Simultaneous utilization of methanol–glucose mixtures by Hansenula polymorpha in chemostat: Influence of dilution rate and mixture composition on utilization pattern

Abstract

The utilization of mixtures of methanol (C₁) and glucose (C₆) of different composition by the methylotrophic yeast Hansenula polymorpha was studied in carbon-limited chemostat culture. For all mixtures tested a similar utilization pattern was observed: At low dilution rates both carbon sources were utilized simultaneously, but at high dilution rates the cells used glucose only and the unutilized methanol accumulated in the culture medium. When grown with C₁ only, the cells exhibited a critical dilution rate D_c(C₁) of 0.19 h⁻¹, but when C₁–C₆ mixtures were used as the carbon and energy substrate, the yeast was able to completely utilize C₁ at dilution rates considerably higher than D_c(C₁). The dilution rate at which the transition from C₁–C₆ growth to C₆ growth occurred (D_t) was strictly dependent on the composition of the C₁–C₆ mixture in the feed, and D_t increased with decreasing proportions of C₁ in the mixture. During mixed substrate growth the formation of biomass from the two substrates was additive. The results reported indicate that the utilization of C₁–C₆ mixtures and hence D_t in H. polymorpha are subject to two different regulatory regimes. When the cells were growing with C₁–C₆ mixtures containing more than 60% C₁, the transition form C₁–C₆ to C₆ growth was most probably influenced by the maximum C₁ oxidizing capacity of the cells, whereas for growth with mixtures containing less than 40% C₁, a growth rate of 0.28–0.30 h⁻¹ seemed to be the limiting barrier for the simultaneous utilization of the components of the binary carbon and energy substrate mixture.