Asymmetric reduction of hydroxyacetone to propanediol in immobilized halotolerant microalga Dunaliella parva

Cells of the halotolerant microalga Dunaliella parva immobilized in calcium -alginate gel showed morphological characteristics and photosynthetic activ ity similar to those of free cells. The photosynthetic activity in immobili zed cells fell only about 20% during 2 weeks incubation at 5 degrees C in t he light. The asymmetric reduction of hydroxyacetone to (R)-propanediol was first examined in batch reactions with both free and immobilized cells. Si nce the activity of immobilized cells was as high as that of free cells, th e optimum conditions for the column reactor were investigated using immobil ized cells. The productivity of propanediol in immobilized cells increased in proportion to increases in the light intensity, suggesting that NADPH re generated through photosynthesis was used for the asymmetric reduction. Ele ctron microscopic observation of thin sections of immobilized cells after t he reaction suggested that NADPH used for asymmetric reduction in the dark was produced through the metabolism of starch granules. The use of a CaCO3- free medium in the column reactor markedly prolonged the period of reductiv e activity and photosynthesis.