Factors affecting the output rate of Spirulina platensis with reference to mass cultivation

Abstract

The disappointingly low yields of Spirulina cultivated in large, commercial-scale reactors prompted our attempt to elucidate some of the reasons for this phenomenon. The results show the importance of maintaining the pH as high as possible, at a point, however, where it does not yet limit growth. To check the growth of contaminating algae, such as Chlorella spp., the pH was elevated to a point higher than the optimal for Spirulina. Close to pH 11·0, signs of deterioration became evident, but the culture recuperated readily when the pH was lowered back to the optimum (pH 10·5). The depth of the culture had no effect on the maximal areal output of dry biomass, but exerted a very marked effect on the optimal population density. At a culture depth of 150 mm the optimal cell density was at an optical density of 0·5 absorbancy units, whereas at a culture depth of 75 mm the optimal cell density was 0·7 absorbancy units. The stirring rate also affected the optimal population density. At slow stirring speeds (58 cm s −1, which were neverthe3ess approximately twice as fast as that maintained in large commercial ponds) there was a substantial reduction in the output rate at high population densities. Similar results were obtained at high stirring rates, but the effect was much reduced and was only observed at the highest cell densities. The chlorophyll: phaeophytin ratio altered signficantly throughout the year, being highest in mid-winter and declining in summer. A practical conclusion drawn from these results was that there is a clear advantage in operating the slow flowing cultures in large reactors at as shallow a culture depth as practically possible. This increases the optimal cell density to a level that may be easier to harvest and maintain in large-scale reactors. The inefficiency of harvesting large volumes of low cell densities has lead to the general practice of allowing cell densities to increase far above the optimum. Our results indicate that this may have detrimental effects on the output rate considering the relatively low stirring rates which commonly prevail in large reactors.