attempts 

 to relate fluctuations in algal populations to vari- 

 ations in chemical constituents of the water or to 

 physical factors of the environment. The problem 

 is most difficult, and the answers least satisfac- 

 tory, when attempts are made to explain the rapid 

 development of bloom-producing species and the 

 apparent decline of other species under environ- 

 mental conditions which appear to be equally sat- 

 isfactory for each organism. Hutchinson (1941, 

 1944) in discussing these problems some years ago 

 pointed out that a clear-cut correlation between 

 chemical conditions and the qualitative composi- 

 tion of the phytoplankton is not to be expected . He 

 suggested the possible importance of accessory or- 

 ganic substances in development of specific popu- 

 lations . 



As early as 1931 it was suggested by Ake- 



hurst (1931) that toxins produced by algae were re- 

 sponsible for fluctuations in phytoplankton popula- 

 tions . He theorized that changes in the rate of 

 development of certain algal genera were in re- 

 sponse to excreted toxins which could serve as 

 "accessory food" and could inhibit or stimulate 

 growth . Phytoplankton was divided into two groups, 

 "starch" and "oil", according to the chemical na- 

 ture of their food reserves . The autotoxins pro- 

 duced by the oil group were postulated as an ac- 

 cessory food of the starch group. It was thus pos- 

 sible to account by this theory for many succes- 

 sional patterns shown by algal communities . 



This proposal was advanced with essentially 

 no direct evidence for the production or existence 

 of such toxic substances and was based almost en- 

 tirely on observations of the sequential changes in 

 phytoplankton populations over the seasons. Data 

 from many investigators covering a variety of geo- 

 graphical locations were gleaned from the litera- 

 ture and analyzed in support of his proposal. 



Direct evidence for the existence of such ex- 

 tracellular growth modifiers in natural waters has 

 been slow to accumulate. Even now, most of the 

 experimental data confirming the production of ex- 

 tracellular substances are based on results from 

 investigations of algae in laboratory cultures. 

 Physiologically active organic substances from 

 algae may originate in nature in either of two ways: 

 (1) by extracellular excretion from living algal cells 

 or (2) from the decomposition of the remains of dead 

 cells . While it may be possible to demonstrate the 

 existence of such active substances in nature, it 

 is almost impossible to determine the specific 

 source of the active materials. Furthermore, even 

 though many intensive studies have been carried 

 out in the laboratory with certain algal species, 

 little has been accomplished in determining the 

 actual chemical composition of the active sub- 

 stances or in understanding the nature of the physi- 

 ological response of algae to these substances. 



SOLUBLE EXTRACELLULAR SUBSTANCES 



The excretion of soluble, extracellular sub- 

 stances by algae has received the attention of a 

 number of investigators . Studies have been con- 

 cerned with algae from various taxonomic groups, 

 with the chemical nature of the excreted products , 

 and with the environmental and internal conditions 

 influencing excretion. Myers (1951) after review- 

 ing the work of Krogh , Lange and Smith (19 30) on 

 Scenedesmus and of Myers and Johnston (1949) and 

 Spoehr and Milner (1949) on Chlorella concluded 



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