SUMMARY: Product excretion by sp. NCIB 12047, NCIB 9399 and during growth on methanol was examined. These organisms possess the ribulose monophosphate pathway, the serine pathway and the dihydroxyacetone pathway of C assimilation, respectively. Only 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 CO. The maximum yield for polysaccharide production by sp. was 0·34 g (g oxygen) and 0·30 g (g methanol). The maximum specific rate of polysaccharide production was 0·18 g (g protein) h. 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 C fixation routes indicates that the ribulose monophosphate pathway is energetically the most favourable. Polymer production by 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 C assimilation strongly influence both the rate of synthesis and the composition of exopolysaccharides produced by methylotrophs.


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