268 Marine Microbiology 



awkward sequence of: nutrient assimilation followed by the pro- 

 duction and liberation of a metabolite which may then be as- 

 similated or favorably alter the physical environment. 



The second group of phyto-stimulatory compounds com- 

 prises those absolutely required for growth, such as vitamin B12 

 or thiamine (9, 36). Since a requirement, or its lack, for these 

 substances is absolute (9, 36), as it is for nitrogen or phosphorus, 

 minimal quantities may limit growth because of depletion, not 

 as a phyto-toxin, while high concentrations may prolong growth, 

 but not as a phyto-stimulant in the sense of the ectocrine theory 

 as it is applied to succession. As accessory substances they fall 

 within the realm of potential limiting factors along with inorganic 

 nutrients. 



What are some consequences of inhibitory substances on 

 succession? Assume Species A produces hetero-inhibitory sub- 

 stances during its advancing senility. Adherence to the ectocrine 

 theory might suggest that Species A is protecting its dominance 

 through inhibition of competitors. But since senility undoubtedly 

 results from an inadequate environment. Species A enjoys no real 

 competitive advantage, for inhibition of its competitors will 

 neither permit its own rejuvenation nor alleviate the conditions 

 inducing senility. 



It is recognized that phytoplankters are characterized by 

 intrinsic differences in specific growth rates (3) which un- 

 doubtedly vary with environmental conditions. Assume now tliat 

 vigorous cells of Species A liberate hetero-toxic substances, prob- 

 ably in less copious amounts than during senescence. Notliwith- 

 standing these metabolites, or even stimulatory substances, it is 

 obvious that Species A, or a stimulated species, may prolong its 

 "chemically" induced dominance provided its basic metabolic 

 requirements are met, and the environment favors its intrinsic 

 growth rate over that of its competitors. But recall that light, 

 temperature, nutrients and the general environmental regimes 

 are likewise changing during the temporal progression of succes- 

 sion, i.e., conditions also causing simultaneous alterations in tlie 

 growth rates of all species. Once again it seems likely that unless 

 a metabolite is produced in prodigious amounts its influence on 



