190 PROVASOLl [CHAP. 8 



4. Nutritional Requirements of Marine Algae 



The nutritional requirements of algae have been reviewed recently (fresh- 

 water algae, Krauss, 1958; nutrition and ecology, including marine algae, 

 Provasoli, 1958). Several marine algae have been grown bacteria-free in 

 artificial media of known composition (Provasoli, McLaughlin and Droop, 

 1957). These media seem adequate for most of the algae growing in the littoral 

 zone, including the red seaweeds (Fries, 1959; Provasoli and Iwasaki, un- 

 published). The photosynthetic marine algae are fundamentally photoauto- 

 trophic, i.e. synthesizing their organic carbon almost exclusively from CO2. 

 Many species, however, have acquired needs for growth factors while retaining 

 their basic photoautotrophic abilities. 



A. Inorganic Requirements 



The tolerance to salinity variations are indeed quite great; most neritic 

 marine flagellates can grow from 12-40% salinity (optima between 20-24% o ). 

 Only species obtained from the Caribbean and Gulf of Mexico seem to prefer a 

 salinity of 33 % . Tolerance to variation in ratios and concentrations of Na, K, 

 Ca and Mg, is, in general, high ; this versatility has been exploited in com- 

 pounding non-precipitating artificial media low in Ca and Mg (Ca 10-20 mg % ; 

 Mg 50-70 mg %). Iron, Mn, Mo, Cu, Co and V have been demonstrated to be 

 essential for fresh-water algae ; B, Zn and S are also needed but their essentiality 

 has not been proven. We may assume that this holds for marine algae also, 

 even though no available data meet the rigorous requirements for purity of 

 chemicals for these studies. In artificial media the addition of metal-buffered 

 (chelated) trace-metal solutions is favorable for sustained growth despite the 

 trace metals introduced as impurities of the "chemically pure" major salts. 1 

 Studies on single-trace-metal requirements may be misleading when metal- 

 chelators are present in the media, because addition or removal of single heavy 

 metals affects simultaneously the hold of the chelator on all other chelatable 

 ions in the medium. Removal of chelators results in precipitates if the trace 

 metals are added before sterilization ; separate aseptic additions do not guaran- 

 tee absence of precipitates and are cumbersome for extensive nutritional 

 studies. However, a precise study of the trace-metal requirements is un- 

 avoidable in view of the results of Johnston (see pp 200-201). Almost all photo- 

 synthetic marine algae utilize nitrates. Ammonia is utilized, and usually is not 

 toxic at the average ecological levels of N, but at higher levels it becomes 

 rapidly toxic in the alkaline range (Shilo and Shilo, 1953; McLaughlin, 1958). 

 Ammonia toxicity is unlikely to be ecologically important and may be signifi- 

 cant only in highly polluted zones. The species which invaded and replaced 



1 Chelators improve growth of higher plants and plant tissues (Wallace, 1960; Klein 

 and Manos, 1960). The finding of Goldberg (1952) that the diatom Asterionella japonica 

 utilizes only particulate iron has not been confirmed yet ; the other species of algae so far 

 tested grow poorly or do not grow in artificial media when precipitates occurred during 

 sterilization. Oceanic forms may behave differently as postulated by Provasoli (1956). 



