THE ROLE OF FORMATE FOR THE GROWTH OF RHODOCOCCUS-OPACUS UFZ B-408 ONPYRIDINE

R. opacus UFZ B 408 is able to use pyridine, a potentially growth-inhi biting substrate, as the sole source of carbon, energy and nitrogen. I n a previous publication [1] we reported that with the simultaneous ut ilization of a second carbon and energy source in carbon-substrate-lim ited chemostat culture, stable steady states could be achieved at high er dilution rates than with growth on pyridine as the sole substrate. Owing to the higher growth yield during growth on such a substrate mix ture, both the specific pyridine consumption rates and the residual py ridine concentrations were lower at similar dilution rates than with g rowth on pyridine alone. Therefore, the critical growth-inhibitory pyr idine concentration was only achieved at a higher dilution rate. With the investigations presented here in carbon-substrate-limited continuo us culture, the simultaneous utilization of pyridine and formate by R opacus UFZ B 408 was studied. The yield coefficient during growth on p yridine as the sole substrate amounted to about 0.55 g dry mass/g pyri dine. Theoretically, however, the carbon-metabolism-determined yield c oefficient should have been about 0.915 g dry mass/g pyridine. Because of the difference between these two values the conclusion was drawn t hat pyridine is energetically deficient. That means that during growth on pyridine a part of the substrate was dissimilated to supply the en ergy required for the incorporation of the pyridine carbon into biomas s. Formate cannot be used as a carbon source for growth by R. opacus U FZ B 408. However, with growth on pyridine, formate was oxidized simul taneously. During growth on pyridine/formate mixtures, the yield coeff icient could be enhanced up to 0.7 S dry mass/g pyridine. That means t hat biologically usable energy, generated in the course of the formate oxidation, was used for the assimilation of pyridine carbon. The incr ease in the yield coefficient was related to the utilization ratio of formate to pyridine in a linear manner. However, the carbon-metabolism -determined yield coefficient of 0.915 g dry mass/g pyridine could not be achieved. That can be put down to the fact that R. opacus UFZ B 40 8 possesses only a limited capacity to oxidize externally supplied for mate. Because of the limited formate oxidation capacity the probabilit y is low that, with simultaneous utilization of formate, stable steady states could be achieved at substantially higher dilution rates than with growth on pyridine alone. Enzymatic studies revealed the inductio n of both NAD(P)(+)-linked glutaric dialdehyde dehydrogenase and isoci trate lyase during growth on pyridine. Therefore, the conclusion was d rawn that pyridine is metabolized by R. opacus UFZ B 408 via the same pathway described for the utilization of pyridine by Nocardia Z1 [2]. This conclusion implies that the ability to oxidize formate represents a metabolic performance which seems not to he directly related to the pyridine metabolism of R. opacus UFZ B 408.