264 Marine Microbiology 



LIBERATION AND ENVIRONMENTAL CONTROL 

 OF METABOLITES 



It is uncertain just how conservative natural phytoplankton 

 populations are in the liberation of metabolites during active 

 growth ( 13, 12 ) . It is known that the production and liberation 

 of certain organic substances by healthy populations are goxenied 

 by the prevaihng environmental conditions, including light in- 

 tensity ( 12 ) , temperature and nutrients ( 23 ) . The regulation of 

 metabolite production and liberation, apparently existing during 

 active growth when cells would appear to be especially vulner- 

 able to such substances, f)i"ogressively declines, resulting in an 

 increased liberation during senescence (37; among others). In- 

 deed, Duursma (10) has concluded that dissolved organic sub- 

 stances emanate primarily from dead cells. 



Although the data are sparse, they consistently suggest that 

 the production and secretion of metabolites are regulated by 

 environmental conditions and are related to cell vigour— increas- 

 ing with senescence and followed by copious liberation upon 

 death. The ecological implications of this will be returned to. 



Not only does the production and liberation of organic sub- 

 stances appear to be regulated during certain growth stages, but 

 following their secretion as well. There is presumptive evidence 

 that environmental forces restrict the accumulation and attenu- 

 ate, if not completely neutralize, the potential biological effects 

 of metabolites. Accordingly, the potential activity of a metab- 

 olite as a nutrient or growth regulator appears to be determined, 

 inter alia, by its: 1) degree of intrinsic stability, 2) degree of 

 non-adsorptiveness, and 3) resistance against biological degrada- 

 tion—all being coupled with the capacity of the watemiass to 

 minimize loss through dispersion (4, 47). 



Stability 



It is obvious that a substance which is thermo- or photo- 

 labile under the environmental conditions encountered upon 

 liberation may become less potent. It appears tliat metabolites 

 are particularly amenable to environmental modification. Kashi- 

 wada et al. (24) observed that 50-81 per cent of vitamin Bii> 

 decomposed in the upper 20m upon exposure to solar irradiation. 



