Abstract
Sphingomonas sp. S213 is a soil oligotrophic, Gram-negative bacterium capable of degrading lignin-related ferulic acid. In this study, the capacity of strain S213 to degrade ferulic acid was characterized from its growth parameters in batch and continuous cultures and the first steps in the degradation pathway were identified. Batch cultures with different concentrations of ferulate (0-9.8 mM) showed a profile of substrate inhibition of growth; i.e. the growth rate increased with ferulate concentration up to an optimum value (0.15-0.3 mM), after which the inhibitory effect became dominant. In contrast, the ferulate degradation rate decreased progressively with increasing concentration. HPLC analysis and oxygen uptake experiments on resting cell suspensions indicated that the first steps in ferulate degradation involve the conversion to vanillate followed by demethylation to protocatechuate, then ring fission of protocatechuate. In chemostat cultures, the specific rates for converting ferulate to vanillate and for converting vanillate to protocatechuate were a linear function of the growth rate. At growth rates above 0.044 h−1 (medium with 0.02% yeast extract) or 0.054 h−1 (with 0.05% yeast extract), the increase in the rate for converting ferulate to vanillate was not accompanied by a proportional increase in the rate for converting vanillate to protocatechuate. This implied that strain S213 had an overcapacity for the conversion of ferulate to vanillate during ferulate metabolism.
