L-Lactate production from carbon dioxide in the red alga Cyanidioschyzon merolae

Abstract

Lactate/lactic acid is a commodity chemical used in various products such as bioplastics. The optical isomers of lactate are l- and d-lactate. Both the lactate enantiomers are required to manufacture lactate derived bioplastics. Eukaryotic microalgae and cyanobacteria can produce l-lactate or d-lactate using carbon dioxide as their sole carbon source. However, eukaryotic microalgae and cyanobacteria with high l-lactate productivity have not yet been identified. Cyanidioschyzon merolae is a hot-spring red alga preferring an acidic pH and high temperature. The entire genomic sequence was determined. C. merolae contains five l-lactate dehydrogenases that catalyze l-lactate-generating reactions during lactate fermentation. This study revealed that C. merolae produces l-lactate with high productivity during dark anaerobic cultivation. Nitrogen starvation before dark anaerobic cultivation increased the l-lactate productivity in C. merolae. l-Lactate accounted for over 90% of the total organic acids excreted by C. merolae cells. C. merolae exhibited a high l-lactate productivity under cultivation at a neutral pH. The l-lactate productivity in C. merolae at 16.0–19.4 mg/L/h was higher than that in a unicellular cyanobacterium at 2.7 mg/L/h. These findings indicate that C. merolae is one of the candidates for a host of l-lactate production from carbon dioxide.