Mixotrophic Cultivation Of Synechocystis Sp. PCC6803 In An Enclosed Photobioreactor And Its Physiology

The development of cyanobacterial genetic engineering makes it an increasing necessary to develop high cell density cultivation of cyanobacteria in the enclosed photobioreactors. Synechocystis sp. PCC6803, which is important material for studies on the mechanism of photosynthesis and molecular biology as well as the most popular host system for cyanobacterial genetic engineering, has special and representative growth modes in cyanobacteria. It can grow under photoautotrophic and photoheterotrophic but heterotrophic conditions. It is full of promise to obtain high cell density cultivation by mixotrophy, especially for cyanobacteria like Synechocystis sp. PCC6803. Up to now, few papers have been published on the mixotrophic cultivation of Synechocystis sp. PCC6803 in an enclosed photobioreactor. In this thesis, the factors affecting the mixotrophic growth of Synechocystis sp. PCC6803, especially in an enclosed photobioreactor, and the physiologic characteristics of cells grown under mixotrophic conditions were studied and the mechanism of Synechocystis sp. PCC6803 utilizing glucose for growth was also discussed.Organic carbon source and light were two most important factors affecting mixortophic growth of Synechocystis sp. PCC6803. In this study, glucose was the most suitable organic carbon source, and its optimal concentration for mixotrophic growth was about 1-3g/L. The saturated light intensities of mixotrophic growth of Synechocystis sp. PCC6803 in shake flasks and a 2.5L enclosed photobioreactor were 47.3 μ Em^-2s^-1 and 78.8 μ Em^-2s^-1, respectively. When the light intensity was higher than the saturated level, the mixotrophic growth rates would decrease. The saturated level of light intensity in mixotrophic culture was almost the same as that in photoautotrophic culture. At the same level of light intensity, the growth rate, cell density and energy efficiency in mixotriophic conditions were higher than those in photoautotrophic conditions.The light attenuation inside photoautotrophic culture was faster than that in mixortophic culture. In a given bioreactor, when incident light intensity and biomasswere identical, the cells inside mixotrophic culture would receive more light than that inside photoautotrophic culture.Studies on mixotrophic batch cultivation were carried out in a 2.5L enclosed photobioreactor, and its incident light intensities were from 9.1 U Em'Y1 to 111.6 u Em'V '. It was concluded that mixotrophic culture system was better than photoautotrophic culture system for Synechocystis sp. PCC6803. In the enclosed photobioreactor, after 58.5h mixotrophic cultivation, 2.50g/L cell density, l.Og/L.d productivity, 15.3 u g/ml chlorophyll concentration and 25.3% energy efficiency were achieved, which were respectively 8.9 times, lltimes, 2.3 times and 4.9 times as much as those in photoautotrophic culture.In the 2.5L enclosed photobioreactor, the growth of Synechocystis sp. PCC6803 under photoautotrophic conditions was linear after one-day delayed phase. The mixotrophic growth process of Synechocystis sp. PCC 6803 could be divided into two phases: the first, from beginning to 48h or so, was the exponential growth phase, and the second, from 48h or so to the culture time when residual glucose concentration deceased to zero, was the linear growth phase. The exponential growth phase was only influenced by incident light intensity and could be described by the Andrews model. The linear phase was effected by both of incident light intensity and glucose concentration. Both incident light intensity and glucose concentration could make effects on synthetic rate of chlorophyll, which had close relation to the growth of Synechocystis sp. PCC6803. The variation of pH in the mixotrophic cultivation process was the results from the balance between photoautotrophic metablism and heterotrophic metabolism.The physiologic characteristics of Synechocystis sp. PCC6803 were investigated and it was observed that the oxygen evolution rate, respiratory rate, photosystem II actual photochemical efficiency ( II ) and the slow phase of millisecond delayed light emission of mixotrophic cells were higher than those of photoautotrophic cells. On the basis of the above results and the influences of inhibitors of photosynthetic electron transport DCMU, DBMIB on the mixotrophic growth of Synechocystis sp. PCC6803, it would be concluded that there were two reasons for the higher growth rate of Synechocysis sp. PCC 6803 under mixotrophic conditions than that under photoautotrophic conditions. The first reason was that glucose would not inhibited butaccelerated the photoautrophic growth in mixotrophic cells, and the second reason was the conversion of energy was promoted in photosynthetic system, which increased the anabolism of cells.The above results are useful for the cultivation of transgenic cyanobacteria in enclosed photobioreactors, and would promote the application of mixotrophic cultivation system in microalgae cultivation. Besides, they are also useful to understand the interrelation between photoautotrophy and heterotrophy in cyanobacteria.