## 1 Introduction
Natural desulfurizing Gram-positive and -negative bacteria include several species belonging to the genera of _Bacillus_[15], _Gordonia_[16; 17; 18; 19; 20; 21], _Mycobacterium_[22; 23], _Microbacterium_[24], _Paenibacillus_[5], _Pseudomonas_[25; 26; 27], _Ralstonia_[28], _Sphingomonas_[29], _Staphylococcus_[30] and _Rhodococcus_[30; 31; 32; 33; 34; 35; 36; 37]. BDS is advantageous compared with traditional HDS since it operates at a low temperature and atmospheric pressure and has no need for H\({}_{2}\) or metal catalysts. Moreover, in a biorefinery context, BDS can be combined with the co-production of high-value products like carotenoids, biosurfactants and other biomolecules, all of which can help to improve the bioprocess's sustainability [21; 38]. Due to its inherent biological nature, the BDS process has some limitations, including (a) low reaction rates (slower process) [39], (b) inhibition by easily metabolized sulfur sources such as sulfate (SO4\({}^{2-}\)) [40], (c) the biocatalysts (the microorganisms) must be maintained under optimal conditions for their growth/metabolic activity and can be affected by the toxicity of compounds present in the fuels, as mentioned by [PERSON] et al. [20]. Several authors propose genetic engineering as a tool to bypass the above limitations [41; 42; 43]. Even if this approach is successful, the transformations can be partially or completely lost in a few generations, since heterologous genes constitute a metabolic burden for the microorganism [44]. Furthermore, plasmid instability necessitates antibiotic addition, a practice prohibitive from both an environmental and a sustainability point of view [39]. Another approach, implemented in the literature, includes changes in process design. In that context, BDS can be conducted by growing cells or resting cells. In the former approach, microorganisms are cultivated directly in oil and the OSCs act as the sulfur source for their proliferation [28; 45; 46]. BDS by resting cells differentiates the step of cell proliferation from that of biodieselurization. Two approaches are described in the literature based on the biocatalyst production method: the \"one step process\" where the biocalayst is produced in sulfur sources such as dimethyl sulfoxide (DMSO) or DBT that do not inhibit the BDS activity of the microorganism [18; 26; 31; 33] and the \"two-step process\" where the biocatalyst is produced in an easily metabolized sulfur source, followed by the induction of the BDS activity with molecules like DBT [47; 48]. Since the desulfurization of oil needs to be carried out continuously on a massive scale, the mass production of DBT-degrading bacteria with high desulfurization activity is required. Cultivation processes such as repeated batch and fed-batch can fulfill the requirement of high cell mass production. The bacterium _R. qingshengii_ IGTS8, used in the present study, is a well-characterized and widely studied biocatalyst for biodieselurization. It offers several advantages, including high efficiency in selectively removing sulfur from refractory OSCs such as dibenzothiophene (DBT) via the 45 pathway, without degrading the hydrocarbon skeleton. Moreover, _R. qingshengii_ IGTS8 exhibits a strong tolerance to toxic compounds and the harsh conditions typical of petroleum environments, largely due to its robust and hydrophobic cell wall [42]. In a previous study [33], we have demonstrated that in batch cultivation the cell mass concentration was quite low and also the desulfurization activity of _R. qingshengii_ IGTS8 was dependent on the age of the cells and maximized at the beginning of the exponential growth phase. These limitations restricted biomass yield and overall desulfurization efficiency, which are critical factors for practical industrial applications. The current study aims to overcome these challenges by systematically investigating different cultivation processes, batch, repeated batch and fed-batch, in order to enhance the biomass production of _R. qingshengii_ IGTS8.