A new impetus for biodesulfurization: bypassing sulfate inhibition in biocatalyst production

Abstract

Biodesulfurization is a biotechnological process that employs microorganisms as biocatalysts to remove sulfur from fuels usually at mesophilic conditions, targeting recalcitrant organosulfur compounds without affecting their hydrocarbon structure. One of the bottlenecks hindering its large-scale application is the inhibition of biodesulfurization activity by easily metabolized sulfur compounds, such as sulfates, even at residual concentrations. This increases production costs by requiring high-purity sulfur-free nutrients or complex induction steps to prevent/revert inhibition. The objective of this work was to bypass this limitation and demonstrate that it is possible to produce biocatalysts with biodesulfurization activity using sulfate as the only sulfur source, without employing inducers or genetic manipulation, simply by adjusting the sulfur : carbon ratio in continuous culture. With this goal, the bacterium Gordonia alkanivorans strain 1B was cultivated in a chemostat with a medium containing 10 g L⁻¹ of fructose as the carbon source and different sulfate concentrations (12–50 mg per L SO₄²⁻) using Na₂SO₄. Then the bacteria were employed as biocatalysts in biodesulfurization assays with a recalcitrant organosulfur compound (dibenzothiophene). Under these conditions it was observed that 2.2 mg_sulfate g_fructose⁻¹ ensured a biodesulfurization activity of 6.1 μmol g_DCW⁻¹ h⁻¹, 15% greater than previously reported for this strain with an inducer, without limiting biocatalyst production. This novel procedure was further applied to another biocatalyst, Rhodococcus erythropolis strain D1, validating its wide applicability to other desulfurizing microorganisms. Overall, these results indicate a previously unknown regulation mechanism dependent on relative sulfur concentration, which influences cellular responses and regulates biodesulfurization activity, allowing the use of easily metabolized sulfur sources to produce cost-effective biocatalysts for biodesulfurization.