Light requirement and photosynthetic cell cultivation - Development of processes for efficient light utilization in photobioreactors

Although the potential of photosynthetic microorganisms for production of v arious metabolites and in environmental bioremediation is recognized, their practical application has been limited by the difficulty in supplying ligh t efficiently to photobioreactors. Various types of photobioreactor with hi gh illumination to volume ratios have been proposed, but most are limited b y cost, mass transfer, contamination, scale-up or a combination of these. The problem of light supply to photobioreactors can be solved by developing photosynthetic cell cultivation systems where light is either substituted or supplemented. Many strains of photosynthetic cells are capable of hetero trophic growth under dark conditions and their heterotrophic culture can be used for efficient production of biomass and some metabolites. However, li ght is absolutely required for efficient production of some metabolites. In such cases, there is a need to supplement the heterotrophic with photoauto trophic metabolism. In photoheterotrophic (mixotrophic) culture, the photoa utotrophic and heterotrophic metabolisms can be exploited for efficient pro duction of useful metabolites but it has many problems such as process opti mization in terms of making a balance between the photoautotrophic and hete rotrophic metabolism. Another promising system is the sequential heterotrop hic/photoautotrophic cultivation system, where the cells are cultivated het erotrophically to high concentrations and then passed through a photobiorea ctor for accumulation of the desired metabolite(s). Furthermore, cyclic pho toautotrophic/heterotrophic cultivation system can be used to achieve conti nuous cell growth under day/night cycles. This involves cultivating the cel ls photoautotrophically using solar light during the day and then adding co ntrolled amount of organic carbon source during the night for heterotrophic growth. In this review, these various systems are discussed with some spec ific examples.