Research On Growth And Biochemical Characteristics Of 5 Microalgae Under Different Nitrogen Concentrations

Nitrogen holds an essential status in proteins,enzymes,chlorophyll,genetic material and energy transfer molecules.For the growth of microalgae,nitrogen is of paramount value.But there are some differences in the ability of microalgae to absorb and utilize nitrogen.To demonstrate the response and pattern of microalgae growth to nitrogen nutrition,five strains algae were selected as experimental materials,namely Nannochloropsis gaditana,Kirchneriella sp.,Scenedesmus acuminatus,Scenedesmus obliquus and Chlorella vulgaris.Ten initial nitrogen concentrations?0,1.8,3.6,5.4,7.2,9.0,10.8,14.4,21.6,25.2 m M?were set,and the nitrogen concentration of 18.0 m M was used as a control group.The cells were cultured in a columnar photo-bioreactor of?3.0 cm×60 cm under the conditions of 300?mol photons/?m²·s?illumination.?1?The relationship between biomass and nitrogen concentration of five strains algae was analyzed.?2?The characteristics of microalgae absorption of nitrogen nutrition was obtained through the change of NO₃^-,NO₂^-concentration and microalgae nitrate reductase activity.?3?The dynamic changes of oil,carbohydrate and soluble protein of five strains algae were analyzed.The research results are as follows:?1?There were differences in initial nitrogen concentrations suitable for growth of 5 strains of algae.The biomass of N.gaditana and K.sp.increased with the initial nitrogen concentration?0-18.0 m M?.The biomass did not increase significantly?p>0.05?when the initial nitrogen concentration was higher than 18.0 m M.The biomass of S.acuminatus and C.vulgaris was the largest when the initial nitrogen concentration was 7.2 m M,but below this concentration,the biomass increased with the increase of the nitrogen concentration.If it was higher than this concentration,it would be just the opposite.The biomass of S.obliquus gradually increased when the initial nitrogen concentration was raised from 0 to 10.8 m M.The biomass decreased if the initial nitrogen concentration was higher than 10.8 m M.The response of the biomass changes of the five strains of microalgae to the initial nitrogen concentration showed two types:the first type?type I?,when the initial nitrogen concentration was higher than the suitable nitrogen concentration,the growth of microalgae was not affected?N.gaditana and K.sp.?;The second type?type II?,when the initial nitrogen concentration exceeded the appropriate nitrogen concentration,the growth of microalgae was inhibited,and the biomass was significantly reduced?p<0.05??S.acuminatus,C.vulgar and S.obliquus?.?2?There were differences in the ability of five strains algae to absorb and utilize nitrogen.The concentration of NO₃^-decreased in the different initial nitrogen concentration culture medium while the five strains of algae absorbed a large amount of nitrate in the first four days.But the change of nitrate reductase activity in the algae was similarity,the activity was the highest in the first 3 days,then decreased sharply,and finally remained at low level.Among them,the nitrate reductase activity of K.sp.and S.acuminatu was the highest,which was significantly higher than the other three microalgae.The microalgae released NO₂^-extracellularly while absorbing nitrate.In the culture medium with low nitrogen,NO₂^-was reabsorbed after the nitrate was completely consumed and it existed in the culture solution for 2to 4 days.In the culture medium with high nitrogen content,the NO₂^-existed in the culture solution for more than 10 days except for N.gaditana.The results showed that the NO₂^-production in the nitrogen concentration of the type I microalgae?N.gaditana and K.sp.?was much lower than that of the type II microalgae?S.acuminatus,C.vulgar and S.obliquus?,therefore Type I microalgae had a higher utilization efficiency for absorbed NO₃^-.The content of nitrogen in the five strains algae ranged from 4.21%DW to 7.34%DW,the lowest N content range for maintaining normal growth was 0.76-2.16%DW.When the 18.0 m M control group was cultured until the 10th day,the nitrogen uptake rates of the five strains of algae were 100%,75.75%,99.28%,65.08%and 91.65%,respectively.?3?On the 15th day of culture,the maximum carbon accumulation of each strain in different nitrogen concentration groups was 3.02-5.46 g/L,1.18-3.94 g/L,4.35-5.49 g/L,3.12-3.81 g/L,2.12-2.89 g/L.The intracellular carbon accumulation of N.gaditana and S.acuminatus was the highest,much larger than the other three strains.The ratio of C/N in the different nitrogen concentration groups continued to increase with time.The maximum C/N values of five strains algae were 54.13?3.6 m M?,22.50?7.2 m M?,83.39?3.6 m M?,37.58?3.6m M?and 40.79?3.6 m M?,respectively.It is indicated that suitable low nitrogen conditions was beneficial to increase the intracellular carbon content and promote the carbon flow to the carbon-rich lipid in the microalgae.?4?When nitrogen was restricted or deficient,the intracellular nitrogen content of microalgae decreased,the synthesis of intracellular nitrogenous compounds such as proteins was slowed or stopped,and photosynthesis helped to accumulate the synthesis of carbohydrates or lipid.The intracellular N content of the 5 strains was sufficient for the first 3 days,and the photosynthetic assimilation C was used for the synthesis of proteins,carbohydrates and lipid.After 6 days,the intracellular N content decreased,protein synthesis slowed down,and N.gaditana used photosynthetic assimilating C for the synthesis of carbohydrates and lipid.K.sp.and S.acuminatus used photosynthetic assimilating C for the accumulation of lipid.The accumulation of carbohydrates in the early stage of S.obliquus and C.vulgaris reduced.After the nitrogen in the environment of the low nitrogen group was completely consumed,the carbon of the five algae assimilated was mainly used for the synthesis of lipid.Type II microalgae?S.acuminatus,C.vulgar and S.obliquus?accumulate more C and lipid in the appropriate low nitrogen group,contributing to biomass growth.