Research On Improving The Recyclability Of Spirulina Medium Based On Ultrafiltration Technology

Spirulina is one of the main microalgae in large-scale industrial production in the world,which has rich nutritional value.However,the cultivation of Spirulina needs a lot of medium with high salinity,and the production of 1 ton of algae powder requires 1000-2000 tons of water.If the medium can be recycled,it can not only reduce the waste of water resource and unconsumed inorganic nutrients,but also reduce the cost of wastewater treatment.However,the existing research shows that Spirulina will release the algal organic matter?AOM?which has inhibitory effect on itself in the growth process.Therefore,eliminating the negative effect of AOM is the premise of the cyclic utilization of medium.Currently,the treatment technologies for algae circulating medium include coagulating sedimentation,activated carbon adsorption,advanced oxidation and so on.There are risks of secondary pollution and loss of inorganic nutrients in coagulating sedimentation and activated carbon adsorption,and the cost of advanced oxidation is too high to realize large-scale industrial application.Ultrafiltration technology has developed rapidly in recent years,which can effectively remove organics with different molecular weight in water.Ultrafiltration treatment will not introduce secondary pollution and can retain the original inorganic salts.In this paper,the algae species studied was Spirulina maxima?S.maxima?and ultrafiltration membranes with different molecular weight were used to treat the medium of S.maxima,and long-term cycle culture of S.maxima was carried out.Based on the growth change of S.maxima,the effect of ultrafiltration on improving the recyclability of S.maxima medium was evaluated and its mechanism was further explored.Based on fresh medium,its growth rule was observed,the culture conditions was optimized,the scheme of nutrient supplement was determined,the negative effect of direct reuse of medium on the growth of S.maxima was studied,the properties of AOM in the culture medium were analyzed,and the foundation for the subsequent circulation culture of S.maxima was laid.The initial inoculation concentration of S.maxima was 0.3(OD₅₆₀),and it was harvested on the eighth day.If the remaining medium was reused directly,S.maxima with short filament and AOM would remain,which would affect the biomass,filament length,harvest rate and intracellular composition of S.maxima.The molecular weight distribution of AOM and the main components of AOM were extensive.The medium of S.maxima was treated by microfiltration and ultrafiltration with 100 k Da,30 k Da,10 k Da,5k Da and 1k Da respectively,and S.maxima was cultured for five consecutive cycles.The results showed that the accumulation of AOM in the microfiltration group resulted in the inhibition of the biomass and the filament length of S.maxiam,the decrease of the yield,the decrease of intracellular chlorophyll a content,phycocyanobilin,protein and oil,and the increase of intercellular polysaccharide content,indicating that S.maxima was stressed by the external environment.The growth of S.maxima in each ultrafiltration group recovered to some extent and the stress effect of the medium after ultrafiltration treatment on S.maxima decreased.The results were as follows: 10 k Da ultrafiltration group > 5 k Da ultrafiltration group > 1 k Da ultrafiltration group > 30 k Da ultrafiltration group > 100 k Da ultrafiltration group > microfiltration group.Based on the results of cyclic culture,the mechanism of ultrafiltration to improve the circulability of spirulina culture medium was further explored.By analyzing the changes of DOC,protein and polysaccharide content before and after the treatment of medium in each group,it was found that the smaller the molecular weight was,the better the control effect of ultrafiltration membrane on AOM was;the AOM of macromolecule might inhibit the growth of Spirulina,and the accumulation of AOM with molecular weight of 1?10 k Da might stimulate the growth of S.maxima;the polysaccharide of macromolecule weight might have a certain inhibition on the growth of S.maxima;the protein had no obvious inhibitory or stimulating effect on the growth of S.maxima.The three-dimensional fluorescence spectrum of medium in each group was monitored and analyzed by EEM-PARAFAC method.It was found that there were three main fluorescence components in the medium,C1 component belonged to humic acid,C2 component belonged to fulvic acid,and C3 component belonged to protein.The large molecular weight part of C1 has a certain inhibitory effect on the growth of S.maxima;C2 has little effect on the growth of S.maxima;the change results of relative content for C3 component are basically consistent with the quantitative monitoring results of protein.The activities of intracellular SOD and MDA content of S.maxima were monitored and compared in each group,and it was found that AOM accumulation could cause intracellular oxidative stress and oxidative damage in S.maxima,thus affecting the growth of S.maxima.Ultrafiltration treatment treatment mainly alleviates the oxidative stress of AOM on algal cells by controlling AOM,so as to improve the recyclability of medium.