A major issue in microalgae mass cultures is the understanding of the factors that determine species dominance. A considerable theoretical background can be gleaned from the ecological and ecophysiological literature, where this problem, as it applies to lotic systems (ponds, lakes, rivers, oceans) is the subject of an enormous literature (briefly reviewed in Benemann and Tillett [1987]). Fundamental to this is Hutchins’ so-called “Paradox of the Plankton.” Hutchins pointed out the fact that only a limited number of nutrients could limit algal growth, and thus, applying Liebing’s Law of the Minimum, only a limited number (one per nutrient) of algal species that should be able to compete in any environment. Rather, we find literally hundreds, if not thousands of species and uncountable strains in even the smallest and most uniform of environments. Indeed, even Hutchins did not realize the greatness of microbial biodiversity in nature, hinted at by earlier work in clonal variations (See Section II. B.), but only recently revealed in ever-increasing detail using modern tools of molecular phylogenetics. The solution to the Paradox is that natural environments are not steady-state systems, but are exposed to periodic and random fluctuations in physical-chemical (let alone biotic) environmental parameters. These fluctuations allow for additional niches, allowing for organisms specialized in the exploitation of particular temporal combination of limiting factors. Also, non-steady-state conditions would select for different species and strains. For example, in a continuous cultivation using natural samples for inoculum into enriched media, the first algal species to appear and dominate are soon replaced by other species, which are slower growing but better at exploiting a light-limited (dense culture) environment.

Continuous algal cultures in 1-L vessels were set up in the laboratory, which allowed operations under fluctuating conditions of temperature, O2 concentrations, and pH. In these experiments, several algal strains were inoculated together, then productivity and species dominance were observed for as long as 3 weeks (over 10 dilution times). In initial experiments, Chlorella and Chaetoceros co-dominated; Cyclotella was lost or greatly diminished, possibly because of different light spectral use of the two types of algae (greens and diatoms). Many other experiments were carried out, with fluctuating pH, dilution rates, and light intensities and even gas sparger types on the dominance of these and one additional algal strain (Ankistrodesmus). All these factors tended to affect species dominance, even the gas sparger, and the results were not clearly interpretable in terms of major dominance factors. One conclusion from these initial experiments was that several factors, alone and in combination, can determine species dominance.