Microalgal mass culture systems and methods: Their limitation and potential

Cultivation of microalgae using natural and man-made open-ponds is technologically simple, but not necessary cheap due to the high down stream processing cost. Products of microalgae cultured in open-ponds could only be marketed as value-added health food supplements, speciality feed and reagents for research. The need to achieve higher productivity and to maintain monoculture of algae led to the development of enclosed tubular and flat plate photobioreactors. Despite higher biomass concentration and better control of culture parameters, data accumulated in the past 25 years have shown that the illuminated areal, volumetric productivity and cost of production in these enclosed photobioreactors are not better than those achievable in open-pond cultures. The technical difficulty in sterilizing these photobioreactors has hindered their application for the production of high value pharmaceutical products. The alternative of cultivating microalgae in heterotrophic mode in sterilizable fermentors has achieved some commercial success. The maximum specific growth rates of heterotrophic algal cultures are in general slower than those measured in photosynthetic cultures. The biomass productivity of heterotrophic algal cultures has yet to achieve a level that is comparable to industrial production of yeast and other heterotrophic microrganisms. Mixotrophic cultivation of microalage takes advantage of their ability to utilise organic energy and carbon substrates and perform photosynthesis concurrently. Moreover, production of some algal metabolites is light regulated. Future design of sterilizable bioreactors for mixotrophic cultivation of microalgae may have to consider the organic substrate the main source of energy and light the supplemental source of energy, a change in mindset.