PHYSIOLOGY OF POLY-3-HYDROXYBUTYRATE (PHB) PRODUCTION BY ALCALIGENES-EUTROPHUS GROWING IN CONTINUOUS-CULTURE

Alcaligenes eutrophus was grown in continuous culture (34 degrees C, p H 6.8) under various conditions with respect to dilution rate, nutrien t limitation and carbon substrate. Poly-3-hydroxybutyrate (PHB) conten t, the rate of PHB production (q(PHB)) and the rate of carbon substrat e utilization (q(s)) during growth on glucose were maximum at low dilu tion rate under ammonia limitation (ammonia limitation > potassium/oxy gen limitation > glucose limitation), PHB content decreased in a linea r manner as a function of dilution rate, from approximately 80% at D 0 .025 h(-1) during ammonia-limited growth to approximately 5% during gr owth at the maximum specific growth rate (mu(max)) in batch culture, P HB content, q(PHB) and q(s) varied with the nature of the carbon subst rate during ammonia-limited growth at fixed dilution rate, and were ma ximum during growth on lactate [lactate > pyruvate > glucose/gluconate > fructose; highest q(PHB) 0.38 g PHB (g non-PHB biomass)(-1) h(-1)]. q(PHB) was related in an approximately linear manner to the q, in exc ess of that required solely for the production of non-PHB biomass, Thi s surplus q(s) was higher during growth on lactate than on glucose bec ause q(s) was approximately equal to the maximum rate of carbon substr ate utilization (q(smax)) during growth on lactate, but much lower tha n q(smax) during growth on glucose. The relationship between q(PHB) an d surplus q(s) was confirmed by the effect of adding formate (as an ad ditional source of NADH and/or ATP) and the uncoupling agent carbonyl cyanide-m-chlorophenylhydrazone (CCCP) to ammonia-limited cultures. It is concluded that A. eutrophus is unable to regulate the rate at whic h it takes up excess carbon substrate to match that required solely fo r growth, particularly during growth on lactate at low dilution rate, and thus produces PHB as a means of avoiding the potentially deleterio us effects of generating high concentrations of intracellular metaboli tes. Possible ways of further increasing PHB production are discussed.