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Effects Of Bacteria On The Harvest Of Microalgae And Role Of Intercellular Communication Under Nitrogen Starvation

Posted on:2017-01-21Degree:MasterType:Thesis
Country:ChinaCandidate:Y YangFull Text:PDF
GTID:2180330482496922Subject:Environmental Engineering
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At present, the global economy is restricted by energy shortage. In order to achieve sustainable economic development, microalgae biofuel as a renewable energy has attracted widespread attention. However, nutrients(C, N, and P) occupies a large proportion of the total cost of culture, which increases total costs of culture. Thus, wastewater treatment and microalgae culture coupling technology has aroused great attention of researchers. Nevertheless, autotrophic microalgae is susceptible to heterotrophic bacteria of higher growth rates in coupling technique. As microalgae heterotrophic growth characteristics is found, co-culture of microalgae and bacteria is increasingly prevalent in the coupling technology. The present study focused on the treatment capacity and treatment effect of consortiums regularly. Whether microalgae can keep a favorable position or not does not get enough attention, and research on flocculation of microalgae and accumulation of biofuel is barely reported as well.This study investigated aggregation, bacterial growth, lipid production and pollutant removal during cultivation under nutrient starvation stress, in which the C/N/P ratios of the medium to 14/1.4/1(MB2.5) and 44/1.4/1(MB4.0) respectively. Granules of 500-650 μm were formed in the bacteria contaminated cultivation. However, purified Chlorophyta sp. were generally suspended freely in the medium, indicating that bacterial presence was a prerequisite for granulation. Extracellular Polymeric Substances(EPS) analysis showed that polysaccharides were dominant in granules, while protein mainly distributed in the outer layer. Denaturing-gradient-gel-electrophoresis(DGGE) results revealed Sphingobacteriales bacterium and Sphingobacterium sp. are vital organisms involved in the flocculation of microalgae and nitrifiers(Stenotrophomonas maltophilia) could co-existed in the granular. Both EPS and DGGE results further supported that bacteria played key roles in granulation. Chlorophyta sp. was always dominant and determined the total biomass concentration during co-cultivation, while bacterial growth was limited owing to nutrient deficiency. Starvation strategy also contributed to enhancement of lipid accumulation, as lipid content in MB4.0 with a greater C/N/P rate led to the greatest increase in the starvation period, and the maximum lipid productivity reached 0.057 g/(L·d). Chemical oxygen demand and nitrogen removal in MB4.0 reached 92% and 96% respectively after 3 days of cultivation. Thus, cultivation of microalgae using high C/N/P rate wastewater enabled simultaneous realization of biomass granulation, bacterial over-growth limitation, enhanced lipid accumulation, and wastewater purification.In perspective of practical application, the activated sludge was cultured by nitrogen starvation stress and the signal molecules(AHLs) were extracted to explore mechanism of AHLs effect on microalgae growth and flocculation.Three kinds of activated sludge(AS I, AS II, AS III) were cultured in four nitrogen starvation conditions for 3 days, and C/N was 0.0/100(Medium I), 4.5/100(Medium II), 12.3/100(Medium III), 18.5/100(Medium IV) respectively. Activated sludge growth characteristics showed nitrogen starvation effectively controlled the growth rate of the activated sludge, activated sludge biomass increased with increasing nitrogen concentration. Three kinds of active sludge AHLs played no significant role in promoting growth. Meanwhile, only AHLs of ASI could promoted flocculation, particularly in Medium I and III which flocculation efficiency was about 0.4 g/g. Polysaccharide was still dominant and protein concentration was much less than that of polysaccharide. Combined with three-dimensional fluorescence spectroscopy(EEM) and gel permeation chromatography(GPC) analysis, aromatic hydrocarbons protein and tryptophan-like protein increased, which strengthened flocculation of suspension cells of Chlorophyta sp. and formed flocs. The results showed that the bacteria AHLs is able to stimulate algae secrete macromolecules to promote the flocculation of microalgae.
Keywords/Search Tags:Nitrogen starvation stress, Bacterial signaling molecules, microalgae growth, microalgae flocculation, lipid accumulation
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