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Bioethanol production from brown seaweed, Laminaria japonica, using Enterobacter sp. JMP3: Study focusing on carbon flux and metabolic pathway

Bioethanol production from brown seaweed, Laminaria japonica, using Enterobacter sp. JMP3: Study focusing on carbon flux and metabolic pathway
Wang Jing
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Biorefinery from seaweed has become one of the most popular research topics for alternative energy development. Among various biorefinery products from seaweed biomass, ethanol is an attractive product for many biotechnological scientists and engineers. Laminaria japonica is the major seaweed species in South Korea. In our previous research, a bacterium, Enterobacter sp. JMP3, was isolated from the gut of turban shell, Batillus cornutus, which could produce ethanol from carbohydrates of Laminaria japonica with high yield. In this work, the feasibility of biorefinery with Enterobacter sp. JMP3 was evaluated by using various carbon and energy sources such as mannitol, alginate - two major components of carbohydrates in Laminaria japonica, glucose, and powdered Laminaria japonica biomass itself. While alginate has proved to be not fermentable with this bacterium, highest ethanol production was achieved in anaerobic fermentation of mannitol with a yield as high as 0.383 C-mol/ C-mol mannitol and a concentration of 1.412 g/l from 5 g/l mannitol. Carbon distribution and metabolic flux were analyzed using carbon and redox balances of products during the fermentation. In terms of ethanol production, mannitol is the better carbon source than glucose under the same condition
0.27 g ethanol could be produced from 1 g of mannitol while 0.14 g from 1 g glucose. The better yield of ethanol from mannitol can be explained by the different redox states of the two substrates: mannitol is more reduced than glucose, which leads to generation of more NADH, resulting in shift of carbon flux to the product consuming more NADH such as ethanol. Moreover, the mannitol metabolic pathway in Enterobacter sp. JMP3 was examined by testing the specific oxidation reduction reaction of two possible pathways and verified to follow the mannitol-1-P dehydrogenase route. In addition, extra NADH and NAD+ were externally added into the culture to explore the possibility of controlling the ethanol production by changing the redox potential in mannitol fermentation but failed. The morphology of Laminaria japonica was slightly changed during the fermentation when the powdered Laminaria japonica was used for fermentation.
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