A key underlying assumption in this work was that monoculture productivity is an indicator of competitiveness in mixed cultures. That is, the most productive culture will also be the most competitive. Another assumption is that productivity and competitiveness will be affected by fluctuations in environmental variables and cannot be predicted from single parameter variations or steady-state operations. Testing these hypotheses was a central objective of these experiments, which were carried out with continuous and then semicontinuous (once a day dilution) laboratory cultures (1-L bottles).

The continuous cultures were used to test productivity of several strains under simulated outdoor conditions (approximately sinusoidal diurnal temperature changes from 15°-32°C overnight, a 14-h constant light period, 0.6-0.7 d-1 dilution, and 6-h gassing with pure O2 rather than air). As in earlier studies, Chlorella and Monoraphidium exhibited higher productivities (250-300 mg/L/d) than the other strains tested (Porphyridium, Ankistrodesmus, and Chaetaceros, with some 150-200 mg/L/d productivity). Under constant conditions, similar ranking and differences were observed between the three fastest-growing strains. Mixed culture experiments between Chlorella and Chaetoceros were then carried out, with the results comparing fairly well with the APM results (Section III. B.6.b.). Figure III. B. 12. shows experiments in which a 90/10 Chaetaceros/Chlorella mixed inoculum grew in continuous cultures, with cell species changing faster experimentally than in the model, which, however, predicts the overall trend. Overall results with these strains were in accord with prior experience (Weissman and Goebel 1985, 1988).

However, the continuous cultures proved logistically and experimentally very difficult, and the experimental design was switched to a simpler, once a day (before dawn), dilution. Fluctuating conditions were achieved primarily by timers and actuators (e. g., for O2 versus air supply, for temperature control in the water baths). This allowed greater reproducibility and simultaneous operation of many more cultures, as many as 16 in four water baths, allowing collection of an extensive data set. The effects of constant versus fluctuating temperature, pO2, and light, on culture productivities and dominance in mixed cultures were studied. In summary, the overall result was that fluctuating temperature and other conditions had major effects and were different from those at constant temperature. Also, growth responses for unialgal cultures can predict the outcome of species competition experiments. Still, much more experimental work is required on these problems.

Figure III. B.12. Species competition between Chaetoceros and Chlorella in continuous cultures.

Mixed culture results for a 90/10 cell density mixture of Chaetoceros and Chlorella.

a. ) Top: Culture productivity.

b. ) Bottom: Species cell concentration (solid line represents model predictions).

(Source: Benemann and Tillett 1990.)