Cultivation of the PHB-producing cyanobacterium Synechococcus leopoliensis in a pilot-scale open system using nitrogen from waste streams

The cultivation of PHB-producing cyanobacteria is a promising approach to obtain the raw material for bio-based and biodegradable plastics. To commercialise photoautotrophic PHB production, the cultivation needs to be scaled up in open cultivation systems and to improve sustainability of the process, nutrients must be obtained from waste streams. Different PHB-producing cyanobacteria were compared in laboratory-scale cultivations using either water from recirculating aquaculture systems or pre-processed liquid digestate as nutrient sources. The species Synechococcus leopoliensis was cultivated in an open thin-layer photobioreactor (18 m², 200 L), were growth in mineral Z-medium was again compared to said waste streams. Cultivation in mineral medium resulted in both the highest final biomass yield (6 g L^-1) and productivity (0.7 g L^-1 d^-1). Both waste stream-based media showed lower biomass yields and productivities (2 g L^-1 and 0.25–0.3 g L^-1 d^-1). However, due to differences in the cultivation conditions (e.g., temperature, nutrient supply), final biomass yield and productivity do not represent the performance of the cultivations adequately. Relative parameters such as nitrogen and energy conversion ratios indicate that cultivation with aquaculture water suffered from insufficient nitrogen supply to the culture, whereas the use pre-processed liquid digestate resulted in a substantial loss of nitrogen due to volatilization. All cultivations in the open system were continued in the laboratory, where cultures were starved for ten days under nutrient-depleted conditions (without nitrogen, phosphorus, or both). While PHB accumulation occurred, concentrations were comparatively low (< 1 %_dw). The comparison of the results suggests that PHB yields were influenced more by the initial cultivation condition than by the specific type of nutrient depletion. Thus, while the cultivation with waste streams in an open system is feasible, environmental parameters seem to influence PHB yields considerably and must be considered for the optimisation of the complete process.