Efficient secretion of (R)-3-hydroxybutyric acid from Halomonas sp. KM-1 by nitrate fed-batch cultivation with glucose under microaerobic conditions

To establish a sustainable society, commodity chemicals need to be developed from biomass resources. Recently, (R)-3-hydroxybutyric acid ((R)-3-HB), a monomer of bioplastic poly-(R)-3-hydroxybutyric acid (PHB), has attracted attention for its possible use in the chemical industry. Halophilic bacteria have been considered for bioprocess applications due to certain characteristics such as the ability to grow in media containing high levels of the starting carbon source and the ability to be rarely contaminated. A halophilic bacterium Halomonas sp. KM-1 stores PHB intracellularly under aerobic conditions and secretes (R)-3-HB under microaerobic conditions. In this study, we optimized culture conditions to maximize (R)-3-HB secretion by KM-1 cells. By a simple nitrate fed-batch cultivation, Halomonas sp. KM-1 secreted 40.3 g/L (R)-3-HB with a productivity of 0.48 g L−1 h−1 with 20% (w/v) glucose. This level is one of the highest recorded productivity of (R)-3-HB to date.