Microalgal mass culture systems and methods: Their limitation and potential

Cultivation of microalgae using natural and man-made open-ponds istechnologically simple, but not necessary cheap due to the high downstream processing cost. Products of microalgae cultured in open-pondscould only be marketed as value-added health food supplements, specialityfeed and reagents for research. The need to achieve higher productivityand to maintain monoculture of algae led to the development of enclosedtubular and flat plate photobioreactors. Despite higher biomassconcentration and better control of culture parameters, data accumulatedin the past 25 years have shown that the illuminated areal, volumetricproductivity and cost of production in these enclosed photobioreactors arenot better than those achievable in open-pond cultures. The technicaldifficulty in sterilizing these photobioreactors has hindered their applicationfor the production of high value pharmaceutical products. The alternativeof cultivating microalgae in heterotrophic mode in sterilizable fermentorshas achieved some commercial success. The maximum specific growth ratesof heterotrophic algal cultures are in general slower than those measured inphotosynthetic cultures. The biomass productivity of heterotrophic algalcultures has yet to achieve a level that is comparable to industrialproduction of yeast and other heterotrophic microrganisms. Mixotrophiccultivation of microalage takes advantage of their ability to utilise organicenergy and carbon substrates and perform photosynthesis concurrently. Moreover, production of some algal metabolites is light regulated. Futuredesign of sterilizable bioreactors for mixotrophic cultivation of microalgaemay have to consider the organic substrate the main source of energy andlight the supplemental source of energy, a change in mindset.