## 3 Results and Discussion
### Cultivation of R. qingshengii IGTS8 Under Fed-Batch Mode
Biomass productivity, calculated at the time point of maximum cell mass concentration, was found to be three times higher (0.09 \(\pm\) 0.001 g DCW/L/h) compared with batch cultivation (Table 1). As mentioned above, the DO at the end of the batch stage was approximately at 35% of saturation (air flow 0.8 L/min). When the fed-batch process started, the air flow was adjusted to 1.25 L/min and kept constant until the end of the process. Further changes in DO were accomplished by changing agitation speed, when necessary. It should be noted that the agitation speed was kept in the range of 200 rpm (initial conditions) to 380 rpm (maximum agitation speed) (Figure 3b). Changes in both agitation rate and air flow cause changes in the hydrodynamic conditions and those in turn may negatively affect the growth of the microorganism due to the possible cell damage caused by shear effects. [PERSON] et al. (2007) reported that the growth of _R. erythropolis_ under different fluid dynamic conditions (air flow and agitation rate) in the bioreactor is negatively influenced at high agitation rates (\(>\)450 rpm), while the change in the air flow rate (1-10 L/min) hardly has any influence on the growth rate when agitation rate is kept at 250 rpm. The DO continued to decrease after the change in air flow to 1.25 L/min and at the minimum point, changes in agitation rate resulted in an increase in DO. As feeding continued, the DO started to decrease again, indicating the demand for oxygen by the cells (Figure 3b). Figure 4 depicts the evolution of CDA during the fed-batch process. CDA at the onset of the fed-batch process had an average value of 22.6 \(\pm\) 0.5 U/mg DCW. CDA was almost stable during the fed-batch process, while the value of 20.7 \(\pm\) 0.09 Units/mg DCW was achieved at the time point of maximum cell mass concentration (89 h). Furthermore, CDA in the fed-batch process was found to be approximately 40% higher at the end of the fed-batch process compared with the batch and repeated batch processes (Table 1). Figure 4 also depicts the logistic model fitting. The specific growth rate (\(\mu\)) improved significantly compared with the batch and repeated batch cultivation (Table 1). Overall, the fed-batch process was proved to be the best cultivation mode to obtain a high biomass concentration with a high CDA. Furthermore, the analysis of variance revealed statistically significant differences (\(p<\) 0.0001) across all evaluated process parameters, namely biomass concentration (X), specific growth rate (\(\mu\)), biomass yield (Yx/s), volumetric productivity and cells' desulfurization activity (CDA), among the different cultivation strategies (Table 1). Fed-batch cultivation consistently exhibited superior performance, with significantly elevated values in each metric compared with the batch and repeated batch modes. Different fed-batch approaches for the production of high cell density cultures with high desulfurization activity (CDA) have been reported in the literature. A two-step process was reported by [PERSON] et al. [47], in which the step of growth (fed-batch cultivation) of _R. erythropolis_ IGTS8 and that of the desulfurization activity induction were separated. The application of a pH-stat feeding method using acetate as the carbon source, resulted in 33 g DCW/L after 28 h of cultivation. Following completion of the fed-batch process, biodegradation activity was induced by incubating the cells for 3 to 4 h in an induction medium containing DBT. Cells exhibited a desulfurization activity of 6.1 mmol 2-HBP/kg DCW/h (corresponding to 6.1 Units/mg DCW). A two-step process was also reported by [PERSON] et al. [48] for the bacterium _Gordonia nitida_ CYKS1. A fed-batch process using sucrose as a carbon source resulted in 92.6 g DCW/L at 120 h of cultivation. Cells were subjected to a 4-8 h induction in the presence of DBT and tested in low-sulfur diesel oil (250 mg sulfur/L) where the specific desulfurization efficiency was found to be equal to 10.8 mmol sulfur/kg DCW/h.