that "conservation of carbon and a minimal level of 

 excretion is probably generally characteristic of the 

 green algae". Fogg (1953), on the other hand, 

 notes that the amount of extracellular carbon pro- 

 duced by species of green algae may be relatively 

 high, with quantities as high as 12.5 per cent 

 being given off by old cultures . 



Studies of motile green algae, particularly 

 species of Chlamydomonas , indicate that produc- 

 tion of soluble organic materials in the media by 

 these organisms Is quite extensive. Lewin (1956) 

 using cultures of eighteen species isolated mostly 

 from soil samples, showed that all species tested 

 liberated some polysaccharides into the growth 

 medium. Chlamydomonas mexicana released as 

 much as .1 gram per liter, equivalent to 25 per 

 cent of the total organic matter produced by the 

 cells of this organism. Allen (1956) similarly 

 demonstrated excretion of oxidizable organic mat- 

 ter by six species of Chlamydomonas . Organic 

 acids as well as polysaccharides were identified 

 in the media . It was apparent from the results of 

 the studies on Chlamydomonas that the appearance 

 of organic substances in the media did not result 

 primarily from decomposition of dead cells. In- 

 creased quantities of extracellular organic material 

 in the media has been shown to parallel growth of 

 the algae . 



In mucilaginous species it is obvious that 

 much, perhaps most, of the soluble polysaccharide 

 arises from disintegration of the external portion of 

 the enveloping sheath. Lewin (1956) states "a 

 clear distinction cannot be readily drawn between 

 a soluble liberated polysaccharide and diffluent 

 sheath, between the latter and a firm capsule, or 

 between a capsule and the cell wall itself" . 



The production of extracellular nitrogenous 

 materials in the growth medium is characteristic of 

 many blue-green species, for example: Nostoc 

 spp. (Henrikson, 1951); Tolypothrix tenuis , 

 Calothrlx brevissima , Anabaenopsis sp . , and 

 Nostoc sp . (Watanabe, 1951) Anabaena variablis , 

 A. gelatinosa (De , 1939): and A. cylindrica (Fogg, 

 1942) . Filtrates from healthy cultures of these 

 algae were shown to contain up to 50 per cent of 

 the total nitrogen fixed by these organisms. These 

 soluble nitrogenous compounds appear to be pres- 

 ent largely as polypeptides, but amides (Fogg, 

 195 2) and amino acids (Watanabe, 1951) have also 

 been identified in the filtrates. Phormidium unci- 

 natum has also been shown by Lefevre and Nisbet 

 (1948) to produce quantities of soluble organic 

 material In the medium (as determined by oxidation 

 with KMn04) , increasing in 40 days from 4.8 mg/1 

 to 30.4 mg/1 . 



It has been suggested (Fogg, 1953; Fogg and 

 Westlake, 1955) that extracellular nitrogenous 

 compounds are liberated by most algae. Further 

 evidence in support of this was supplied by Fogg 

 and Boalch (195 8) who showed that the brown algae 



(Ectocarpus confervoides) growing in a bacteria- 

 free culture liberated sizeable amounts of nitro- 

 genous material to the media. The ecological 

 significance of the presence of extracellular nitro- 

 genous compounds in natural waters, has been dis- 

 cussed by Fogg and Westlake (1955) and Fogg 

 (1956) who have emphasized the possible role of 

 excreted polypeptides in forming complexes with 

 sparingly soluble salts and with toxic ions . The 

 availability of the nitrogenous materials to organ- 

 isms unable to fix nitrogen may also be of great 

 ecological importance. It has been pointed out, 

 however, that not only is Anabaena in bacteria- 

 free culture unable to make use of its own excre- 

 tion products, but that Chlorella sp. and Oscilla - 

 toria sp. are likewise unable to use these materials 

 as sources of nitrogen. 



Of ecological importance, at present not fully 

 evaluated in the case of algae, is the production of 

 extracellular substances capable of influencing 

 growth of the producing organism itself or of assoc- 

 iated species. Harder (1917) has been credited 

 (Jpfrgensen, 195 6) with the first report of the pro- 

 duction of growth-inhibiting substances, in this 

 case as autoinhibitor produced by Nostoc puncti - 

 forme . The classical work of Pratt and Fong (1940) 

 established firm evidence for the production of an 

 autoinhibitor by Chlorella vulgaris . Subsequent 

 investigations by Pratt and his co-workers (Pratt 

 1942, 1943; Pratt etai 1944, 1944a, 1945, 1948) 

 and by various other investigators over nearly a 

 score of years has resulted in the accumulation of 

 a great deal of information about this species. The 

 inhibiting material, termed "Chlorellin" by Pratt 

 has not been isolated nor chemically defined al- 

 though it has been concluded that the active agent 

 is probably an organic base. 



Evidence for the production of extracellular 

 inhibitory or stimulatory substances by algae has 

 come largely from laboratory studies carried out in 

 either of two ways: (1) by growing certain test 

 species in cell-free media (filtrates) in which an 

 alga had previously been grown or (2) by growing 

 two species in mixed culture using a media which 

 would permit satisfactory growth of either organism 

 when grown alone . 



The use of filtrates permits following the 

 production of active substances throughout dif- 

 ferent growth phases of the culture . Mixed-culture 

 techniques, while providing perhaps a closer ap- 

 proach to the natural situation than procedures 

 using culture fiifrates, produce no concrete data on 

 the rate of production of active metabolites nor do 

 they permit recognition and evaluation of responses 

 resulting from interactions of the mixed strains in 

 culture. Difficulties may arise, for example, 

 when two organisms have different inherent growth 

 rates under a given set of conditions . Active 

 growth and rapid photosynthesis of one species 

 may bring about the pH changes in the media which 



39 



