Strategies For Microalgal Lipid Synthesis And Energy Production By Combination Of Dark Fermentative Bacteria And Microalgae

The use of fossil fuels has caused increasing environmental pollution and severe climate change, driving an intensifying demand for sustainable and renewable energy sources that can reduce the dependency on fossil fuels. Biodiesel exhibits great potential and attracts extensive interest as it is carbon-neutral and environment friendly. Microalgae are considered as one of the most promising feedstock for biodiesel production due to the high area-specific yield relative to agricultural oleaginous crops. However, the lack of fast-growing and lipid-rich microalgae and the high cost of microalgal cultivation seriously restrict the development of biodiesel technology. Dark fermentative hydrogen production could produce a large number of small molecule organic acids and other by-products, which inhibits the hydrogen production process and has potential harm to the environment. Notably, the organic acids are appropriate substrate for microalgal lipid production. Combination of dark fermentation and microalgal lipid production can solve the shortage of raw materials and product inhibition. This can increase the substrate utilization efficiency and the energy production capacity, and has important significance to the industrial appliacation of bioenergy technology.A novel green microalga strain R-16 with high total lipid content was selected from eighty-eight isolates using the high-throughput screening technology, which was named as Scenedesmus sp. R-16 by morphological analysis and molecular identification. The cell growth and lipid accumulation of strain R-16 under different nutrient and ecological factors were studied. Various carbon sources and nitrogen sources can be utilized for microalgal growth and lipid production, and the optimal carbon source and nitrogen source were glucose and sodium nitrate, respectively. The strain R-16 exhibited strong tolerance to high glucose concentration (100 g/L) and can grow in a wide range of pH (4.0-11.0). In addition, nitrogen deficiency led to an accumulation of lipids and the total lipid content was as high as 52.6%. During the lag phase and exponential growth phase, the increase of lipid concentration mainly came from the increase of algal biomass, and during stationary phase most of lipids came from the accumulation of microalgae.To solve the quantitative limitations of the gravimetric method and the traditional Nile red method, a method combining ultrasonic treatment with three-dimensional excitation emission matrix fluorescence spectroscopy was used to detect the lipid content of microalgae. Some key factors that could influence the fluorescence staining were optimized, and a high correlation (R2:0.9957) between the fluorescence intensity and lipid concentration was obtained. Compared with the traditional Nile red method, this method can effectively disrupt the cell walls and significantly improve the staining efficiency. The lipid synthesis and energy conversion of microalgae under different culture modes and metal ions were investigated. Under mixotrophic mode, algal biomass concentration and lipid productivity were higher than those of heterotrophic mode, anaerobic mode and autotrophic mode. Mixotrophic mode consumed a lot of light energy, and heterotrophic mode gave the maximum energy conversion efficiency. The Fe3+、Mg2+ and Ca2+ had great influence on the biomass and lipid production of microalgae. EDTA addition could enhance the solubility of iron and calcium and increase their availability by microalgae, which evidently promote the lipid accumulation. Compared with the control groups, the lipid content and lipid productivity increased 28.2% and 29.7%, respectively.The energy production system of combined dark fermentation and microalgal cultivation was constructed. A two-step process of sequential dark fermentative hydrogen production and microalgal cultivation was applied to produce energy with glucose or various types of starch as the substrates. Ethanol fermentation bacterium Ethanoligenens harbinense B49 and mixed bacteria were used as hydrogen producer in dark fermentation, respectively. The main soluble products in dark fermentative effluent contained acetate and butyrate/ethanol, which were further utilized by microalga Scenedesmus sp. R-16 for cell growth and lipid production. Mixed ratio, starch concentration and initial pH played critical roles on the hydrogen and lipid production of the one-step process. Most soluble metabolites in hydrogen production can be consumed in one-step process. Compared with dark fermentation, the energy conversion efficiencies of two-step and one-step processes remarkably increased. In addition, the microbial communities in dark fermentation and one-step process were examined.