Pilot-scale production of poly-β-hydroxybutyrate with the cyanobacterium Synechocytis sp CCALA192 in a non-sterile tubular photobioreactor

The biopolymer poly-beta-hydroxybutyrate (PHB) can be used as a promising bioplastic. It has a broad range of applications and is degraded relatively rapidly by soil organisms. Like many prokaryotes, the cyanobacterium Synechocystis sp. CCALA192 produces this biopolymer as a storage compound, especially under nutrient limitation. In a 200-L tubular photobioreactor, we cultivated Synechocystis sp. CCALA192 semi-continuously over a period of 75 days with CO2 as sole carbon source. A two-stage cultivation strategy was performed, where after 5-7 days nitrogen was depleted and the culture started to produce PHB and gradually turned from blue-green to yellow. After 16-20 days, 90% of the culture were harvested and the residual 10% were used as inoculum for the following cycle. The harvested culture had an average biomass concentration of 1.0 g/L with an average PHB content of 12.5% of cell dry weight. After restarting with fresh nutrients, the yellow culture turned blue-green again and degraded the PHB within 24-48 h. When nitrogen of the medium was consumed, PHB was produced again and the cycle continued. In the late stage of each production cycle, a ripening process was observed, where no CO2 was consumed but the PHB concentration was still rising at the expense of the existing glycogen rich biomass. Establishing a stable Synechocystis sp. CCALA192 culture under non-sterile conditions turned out to be difficult, as this small unicellular organism is very sensitive and easily grazed by protozoa. Therefore, a special cultivation strategy with partially anoxic conditions was necessary.