The production of a biocatalyst of a genetically modified microorganism (GMO) (Pseudomonas putida CECT5279) that can desulfurize dibenzothiophene (DBT) has been studied. The biomass growth rate and the development of the sulfur-removal capability during microorganism growth have been measured and modeled. Different growth media and different carbon-glucose, carboncitrate, and carbon-glutamic acid sources, as well as different nitrogen-ammonium and/or nitrogen-glutamic acid sources, have been used, in addition to different glutamic acid concentrations (5, 10, 20, and 40 g/L), to study their influence on the growth rate and desulfurizing capability. Experimental results show clear differences both in growth and in the biodesulfurization capability developed by the cells, depending on the media composition. To quantify the desulfurization capability, a parameter has been defined: D BDS , which is the degree of biodesulfurization developed during growth. This parameter is useful not only to compare the results achieved under different media and conditions but also to compare different microorganisms, in regard to the desulfurization capability. Inside the experimental range studied, the best production media is composed of 20 g/L of glutamic acid, with 670 ppm of NH 4 + as the respective carbon and nitrogen sources. A nonstructured kinetic model that describes the growth and desulfurizing capability development is proposed and applied by nonlinear simple response fitting to all the experiments that have been performed. The model can be used to describe all the experimental data with good statistical parameters.

The influence of working conditions on the growth batch of Pseudomonas putida CECT5279 has been studied, in regard to both the growth rate and the desulfurization capability accumulated in the cells. These operational conditions include pH conditions (buffered and nonbuffered media, using different carbon sources (glucose, citrate, and glutamic acid)), operating temperatures (26-32 °C), and different dissolved oxygen concentrations, due to different aeration conditions (different air flows, using enriched air, etc.). Pseudomonas putida CECT5979, which is a genetically modified microorganism (GMO), has the ability to convert dibenzothiophene (DBT) to 2-hydroxybiphenyl (HBP), desulfurizing the organic molecule. To get the best conditions to obtain desulfurizing cells, a parameter (D BDS ) that incorporates both biomass concentration and time to reach a particular percentage of desulfurizing capability (X BDS ) has been used. The optimum value of D BDS has been obtained under the following working conditions: temperature, 30 °C; nonbuffered medium with glutamic acid as the carbon source; and, in relation to the dissolved oxygen concentration, the best conditions for growth are not the same as those required to get the highest desulfurizing activity. A kinetic model based on a logistic equation has been applied to describe biomass concentration during P. putida CECT5979 growth. Kinetic model parameters (µ and C X max ) were obtained under several operating conditions. A model proposed in a previous work [Martin et al., Energy Fuels 2004, 18, 851-857] was applied to describe biodesulfurization capability evolution during growth. Predicted values of biomass concentration and biodesulfurizing capability percentage achieved by the cells can be obtained during bacteria growth, with values very similar to those found experimentally, in a wide interval of operating conditions.

No abstract

The growth rate and desulfurization capacity accumulated by the cells during the growth of Pseudomonas putida KTH2 under different oxygen transfer conditions in a stirred and sparged tank bioreactor have been studied. Hydrodynamic conditions were changed using different agitation conditions. During the culture, several magnitudes associated to growth, such as the specific growth rate, the dissolved oxygen concentration and the carbon source consumption have been measured. Experimental results indicate that cultures are influenced by the fluid dynamic conditions into the bioreactor. An increase in the stirrer speed from 400 to 700 rpm has a positive influence on the cell growth rate. Nevertheless, the increase of agitation from 700 to 2000 rpm hardly has any influence on the growth rate. The effect of fluid dynamics on the cells development of the biodesulfurization (BDS) capacity of the cells during growth is different. The activities of the intracellular enzymes involved in the 4S pathway change with dissolved oxygen concentration. The enzyme activities have been evaluated in cells at several growth time and different hydrodynamic conditions. An increase of the agitation from 100 to 300 rpm has a positive influence on the development of the overall BDS capacity of the cells during growth. This capacity shows a decrease for higher stirrer speeds and the activity of the enzymes monooxygenases DszC and DszA decreases dramatically. The highest value of the activity of DszB enzyme was obtained with cells cultured at 100 rpm, while this activity decreases when the stirrer speed was increased higher than this value.

The specific growth and the xanthan production rates by the bacterium Xanthomonas campestris under different shear levels in shake flasks and in a stirred and sparged tank bioreactor have been studied. The shake flask has been used as a reference for studying the shear effects. An effectiveness factor expressed by the ratio of the observed growth rate and the growth rate without oxygen limitation or cell damage was calculated in both modes of cultures. It was observed that the effectiveness factor was strongly dependent on the operational conditions. A strong oxygen transfer limitation at low stirring rates, indicated by a 54 % decrease in the effectiveness factor was observed. In contrast, at higher stirrer speed, cell damage was caused by hydrodynamic stress in the turbulent bulk of the broth, yielding again a decrease in the effectiveness factor values for stirrer speeds higher than 500 rpm. Cell morphological changes were also observed depending on the agitation conditions, differences in morphology being evident at high shear stress.