Characteristics And Flora Development Of A Microbial Community For Degradation Of Lignocellulose

As reserves of fossil fuels are limited and greenhouse effect caused by excessive use of fossil fuels is getting serious, the human are forced to seek alternatives to fossil fuels. Biomass energy is highly recognized as a result of both energy and environmental benefits. Lignocellulose is the main component of biomass and also the bottleneck for the usage of biomass. Thus, how to achieve efficient hydrolysis and acidification of lignocellulose has been a difficult problem for the usage of biomass resources. With the development of environmental biotechnology, microbial synergy gradually is taken seriously, and the application of microbial community in lignocellulose degradation has become a hot research. The main contributions to the current understanding of this topic are described below:Growth characteristics, morphological characteristics and ecological conditions of the microbial community were studied respectively. Microscopic observation reveals that the microbial community is mainly composed of rod-shape bacteria. Those bacteria are mostly peritrichous flagellation with extracellular mucus layer, and their surface morphologies are diverse. The microbial community demands low oxygen, and DO was kept in 0.07~0.25 mg/L during degradation. The pH shows a trend of increase after initial decrease, which demonstrates the strong buffer capacity of the microbial community for the feedback inhibition of fermentation product.Fermentation characteristics of microbial community were explored in detail and showed stronger capacity of lignocellulose degradation. The degradation rate of rice straw, cellulose, hemi cellulose and lignin may reach 75 %, 81 %, 92 %, and 46 % respectively when the fermentation terminated. The initial 60 h play an important role in degradating process, in which the degradation of rice straw, cellulose, hemi cellulose and lignin reached 59.0 %, 48.5 %, 86.8 % and 44.0 % of total degradation amount respectively. The fermentation is typically acetic acid type owing to 90% metabolite is acetic acid, and there are also a little ethanol, butyric acid and propionic acid in fermentative products. At 108 h, acetic acid and total volatile acids concentration reached maximum, they were 2.58 g/L and 2.90 g/L, respectively. The COD value of reactive system shows a trend that it increased at initial stage and decreased graduately. This demonstrates changes of substrate from solide lignocellulose to liquid volitail fatty acid, then to gaseous CO2 and H2. Furthermore, the change of Total Kjeltic Nitrogen (TKN) and ammonia nitrogen show that degradation capacity of microbial community declined owing to mainly depletion of the substrates, but not ammonia accumulation in 144-216 h.Growth curve of the microbial community includes typical three periods, they are logarithmic phase (0~108 h), stationary phase (108~144 h) and decline phase (144~216 h) respectively. Community composition and development were investigated by the means of DGGE, and results showed that microbial community was constituted of 14 kinds of bacteria and presented fluctuation in some degree with fermention. Furthmore, result of cluster analysis of DGGE profile was accordant with growth curve, and the degradation process was divided into three stages: initial stage (0~12 h), intermediate stage (24~144 h) and end stage (144~216 h).In initial stage, bateria competed fiercely, some bateria amount were dominant, such as Proteolyticum ethanoligenes strain GW,Uncultured Acidobacteria bacterium clone RDM3-B02,Uncultured Bacillus sp. clone 126 and Aneurinibacillus danicus, DO and pH of reactive system decreased rapidly owing to these aerobic bacteria's relative faster grow rate, this will provide fitting conditions for grow and fermentation of facultative bacteria. In intermediate stage, these species were gradually being inhibited and transferred to a dormant state.Straw degradation occurred mainly in intermediate stage. In this stage Clostridium sp. PML14,Clostridium islandicum strain AK1,Brevibacillus borstelensis clone US12,Ureibacillus sp. A3.03,Uncultured bacterium tbr1-10,Bacillus sp. R-30914,Brevibacillus borstelensis strain U404 existed in a large amount. These microorganisms are gram-positive bacteria with endogenous spore, which can adapt to changes in temperature and pH value and produce extracellular hydrolase. Uncultured bacterium tbr1-10,Bacillus sp. R-30914,Brevibacillus borstelensis strain U404,Clostridium islandicum strain AK1 performed as functional species group. Synergy of microbial community can also lift inhibition produced by toxic substances of the lignin degradation. This is one of the important reasons for the effective degradation of lignocellulose.After entering end stage, with gradual depletion of the nutrients in substrate, part of functional species group declined as a result of endogenous respiration, such as Clostridium islandicum strain AK1,Brevibacillus borstelensis clone US12,Ureibacillus sp. A3.03,Uncultured bacterium tbr1-10,Bacillus sp. R-30914;Part of species shows the contary trend,and gradually develop into dominant species group, such as Bacillales bacterium 01QDX,Uncultured Bacilli bacterium clone SHBZ1189 and Aneurinibacillus danicus;While some species have always been to maintain a relatively high number, such as Clostridium sp. PML14 and Brevibacillus borstelensis strain U404。...