Potential of mixotrophic microalgal cultivation in biogas slurry and the effects of using various organic carbon sources
Cultivating microalgae in biogas slurry can recover carbon,nitrogen,and phosphorus nutrients while producing algal biomass.This approach offers a promising way to utilize biogas slurry resources.Mixotrophic culture,compared to photosynthetic autotrophy,can rapidly accumulate algal biomass,flexibly use light and carbon,and is well-suited to biogas slurry with poor light transmittance.Despite its potential,there is currently limited research demonstrating its feasibility.This study initially selected Chlorella sp.,Chlorella pyrenoidosa,and Scenedesmus sp.,using glucose as the carbon source to culture these microalgal strains in pig manure biogas slurry.The results indicated that the mixotrophic strategy significantly enhanced algal biomass accumulation and synergistically improved pollutant removal from biogas slurry,showing promising feasibility.Chlorella pyrenoidosa exhibited the greatest advantages in biomass production and nutrient removal,achieving 1.51 g·L-1 after 7 days of cultivation,which is 6.12 times the value of photoautotrophy.It also increased the removal rates of COD,NH+4-N,TN,and TP by 20%,36%,41%,and 32%,respectively,compared with autotrophy.Additionally,in this study we examined the impacts of organic carbon sources on the mixed culture Chlorella pyrenoidosa in biogas slurry,focusing on two typical carbon metabolism pathways(glucose-tricarboxylic acid cycle and sodium acetate-glyoxylic acid cycle).These findings suggested that glucose,especially at a concentration of 1 g·L-1,was more suitable than sodium acetate as an organic carbon source for biogas slurry,showing a negative correlation with utilization efficiency.Moreover,Chlorella pyrenoidosa maintained high levels of photosynthetic performance by metabolizing glucose,maintaining the maximum quantum yield of PSII,actual quantum yield,regulatory energy dissipation quantum yield,and others,at a high level,which not only makes up for the lack of light energy under weak light transmission of biogas slurry,but it also strengthened the photosystem damage recovery mechanism after continuous illumination.The findings suggest that cultivating Chlorella pyrenoidosa in biogas slurry with the addition of 1 g·L-1 glucose is an effective way to overcome unfavorable factors such as the light attenuation of algae culture in biogas slurry and enhance microalgae biomass production and nutrient recovery.These findings have good application prospects in livestock and poultry breeding,biological consumption,and resource utilization of on-site biogas slurry.
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