The Energetics and Kinetics of Extracellular Polysaccharide Production From Methanol by Micro-organisms Possessing Different Pathways of C1 Assimilation

Abstract

SUMMARY: Product excretion by Methylophilus sp. NCIB 12047, Pseudomonas extorquens NCIB 9399 and Pichia pastoris during growth on methanol was examined. These organisms possess the ribulose monophosphate pathway, the serine pathway and the dihydroxyacetone pathway of C1 assimilation, respectively. Only Methylophilus sp. NCIB 12047 produced significant amounts of extracellular product from methanol under conditions of nitrogen limitation in chemostat culture. This was a low-viscosity extracellular polysaccharide containing glucose and mannose in the ratio 3:1. Maximum polysaccharide production occurred under nitrogen limitation at a methanol/ammonium sulphate ratio > 10. The other two organisms responded to nitrogen limitation by increasing the rate of methanol oxidation to CO2. The maximum yield for polysaccharide production by Methylophilus sp. was 0·34 g (g oxygen)-1 and 0·30 g (g methanol)-1. The maximum specific rate of polysaccharide production was 0·18 g (g protein)-1 h-1. Methylophilus sp. grew readily under oxygen limitation and excreted an extracellular polysaccharide under these conditions. Examination of the biochemical pathways for polysaccharide production via the various C1 fixation routes indicates that the ribulose monophosphate pathway is energetically the most favourable. Polymer production by Methylophilus sp. is energetically neutral in terms of net ATP demand; however, the rate of ATP utilization for polymer production is equivalent to 65 to 80% of that required for cell production at the same growth rate. The results reported suggest that the energetic constraints imposed by the various pathways of C1 assimilation strongly influence both the rate of synthesis and the composition of exopolysaccharides produced by methylotrophs.