Enhancing Biomass Production and Biodesulfurization Efficiency of Rhodococcus qingshengii IGTS8: Evaluation of Batch, Repeated Batch and Fed-Batch Cultivation Techniques

The batch, repeated batch and fed-batch cultivation strategies, in stirred tank bioreactors, were evaluated to maximize biomass production and the cells’ desulfurization activity (CDA) of Rhodococcus qingshengii IGTS8. The batch culture reached 2.62 g DCW/L biomass, with a productivity of 0.03 g DCW·L−1·h−1 and only 26% glycerol consumption. The repeated batch strategy reduced cultivation time during the first cycle, increasing biomass production by 15%, with 30% glycerol consumed and productivity 2.3 times higher than the batch process; however, subsequent cycles showed no further improvement. CDA peaked early in both modes but declined to 12–13 U/mg DCW by the end of the exponential growth phase. Fed-batch cultivation achieved 8.35 g DCW/L with 87% glycerol consumption, resulting in a threefold increase in volumetric productivity and a 1.7-fold higher specific growth rate compared with the batch mode. CDA remained stable during the fed-batch process and was approximately 40% higher compared with the batch and repeated batch processes. The fed-batch culture was used directly in a two-phase bubble column bioreactor for the desulfurization of dibenzothiophene (DBT), 4-methyl-dibenzothiophene (4-MDBT) and their mixture. The complete desulfurization of 1.4 mM DBT was achieved at a rate of 21.6 mmol DBT/kg DCW/h, while 0.9 mM 4-MDBT was fully converted but at a 2.5-fold lower rate. The simultaneous conversion of the DBT/4-MDBT mixture showed reduced efficiencies of 59.6% and 41.2%, respectively.