Carbon material balances and bioenergetics of 2,3-butanediol bio-oxidation by Acetobacter hansenii

The bio-oxidation of 2,3-butanediol to acetoin and diacetyl was studied exploiting the membrane-bound alcohol dehydrogenase activity of whole cells of Acetobacter hansenii MIM 2000/5, which previously proved very effective in the dehydrogenation of primary and secondary alcohols as well as diols. The experimental data of concentrations of C-containing products (acetoin, diacetyl, biomass and carbon dioxide) and 2,3-butanediol, collected versus time during batch oxidations, were used to propose and check a mechanistic model for such a biosystem. Based on the present knowledge on the respiratory chain of acetic acid bacteria, material balances performed at different starting concentrations of the carbon source (1.8-10gl-1) and oxygen levels (10.0-27.5% of the saturation value in water) were used in a bioenergetic study of 2,3-butanediol bio-oxidation. The experimental data at various oxygen levels allowed estimating the P/O ratio making different energetic hypotheses for the catabolism. The values of this parameter were particularly low (0.17-1.04molATP molO-1) if compared with those reported for the growth of different aerobes on simple carbon sources, thus suggesting a dependence of the ATPase activity on the medium composition and the existence of an energy-expensive system for acetoin transfer into the cell. The estimated overall energy consumption for cell growth and maintenance proved to be a decreasing function of the starting carbon source concentration