Effect Of Ammonium On Growth Characteristics And Carbon And Nitrogen Metabolism Of Microalga Spirulina Platensis

Microalgal(including cyanobacterial)biomass has great potential for the production of various bio-products that can be utilizedin breeding,food,energy,pharmacy and healthcare industries.The microalga,Spirulina platensis belonging to cyanobacteria,has been attracted much interest for its high cell growth rate,accumulation of large amount of proteins and easy biomass harvest.The microalga can assimilate various nitrogen sources,but it is preferred to utilize ammonium.When ammonium is available,the microalga does not use other nitrogen source until all the ammonium is utilized by the process of glutamate synthesis.Despite ammonium is the preferred nitrogen source for the microalga,it is toxic to cells at elevated concentrations and can cause growth inhibition or even cell death.Beside,various types of wastewater,such as food processing wastewater,cooking waste stream,and slaughter effluent,contain high concentration of ammonium.The toxicity to microalgae caused by high concentration of ammonium has become a technical bottleneck in biological treatment of wastewater.In this study,the microalga,Spirulina platensis,was selected to cultivate in autotrophic and mixotrophic media.The effect of different concentrations of ammonium on the microalgal growth,cell compositions,photosynthetic performance was investigated.Moreover,the effect of different concentrations of ammonium on microalgal carbon and nitrogen metabolism and energy utilization was explored.These information will be helpful for us to understand the effect of ammonium on the microalga from a macro to micro perspective and can provide scientific basis to enhance microalgal productivity,target metabolites and energy utilization.The main results and conclusions are as follows:(1)The microalgal growth was significantly inhibited by ammonium in autotrophic cultivations.The microalgal growth was inhibited by 50mg/L ammonium to some extent,but the inhibition was increased by elevated ammonium concentration to 100 and 200mg/L.The microalgal biomass productivity was greatly enhanced by mixotrophic cultivation.The microalga could endure 100mg/L ammonium,but it had the highest biomass in 50mg/L ammonium.However,the microalga could not survive in 200mg/L ammonium in both autotrophic and mixotrophic cultivations.(2)Addition of ammonium resulted into gradual decrease of microalgal pigments.Higher concentration of ammonium inhibited the synthesis of microalgal pigments more obviously.Under mixotrophic cultivation,the decrease of microalgal pigments was greater and faster.The total protein content in microalgal cells was increased with the increase of available nitrogen in the media and higher ammonium inhibited the synthesis of carbohytraes.The cell lipids were reduced by ammonium under autotrophic cultivations while increased under mixotrophic cultivations.(3)Under autotrophic and mixotrophic cultivations,microalgal photosynthetic performance was reduced by ammonium,and the inhibition to microalgal photosynthetic performance increased with increasing ammonium concentration.Besides,the microalgal photo-protection mechanism(NPQ)was increased by addition of ammonium,and was intensified strongly under mixotrophic cultivations.However,the microalgal photo-protection mechanism could hardly perform under higher ammonium concentrations,resulting into disrupt of microalgal photosystem.(4)The absorption of light energy by photosystem reaction center was enhanced by ammonium,but the absorbed energy was mainly dissipated through heat or fluorescence,only few was utilized and converted to ATP by the microalga.The effect of mixotrophic cultivation on light energy conversion in microalgal photosystem was similar with ammonium,but lower than ammonium.Besides,the effect on light energy conversion was more obvious in PS II than PS I.(5)50mg/L ammonium had no significant influence on microalgal carbon and nitrogen metabolism.The metabolites in autotrophic microalgal cells were converted only by PGA through Calivin cycle,while in mixotrophic microalgal cells,the metabolites in Calvin cycle and PP pathway were mainly supplied by external glucose.Most importantly,under 100mg/L ammonium conditions,the metabolites in Calvin cycle and PP pathway were all converted from added glucose and the conversion of PGA to GAP was reversed,which was crucial to supplement NADPH for Calvin cycle.(6)50mg/L ammonium only reduced the metabolic flux of SUCC branch in TCA cycle,but didnot affected the metabolic flux of other pathways.The addition of ammonium significantly reduced the metabolic flux of SUCC branch in TCA cycle in mixotrophic cultivation,while the flux varied with the change of ammonium concentrations in other pathways.Based on the absolute metabolic flux,50mg/L ammonium reduced the assimilation of nitrogen and inorganic carbon while increased the utilization of glucose.100mg/L ammonium reduced the assimilation of inorganic carbon while increased the utilization of nitrogen and glucose.(7)The energy was mainly produced by photo-phosphorylation in autotrophic microalgal cells,while generated through oxidative phosphorylation in mixotrophic microalgal cells.In both autotrophic and mixotrophic cells,the energy was consumed mainly by Calvin cycle.Under different cultivation conditions,the highest efficiency of energy utilization was under mixotrophic cultivation with 50mg/L of ammonium.