Abstract
The Synechocystis sp. PCC 6803 is a promising host for ethanol biosynthesis from CO2. In this study, the production of ethanol was performed by utilizing two different engineered Synechocystis strains consisting of pdc-adh (pyruvate decarboxylase and alcohol dehydrogenase) overexpression (▲APX) and glgC-phaA (glucose-1-phosphate adenylyltransferase and PHA-specific β-ketothiolase) knockout (ΔGBK) pathways. This strategy involves destruction of glycogen and polyhydroxybutyrate (PHB) synthesis, both of which act as a storage polymer. The blocking of such biosynthetic pathways enabled Synechocystis to release various metabolites into the medium with an increase of chlorophyll a content and oxygen evolution due to the reduction of glycogen and PHB synthesis. In co-cultivation system, the extracellular metabolites released from knock out strain (ΔGBK) were utilized by the pdc-adh overexpressing strain (▲APX) and produced 4708 mg/L ethanol, which is comparatively higher than pdc-adh overexpressing strain containing glgC and PhaA knock out (ΔGBK-▲APX) (4103 mg/L). Although the concentration of ▲APX cells were lower in co-culture than in mono-culture, the overflow of metabolites from knock out Synechocystis and the consumption of those products by pdc-adh overexpressing Synechocystis in co-cultivation system improved the overall yield.
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