Growth Characteristics And Regulation Of Synchronous Photoautotrophic-heterotrophic Microalgae Biofilm

Nowadays,due to the large consumption of fossil fuels,the world faces the double crisis of energy depletion and climate warming.Countries around the world are striving to find a clean,efficient and environment-friendly new energy source.While microalgae can convert CO₂ into organic biomass through photosynthesis with many advantages such as fast growth rate,high photosynthetic efficiency,high oil accumulation and environmental suitability.The third generation biofuel based on microalgae is considered to be a potential alternative energy source.The premise of energy utilization of microalgae is the large-scale cultivation of biomass.Compared with suspension microalgae culture,biofilm culture system has been widely used due to low water consumption,stable operation,high biomass density and low energy consumption.Microalgal biofilm is a community in which microalgal cells grow to maturity after adsorbing on the carrier surface under the action of van der Waals force,electrostatic force and the adhesion force of extracellular polymers.Previous studies have shown that biomass can be highly concentrated through biofilm culture,but in the late growth stage,with the continuous accumulation of cells,the thickness of biofilm increases,the effective light penetration decreases,and the activity of the underlying cells of biofilm is poor,leading to the shedding of biofilm,the decrease of biomass,and the low lipid content.In order to overcome these shortcomings,a new method of simultaneous photoautotrophical-heterotrophic microalgae biofilm culture was developed.However,at present,the growth mechanism of synchronous photoautotrophic-heterotrophic microalgae biofilm is still unclear,the reaction process is still unclear,the mathematical model research is lacking,and the growth regulation of microalgae biofilm is not studied.Therefore,it is particularly important to study the growth characteristics of microalgae biofilms,establish a biofilm growth kinetics model,and strengthen the growth process on this basis.In this paper,the synchronous autotrophic-heterotrophic culture technique of microalgae biofilm is used as background to strengthen the material transfer and biochemical transformation in microalgae biofilm.first of all,the main factors affecting the growth of microalgae biofilm?light intensity,CO₂,glucose and nitrate?were identified,for each influencing factor,a series of concentration gradient kinetic experiments were set up.Based on the full understanding of the growth characteristics of microalgae biofilm,a comprehensive multi-factor growth kinetics model was constructed.Meantime,absorption kinetics model was established.Then,based on the research conclusions of microalgae biofilm growth kinetics,an optimal regulation scheme for key nutrients in medium was proposed,the effects of different carbon/nitrogen sources,the ratio of inorganic carbon to organic carbon,and the ratio of total carbon to total nitrogen on the growth of microalgae biofilms were mainly studied.In addition,the light penetration ability of photoautotrophic/heterotrophic microalgae biofilm and pure photosynthetic autotrophic biofilm was compared by different illumination experiments.Finally,the distribution of microscopic characteristic parameters in the microalgae biofilm was measured based on the Unisense microelectrode test technology,and the growth of microalgae was further optimized by adjusting the ratio of CO₂ and O₂ in the gas phase environment.The main research results are as follows:?1?When biofilms were cultured with different concentrations of nutrients,the maximum biomass density of microalgae increased with the augment of the substrate concentration until the saturated growth point was reached.Beyond the saturation point,the microalgae biofilm density no longer increased significantly.Then a multi-factor growth kinetic model of microalgae biofilm was established,which could accurately evaluate and predict the production capacity of microalgae biofilm growth system,the degree of coincidence was up to 0.99.In addition,an absorption kinetics model was established by studying the consumption of nutrient substrate by the microalgae biofilm,and the kinetic parameters such as reaction rate constant and half-saturation constant were obtained.?2?Compared with the autotrophic microalgae biofilm,the synchronous photoautotrophic-heterotrophic microalgae biofilm showed a 64%increase in light penetration.Meantime,an optimized molar ratio of total inorganic carbon?CO₂?to total organic carbon?glucose??20:1?and a molar ratio of total carbon to total nitrogen?72:1?were obtained,when microalgae biofilms were cultured with the optimized nutrients,the specific growth rate of biofilm was increased by 78%compared with that before optimization,and the oil content and yield were 47.53%and 41.95 g m^-2,which were increased by 120%and 147%,respectively.In addition,the structure of the synchronous photoautotrophic-heterotrophic microalgae biofilm was looser and more porous,which was conducive to the conversion and utilization of gas and nutrients.?3?pH in the syntrophic autotrophic and heterotrophic microalgae biofilm was about7.6 and it was neutral.Dissolved oxygen concentration was about 8.3 mg L^-1,which increased first and then decreased over time.Under the same amount of glucose,the heterotrophic growth effect of the biofilm cultured with the smear method was stronger,and the oil content increased by 32%.In addition,when the volume ratio of CO₂ and O₂was 1:1,the normal growth demand of the biofilm of syntrophic photoautotrophy-heterotrophic microalgae was met and the maximum biomass density and lipid content were 89.43 g m^-22 and 36.68%respectively.