Experimental Investigation On Microalgal Cell Adhesion And Biofilm-Based Cultivation

Microalgae have been considered to be one of the most promising sources of renewable energy owing to their high growth rates,high lipid contents,and ability to mitigate CO2 emissions effectively.Biofilm-based microalgae cultivation has attracted much interest recently because of its high harvesting efficiency,reduced water consumption,and low energy requirements.Biofilm-based microalgae cultivation involves cell adhesion,biofilm formation and growth,and these processes are influenced by many factors with complex mechanisms.For cell adhesion in biofilm formation,a quantitative criterion to predict microbial adhesion was proposed by identifying the dominant interactions between microorganisms and surfaces.Baesd on the quantitative criterion and the parallel plate flow chamber system,the influencing mechanism of liquid medium properties on biofilm formation were elucidated.A microalgae biofilm cultivation system was developed to explore the effect of light and CO2 conditions on microalgae biofilm.Finally,the biofilm microstructure was interpreted and the influence of culture conditions on the biofilm structure were studied.For the cell adhesion in biofilm formation,a quantitative criterion was proposed.According to the criterion,the dominant interaction in cell adhesion could be identified via comparison of two expressions containing the electron donor characteristics of the microorganism(?mv-)and abiotic surface(?sv-)and their ?potentials(?m,?s).The results revealed that when dominated by the Lewis Acid-Base interaction(AB),adhesion would decrease with increasing(?).Whereas,when the electrostatic interaction(EL)was dominant,adhesion would decrease with increasing(?m+?s)2.We have verified the criterion based on the adhesion of microalgae,bacteria and fungi onto various surfaces obtained via our experiments and available in literatures.The influence mechanism of liquid medium pH,ion and surface free energy on microalgae biofilm formation was analyzed with the quantitative criterion and parallel plate flow chamber system.The results revealed that when adhesions were dominated by AB interaction,and the pH and ion would influence the biofilm formation primarily by affecting the components of surface free energy of cell.Whereas,when the EL interaction was dominant,the pH and ion would influence the biofilm formation primarily by affecting the ? potential of the cell and substratum.When the surface tension of liquid medium(?lv)was approximately equal to the average value of surface free energy of microalgae and substrata,biofilm density reached the minimum.A microalgae biofilm cultivation system was developed to explore the effect of light spectrum,light dark cycles,light intensity,CO2 concentration and velocity on microalgae biofilm(Biomass Yield,Photosynthetic Performance,Cellular Composition and CO2 fixation rate).The results indicated that blue and red light were more efficient than white and green light in enhancing cell growth.Blue light was more suitable for lipid synthesis,while,red light was more suitable for carbohydrate synthesis.Microalgae cultured under blue light had a higher power-to-lipid conversion capabilitiy and CO2 fixation rate.The biomass yield and lipid content of the microalgae cultured under the photoperiod with seconds scale(3:3 s and 5:5 s)increased.With the increasing of light intensity,the N.oculata biofilm biomass,photosynthetic performance and CO2 fixation rate showed a trend of increasing first and then decreasing,reaching the peak at 200 ?mol/m2s.With the increasing of CO2 concentration and velocity,the Chlorella biofilm biomass and photosynthetic performance reaching the peak at 0.5%and 0.3VVM.In order to explore the influence mechanism of biofilm microstructure on the transfer in biofilm,the biofilm structure were investigated via experimental observation and theoretical analysis.Microalgae biofilm microstructure were observed with confocal laser scanning microscopes and the cell-cell interaction were interpreted within the scope of thermodynamics.Due to the low co-adhesion energy,cells with a low value of surface energy(40 to 50 mJ/m2)formed a heterogeneous biofilm with a large number of voids and a high porosity(>20%),while due to the high co-adhesion energy,cells with a high value of surface energy(50 to 65 mJ/m2)resulted in a homogeneous and flat biofilm with a low porosity(<20%).Combined with the biofilm micro structure under different light and CO2 conditions,the influence mechanism among the culture conditions,cell-cell interaction and biofilm structure was investigated.Due to the low co-adhesion energy,the cells cultured under white light formed heterogeneous biofilm with high porosity and roughness.Whereas,due to the high co-adhesion energy,the cells cultured under blue and red lights formed homogenous biofilm with low porosity and roughness.CO2 conditions had little effect on the microalgae cell surface properties and biofilm structure.This study provides a new theoretical tool for predicting and analyzing microbial cell adhesion,and has theoretical guidance and reference significance for the analysis of transmission mechanism and system optimization design for biofilm-based microalgae cultivation.