concentrations. Chu (1942) succeeded ingrowing 

 several oligotrophic fresh-water algae by simply 

 diluting the well known old medium of Benecke . 

 Later, by studying the mineral nutrition of several 

 oligotrophic diatoms and chlorophytes , he com- 

 pounded a medium (Chu 10) of wide use for oligo- 

 trophic organisms. The media experimentally de- 

 signed by Chu, Rodhe and us (Provasoli and 

 Pintner, 1953) are very similar. Further studies 

 (Provasoli, McLaughlin and Pintner, 1954) con- 

 firmed that oligotrophic species in general require 

 very low total-solids concentrations and are steno- 

 bionts toward this parameter. In the same analysis 

 it was found that algae may prefer a mono - or di- 

 valent ion, but that they are in general far less 

 exacting toward the Ca/Mg or Na/K ratio, and that, 

 within large limits, the two monovalent and two 

 divalent ions are interchangeable when the lower 

 limits of the preferred ion have been satisfied , 

 (Provasoli, McLaughlin, Pintner 1954, Droop 1958). 

 However, it is possible that organisms living in 

 dystrophic lakes may show the need for special 

 ratios of major elements besides total solids con- 

 centrations . 



Trace-metal content and pH are related fac- 

 tors because the solubility and availability of 

 trace-metals varies with pH . Iron and Mn seem 

 to be the two trace-metals ions which are quanti- 

 tatively important . Zn, Co, Cu , Mo, and V are 

 also important because required for growth, even 

 though they are generally present in waters in far 

 lower concentrations . They are generally present 

 as impurities of other chemically pure salts and, 

 depending upon the concentrations of major ele- 

 ments employed (especially in sea water media) , 

 one of them may be introduced in media at concen- 

 trations approaching inhibition if not toxicity. The 

 use of metal chelators may be of advantage in che- 

 lating these metals and in giving the limited steady 

 supply allowed by the dissociation constant typi- 

 cal for each divalent ion. Hence in mimicking nat- 

 ural conditions one should try to estimate the kind 

 and content of organic substances in the water . 

 "Humates" in alkaline or peaty waters are trace- 

 metal chelators less strong in their chelating power 

 than the chelator most used in artificial media 

 (EDTA) , but apparently more versatile in that they 

 can be employed at different ph"s without causing 

 toxicity (i.e. soil extract can be employed at pH's 

 form 5-8.5). "Pollution" is generally thought ben- 

 eficial to algae as a source of N and P. However, 

 in the degradation of organic matter by microorgan- 

 isms organic acids may be produced, especially 

 amino acids, many of which are good trace-metal 

 chelators . This is especially important in alka- 

 line waters when the solubility of heavy divalent 

 metals is almost nil. Iron was found by Uspenski 

 and Uspenskaja (1925) to be the important trace- 

 metal for several Volvox species . They were the 

 first to introduce chelators (citrate) in fresh-water 



media to keep the iron available to the algae in 

 neutral and alkaline media, because they sus- 

 pected that this was the mechanism operating in 

 nature. Volvox generally blooms in waters rich in 

 organic matter. Further investigations on the nu- 

 trition of Volvox (Pintner and Provasoli, 1959) show 

 that V. qlobator and V. tertius have scant hetero- 

 trophic abilities and that they do not need any pre- 

 formed organic compound as sources of energy; the 

 only organic compound needed is vitamin Bi2- 

 Their colonizing of water rich in organic matter is 

 therefore due to the need of finding vitamins and, 

 perhaps even more, to the need of having Fe solu- 

 bilized by the "organic acids". 



Generalities on the Nutritional Requirements 

 of Different Algae 



The known requirements of the algae have 

 been recently reviewed (Krauss, 1958; Provasoli, 

 195 8) . We will therefore consider only a few 

 points . 



a) Some nutritional requirements seem pre- 

 dominant or even unique in some algal groups . 

 Silica is important and often a limiting factor for 

 diatoms and perhaps also for the chrysomonads 

 bearing silica plates . High Fe and trace metals 

 are important for many euglenids and cryptomonads 

 colonizing acid waters. Sodium and/or potassium 

 are essential elements for blue-green algae. Dia- 

 toms have calciophilic and calciophobic species . 

 Most algae utilize nitrates preferentially but the 

 euglenids, so far cultured, can only utilize am- 

 monia, and, some, amino acids as N sources. 

 Many blue-green algae utilize atmospheric nitro- 

 gen . 



b) Organisms in the same ecological niche 

 often have common nutritional features. Most oli- 

 gotrophic algae cannot withstand total solid con- 

 centrations above 100-200 ppm.; many are steno- 

 haline . Algae in environments high in some sub- 

 stances (as the high concentrations of Fe and trace 

 metals in bogs and ditches and the high S in heav- 

 ily polluted waters) may merely be withstanding 

 these conditions; some actually require them. The 

 organisms of barnyards and sewage oxidation ponds 

 in general withstand ammonia even at alkaline pHs . 



c) Algae lacking photo synthetic pigments ob- 

 viously need non photosynthetic sources of energy. 

 Acetate, glutamate, aspartate, glycine, and glu- 

 cose seem to be the preferred carbon sources for 

 many. So far we know no chemotrophic algae. 



However the possession of photosynthetic 

 pigments does not exclude the possibility of heter- 

 otrophy, indeed, they may prefer it. While per- 

 haps most pigmented algae are phototrophic , many 

 are heterotrophic . 



The taxonomic position of a species to be 

 cultured offers indications of the probabilities. 



85 



