EFFECT OF SINGLE NUTRIENT LIMITATION ON POLY-BETA-HYDROXYBUTYRATE MOLECULAR-WEIGHT DISTRIBUTION IN ALCALIGENES-EUTROPHUS

The effect of magnesium and phosphate limitation on the molecular weig ht distribution of poly-beta-hydroxybutyrate (PHB) in Alcaligenes eutr ophus in continuous culture has been studied. Conditions of nitrogen l imitation both with glucose excess (above ca. 20 g/L) and without exce ss were investigated. Under N-limitation and glucose excess, M(w) decr eases when the magnesium content is decreased below 50% (19.7 mg/L) of the basal medium content; this also results in a broadening of molecu lar weight distribution (M(w)/M(n)) from 2 to 5 and a decrease in M(w) from 2 x 10(6) to 0.9 X 10(6). Below 20% of the basal content of magn esium (7.9 mg/L) these two trends were reversed. This behaviour was no t observed in the absence of glucose excess. Under N-limitation and gl ucose excess, phosphate had virtually no effect on PHB M(w) or its dis tribution, whereas with no (or little) glucose excess M(w) of the PHB decreased with phosphate concentrations below 50% of the basal level ( 0.705 g/L). Hence, in continuous or fed-batch cultures, in addition to nitrogen limitation to allow for PHB accumulation, it is necessary to control both the addition of glucose (no excess) and also to maintain magnesium limitation (ca. 25% of basal medium level, 9.9 mg/L) and ph osphate above 50% of the basal level (0.705 g/L). Thus, when broadenin g of molecular weight distribution (increase in M(w)/M(n)) is observed at the end of fed-batch culture it is probably caused by phosphate li mitation and/or glucose excess. (C) 1995 John Wiley & Sons, Inc.