Optimization of a Desulfurizating Biocatalyst by Combining Cells of Different Cell Age of Pseudomonas putida CECT 5279 Calzada, J. 1 ; Heras, S. 1 ; Carbajo, J. 1 ; Alcon, A. 1 ; Santos, V.E. 1 ; García, J.L. 2 and García-Ochoa, F. 1 1 Dpto. Ingeniería Química. Facultad de Ciencias Químicas. Universidad Complutense de Madrid. Avda. Complutense s/n 28040 Madrid 2 Dpto. Microbiología Molecular. Centro de Investigaciones Biológicas. Consejo Superior de Investigaciones Científicas. 28040 Madrid Biodesulfurization is proposed as an alternative technology able to reduce sulfur content in fossil fuels, in order to aminish sulfur oxide emissions to the atmosphere. In this work, a genetically modified microorganism, Pseudomonas putida CECT5279, is employed as a desulfurizing biocatalyst. The combination of two cell age biomass can be used in order to optimize an effective biodesulfuring catalyst. Hereby, 5 hours and 23 hours were the selected growth times when cells were collected, combined in different biomass concentrations and tested by carrying out resting cells assays. In these experiments dibenzothiophene (DBT) was used as sulfur model compound. Biodesulfurization percentage, X BDS , initial DBT elimination rate, R DBT 0 , employed biomass, C x , and time for desulfurization, t BDS , were the chosen parameters in order to search for the best biocatalyst. After this study, the mixture of 0,7 and 1,4 gDCW/L biomass concentration were selected for 5 hours and 23 hours growth time cells respectively, because of the best effectiveness achieved with formulation for a biodesulfurizing catalyst. 1. Introduction World energy expense overcame 7900 Mtoe in 2005 and it is showing an annual increase of 2.2% (International Energy Agency, 2007). Population consumption of fossil fuels causes an important emission of sulfur oxides to the atmosphere. These compounds are recognized to be involved in many environmental, health and material stability problems. Therefore, more and more restrictive legal limitations about sulfur content in fossil fuels have been imposed in order to control emissions in combustion processes. For instance, European Union has fixed maximum sulfur content at 10 ppm for 2009 in diesel fuel (European Directive, 2003). Many technologies have been proposed to achieve these low limits (Babich and Moulijn, 2003). Among them, hydrodesulfurization (HDS) has been the most extensively employed. However, the severe conditions of pressure and temperature needed to be employed in deep HDS cause alterations in fuel final characteristics (Babich and Moulijn, 2003).