Study On Growth Regulation Mechanism Of Attached Microalgae And Mass Transfer In Microalgae Biofilm

Microalgae wastewater treatment technology is a green environmental protection technology in line with the concept of sustainable development.It can not only purify carbon,nitrogen,phosphorus and other pollutants in wastewater,but also obtain valuable biomass resources to achieve both environmental and economic benefits.In addition,compared with the traditional suspension mode,the microalgal attachment culture mode can achieve a high concentration of biomass,without the separation of algal water,and lower energy consumption.However,different from the homogeneous suspension system,the nutrient distribution in the solid phase biofilm in the attached system has spatial heterogeneity,and the internal environmental mass transfer law and the growth regulation mechanism of attached microalgae have not been explored.In view of the shortage of research on internal mechanism of microalgae biofilm,the response rule of internal microenvironment of biofilm to changes in external conditions was clarified by regulating environmental conditions,the growth mechanism and metabolic pathway of attached microalgae were analyzed,and the transfer and utilization rule of light and nutrients in microalgae biofilm was explored.The research results are as follows:(1)By constructing the system of metabolism and mass transfer within the algae-biofilm,the general rule of light and nutrient transfer and utilization was obtained:the net photosynthetic rate at a certain depth of the biofilm showed a linear relationship with the second derivative of the distribution of net photosynthetic oxygen concentration.Compared with the suspension culture system,the light attenuation of the attached system is relatively slow due to the difference of media,and the light energy utilization rate is higher.Isotope labeling method proved that the underlying cells in the biofilm still had the metabolic activity of nitrogen assimilation,indicating that the assimilation of nitrogen occurred in the whole biofilm,and the attached microalgae had a certain heterotrophic ability.(2)The concentration of N&P(TN=100 mg/L,TP=8 mg/L;TN=50 mg/L,TP=4 mg/L;TN=25 mg/L,TP=2 mg/L;TN=10 mg/L,TP=0.8 mg/L;TN=5 mg/L,TP=0.4 mg/L)on the growth of attached microalgae.The results showed that the physiological state of microalgae was the best under TN=100 mg/L and TP=8 mg/L,and there was no growth inhibition caused by excessive salt accumulation.Compared with the worst group,the density of microalgae increased by 4.34×107 cells/cm2(cultured to 29 d).The biomass accumulation and photosynthetic capacity of the attached microalgae were positively correlated with the concentration of external nutrients.Nutrients are evenly distributed in the biofilm,and light is a key limiting factor for the efficient growth of attached microalgae and the conversion of pollutants into biomass.(3)The effect of N/P ratio(N/P=2.5:1,N/P=5:1,N/P=10:1,N/P=25:1,N/P=50:1)on the growth of attached microalgae in sewage was investigated.The results showed that N/P=10:1 was the best ratio to improve the growth potential of attached microalgae.The imbalance of N/P ratio would make the algal cells in a non-ideal state.POD content and CAT content in the N/P=50:1 group reached 29.3 ng/1011cells and 49.2 ng/108cells at 24 d.Under the condition of N/P imbalance,the photosynthetic rate of each layer of microalgae biofilm decreased with the culture time,and the photosynthetic advantage of the N/P=10:1 group was throughout the whole culture cycle.(4)In terms of regulating COD concentration(0 mg/L、60 mg/L、150 mg/L、240 mg/L、330 mg/L),it is found that low concentration of COD(60 mg/L)is conducive to the growth of attached microalgae,while 330 mg/L COD has certain inhibition on growth and metabolism.Adding 1～5 mg/L Cmp on the basis of 330 mg/L COD can alleviate stress,ensure biomass yield.By analyzing the proteomic response of the attached microalgae to 60 mg/L COD,the main functions of the proteins were significantly up-regulated,including light reception and cell division,etc.The molecular mechanism of the promoting effect of COD on photosynthetic potential and biomass accumulation of algal cells was further analyzed.