Outdoor production of a nitrogen-fixing photosynthetic microbial consortium in batch and semi-continuous culture in a flat-panel airlift photobioreactor

Cyanobacteria are oxygenic phototrophic organisms that have the capacity to form consortia. One strategy that has been used to improve the productivity of cyanobacteria and algae in photobioreactors is to produce them in semicontinuous cultures. In our research group, a photosynthetic microbial consortium (MC) with nitrogen fixing capacity was studied. It consists mainly of cyanobacteria and could potentially be used as a biofertilizer. The aim of this study was to evaluate the capability to produce biomass of MC in two flat-panel airlift photobioreactors (PBR) (50 y 150 L) in batch culture, and in semicontinuous culture in a PBR with 150 L capacity, outdoors. The maximum biomass concentration in batch culture was obtained in the PBR of 150 L, with a value of 600 mg/L, in comparison with the biomass concentration obtained in the PBR of 50 L, which was equal to 385 mg/L. The biomass productivity at the end of the batch culture in both PBR (50 and 150 L) was 0.77 and 1.19 mg/L/h, respectively. The semicontinuous culture of CM was operated during three cycles (7, 7 and 5 d, respectively), attaining an increase in the biomass productivity following each cycle, with values of 1.86, 1.94 and 2.94 mg/L/h, respectively. Similarly, the maximum values of the nitrogenase activity were obtained at the end of each cycle with values of 4963, 5366 and 5689 nmol ethene/g dry weight/h, respectively. These were higher than the values obtained in batch cultures. In conclusion, it was possible to produce the MC in the PBR in batch and semicontinuous mode outdoors, without affecting the nitrogen fixation capacity.