Screening Of Filamentous Cyanobacteria Using Pig Biogas Slurry As Medium And Its Lipid Production Capacity

The pig industry produces a large amount of heavy polluted sewage,which,if not properly treated,will pose a serious threat to the health and ecological environment of the surrounding residents.At the same time,the world is facing an energy crisis and the development of bio-energy is in the ascendant with huge potential.The results show that microalgae can efficiently treat nitrogen and phosphorus in biogas slurry of pig farm,and algae biomass can be used to produce biodiesel.The coupling of microalgae cultivation and sewage treatment can not only reduce the cost,but also purify the environment,which has good economic and ecological benefits.The present study intends to screen the filamentous cyanobacteria tolerant to pig biogas slurry（PBS）of pig farm through the laboratory flask experiment,evaluate the ability of algae to remove the major pollutants in PBS and the potential of algae biomass cultivated by PBS to produce biodiesel,and explore the tolerance mechanism of resistant algae strains.The main research results are as follows:（1）Screening and identifing of resistant algal strainsIn this paper,three strains adapted to 10%PBS were screened from 36 strains of filamentous cyanobacteria isolated and purified from wetlands in South China.These three strains,numbered as B,J and L in our lab,were identified as Nostoc piscinale CENA21,Nostoc sp.NIES-3756 and Anabaena variabilis ATCC 29413 by morphological and molecular biological methods,respectively.（2）Growth characteristics of filamentous cyanobacteria resistant to 10%PBSThe specific growth rates of B,J and L were 0.14,0.19 and 0.21 d^-1,respectively.The dry biomass was 67.50,196.30 and 140.00 mg L^-1,respectively.Chlorophyll a content was0.53,0.67 and 0.51 mg L^-1,respectively.Based on above results,the tolerance ability of three algae to 10%PBS was J>L>B.（3）Evaluation the pollutants remove ability of algal from biogas slurryThe removal rates of ammonia nitrogen by B,J and L at day 14 were 82.33%,92.46%and 97.97%,respectively.The removal rates of total nitrogen were 54.06%,73.79%and79.90%,respectively.The removal rates of total phosphorus were 97.03%,98.46%and97.14%,respectively.However,for COD,although the removal rates were 61.87%,20.94%and 57.11%at 12d,respectively,the rates decreased after 14d.The removal rates of Pb by B,J and L were 74.94%,80.37%and 82.04%,respectively.The removal rates of As were 50.55%,49.34%and 55.83%,respectively.The results showed that three algae could remove the main pollutants in PBS,and the removal capacity of J and L was stronger than that of B.（4）Evaluation of the lipid production capacity and bioenergy characteristics of the algaeThe biomass production rates of B,J and L in the 10%PBS were 4.82,14.03 and 10.00mg L^-1d^-1,respectively.The lipid contents accounted for 7.97%,4.82%and 4.91%of the dry weight of algal cells respectively.The lipid yield was 0.38,0.69 and 0.48 mg L^-1d^-1,respectively.The cetane values of B,J and L were 37.23,43.14 and 43.26,respectively.The density were 0.88,0.88 and 0.88 g m^-3,respectively.The kinetic viscosity were 3.15,3.33 and 3.28 mm²s^-1,respectively.Therefore,algalρandνvalues met the American biodiesel standards.Based on above results,it showed that the biomass of J was the best chose for biodiesel production,followed by L.In addition,all the three strains can produce relatively high content of heptadecane,which is a potential high-quality bioenergy material as a long chain unsaturated hydrocarbon with high hexadecane number and high oxidation stability.（5）Research on the PBS tolerance mechanism of algae（i）Algal filament and algal cell morphology.When cultured with PBS,algal cells of J secreted a large amount of extracellular substances,which caused algal filaments to gather into clusters,form a small environment conducive to coping with the stress of pollutants from PBS.（ii）Photosystem II.Compared with the Fv/Fm,Fv/Fo and Yield values of B and L,the drop of Fv/Fm and Fv/Fo and Yield values of algal J were very small under the stress of PBS.It indicates that PBS has no effect on the actual photoenergy conversion efficiency of J,and there may be some mechanisms to protect photosystem II.（iii）Antioxidant system.Under PBS stress,SOD and POD activities of J were significantly droped compared with that cultured in normal medium,while CAT and GR activities and MDA contents were not significantly different.The results indicated that PBS did not destroy cell membrane,cause lipid peroxidation,or activate its antioxidant enzyme system.We speculat that the possible reason is that the algal strain used have been domesticated in PBS and the domesticated algae cells have adapted to PBS as non-stress environment conditions.（iv）Lipid composition.Compared with BG11₀medium,monounsaturated fatty acid（MUFA）content of J cells cultured in PBS increased significantly,while polyunsaturated fatty acid（PUFA）content decreased significantly.It is speculated that as stronger antioxidants,PUFA was consumpted under stress conditions and partially converted into MUFA,which may be a strategy of the alga to adapt to PBS stress conditions,beneficial to the oxidation stability of biodiesel.