[Formation of polyhydroxyalkanoates during the dual-nutrient-limited zone by Ralstonia eutropha]

Polyhydroxyalkanoates (PHAs) are a class of polyesters biosynthesized by microorganisms (esp. Ralstonia eutropha) under an unbalanced growth condition, and which are supposed to partly take the place of traditional plastics made from petroleum in the near future since they are harmless to the environment and biodegradable. Organic acids (mainly butyrate, lactate, propionate and acetate) produced from anaerobic digested food wastes, industrial wastes and sewage may be used as cheap carbon sources since the large amounts of the above wastes disposed by industry and family each year. In order to better understand the process of PHAs formation with acids as carbon sources, so as to increase the yields of PHAs. Biosynthesis of PHAs by R. eutropha during the dual nutrient-limitation-zone was investigated with mixed organic acids (the mass ratio of the four component acids was butyrate: propionate: acetate: lactate = 3: 3: 1: 1, which was simulated as once the result of anaerobic digestion of food wastes) as carbon sources and (NH4)2 SO4 as nitrogen source. Two different manners of maintaining the dual-nutrient-limitation zone were adopted by feeding mixed acids and (NH4 )2SO4 at determined rates to the fermentation culture which were free of carbon sources (manner A) or nitrogen sources (manner B) firstly. The results suggest that, first of all, the meaning of the limitation of mixed acids or (NH4)2 SO4 does not mean to limit the supply of them, but mean to feed as more as possible of carbon and nitrogen sources in order to meet the cell growth and PHAs formation of R. eutropha by the largest extent. However, it's indispensable to make the residual concentration of carbon and nitrogen sources as low as possible since organic acids are inhibitive to the cell growth, and most importantly, only under the presence of nitrogen during the PHAS formation period of the fermentation could R. eutropha produce more PHAs than any other unbalanced growth condition. Secondly, with the increase of the width of the dual-nutrient-limitation zone, the yield of PHAs would also increase, it suggest that most of the PHAs were biosynthesized during the dual-nutrient-limitation zone. Finally, in contrast with the dual-nutrient-limitation manner of limiting the nitrogen source at first (manner B), the dual-nutrient-limitation manner of limiting the carbon source at first (manner A) was more favorable for the production of PHAs, and the maximum production of PHAs of these two manners are 3.72 g/L and 2.55 g/L, respectively. It may be because that PHAs formation required enzymes could not be well developed when R. eutropha grow under the state of nitrogen limitation from the beginning of fermentation. Besides, yield of PHAs produced by the dual-nutrient-limitation fermentation is larger than that of the single-nutrient-limitation batch culture. Therefore, it seems that to increase the output of PHAs production, the strategy of maintaining as wide as possible the width of dual-nutrient (C, N)-limitation zone may be effective.