Harvesting Of Nannochloropsis Via Bioflocculation And Reuse Of Aqueous Phase From Microalgal Hydrothermal Liquefaction

Production of renewable energy has received great concern due to the gradual depletion of fossil fuel sources and the environmental problems arised by the utilization of crude oil.Microalgal lipid production has become alternative and renewable energy sources due to the advantages of microalgae in carbon fixation and photoautotrophic growth.However,low biomass and lipid productivity,difficulty in cell harvest,and high cost in cultivation present challenges for microalgal lipid production.The high production cost of microalgal lipid is the barrier for commercialization,where the harvesting cost consists of 20-30%.Centrifugation,flotation and filtration,and flocculation have been commonly used for microalgae biomass harvesting,while bioflocculation is a promising harvesting method with the advantages of eco-friendly and high efficiency.Nannochloropsis has been widely studied as a lipid producer.However,harvesting of its biomass using bioflocculation and subsequent utilization have been poorly studied.In this thesis,marine Nannochloropsis was studied for its lipid production,biomass harvesting and subsequent utilization,and the detailed results were as follows:Firstly,the culture medium and conditions of N.oceanica DUT01 were optimized,and effects of medium replacement ratios on lipid accumulation were also investigated.With 1/5(v/v)fresh medium replacement,the lipid content of N.oceanica DUT01 obtained from f/2 medium was 50%to 64%,1.2-1.6 times higher than 12-day cultivation,and the lipid content in f/2 medium is much higher than that from BG11 medium(40-45%),while the highest lipid productivity was 31 mg/L/d in BG11 medium,and the biomass was improved to 1.4 g/L.Moreover,the accumulation of lipid was accompanied with the decrease of protein and carbohydrate.Secondly,harvesting N.oceanica DUT01 via bioflocculation was investigated.The bioflocculant producing bacterium bacteria were screened,and the best producer was selected and identified as Solibacillus silvestris W01.Maltose and yeast extract were selected as the optimal carbon source and nitrogen source,respectively.Subsequently,the concentrations of maltose and yeast extract were optimized to be 1.9 g/L and 11.0 g/L,respectively,by response surface methodology.The culture broth S.silvestris W01 from the optimized culture conditions exhibited 90%flocculation efficiency on N.oceanica DUT01 without any addition of metal ions.Chemical analysis had revealed that the bioflocculant was proteoglycan consisted of 24.9%(w/w)proteins and 75.1%carbohydrates,and the ratio of neutral sugar,uronic acid and amino sugar was 6:1:12.Further,the recovered bioflocculant exhibited 87%flocculation efficiency on N.oceanica DUT01 and the supernatant was reused without affecting microalgal cell growth.Next,aqueous phase of Nannochloropsis(AqAl)after hydrothermal liquefaction was studied.The AqAl is able to substitute carbon,nitrogen,and phosphorus in the standard medium,and microbial processed 30%AqA1 was used for microalgal regrowth after appropriate dilution.The high-throughput microplates experiments had revealed that N.oceanica DUT01 showed 1.5-1.7 times higher growth when cultivated in 0.5%bacterial processed AqAl,while the 0.5%unprocessed AqAl significantly inhibited the growth of N.oceanica DUT01.Also,most polyculture processed higher growth than monoculture in the presence of 0.5%raw AqAl,while increased species diversity would contribute to a convergence at a high microalgal growth.The presented work provides basis for further development of bioflocculant and biological flocculation to harvest microalgae.The processes described in this study will benefit efficient production of biofuels and lipids from microalgae.