| Microalgae are rich in nutrients and contain a variety of high value-added biologically active compounds.In this paper,the biomass,protein and polysaccharide content were used as indicators to screen the dominant microalgae;the selected Desmodesmus armatus B38 was optimized for culture conditions and nutrient analysis,and its potential value as food or bait was evaluated;protein and polysaccharide were separated and purified.To study the activity and structure identification;the research and development of bait for microalgae,the main results are as follows:15 tropical microalgae were studied and their biomass,polysaccharide and protein content were analyzed and compared.The content of B38 protein and polysaccharide in algae strains was 49.00±1.35%and 5.37±0.03%,respectively.It has the most development potential in 15 tropical microalgae and is suitable for high-density culture.It is identified by morphology and molecular biology as Desmodesmus armatus B38.The nutritional value of Desmodesmus armatus B38 was determined by nutrient analysis.It is rich in nutrients.The carbohydrate,oil,ash,crude fiber and protein accounted for 13.86%,14.00%,4.28%,3.52%and 49.00%of the dry weight of algae powder,respectively.The energy is 377.44 kcal.It can be used as a source of nutrients for animals.Contains a variety of fatty acids and contains C18:2(n-6)(linoleic acid)and C18:3(n-6)(linolenic acid)required for growth of the organism,but at a lower level;contains 18 amino acids,It contains 8 essential amino acids and two umami amino acids,of which the essential amino acid has an SRCAA value of 100,indicating that it has more free amino acids.In addition to VB3 and VE,vitamins are lacking in other vitamins and low in content.It is rich in various mineral elements,among which sodium,potassium,calcium,magnesium,iron and aluminum are rich in content.It can be used as a source of trace elements necessary for animals,and heavy metals such as lead,cadmium,mercury,arsenic,tin,nickel and chromium.The content is below the limit.The culture conditions of Desmodesmus armatus B38 were optimized,and the optimal nitrogen source(nitrate),carbon source(bicarbonate)and light intensity(108?mol m-2s-1)were determined by single factor experiments.The Plackett-Burman(PB)design was used to initially screen for variables that had a significant impact on biomass.Then,the optimized medium composition using central composite design(CCD)-Response Surface Methodology(RSM)for Desmodesmus armatus B38 was reconstituted accordingly to have 0.93 g L-1 nitrate,0.04 g L"1 phosphate,0.15 g L-1 magnesium sulfate and 0.07 g L-1 bicarbonate,and optimum growth conditions of temperature at 27?,light intensity of 108?mol m-2 s-1,pH 7.00 and aeration rate of 0.50 L min-1.After 12 days,the biomass,carbohydrate and protein content increased from 0.96±0.11 g L-1,49±2.37%,13.86±1.45%to 1.65±0.15 GL-1,53.61±1.25%and 16.37±1.03%respectively.Finally,the optimized conditions were applied to the outdoor 800 L photobioreactor for scale-up cultivation.In order to explore the value of protein and polysaccharide utilization,first,the protein and polysaccharide were optimized and extracted,and then the structure and properties of proteins and polysaccharides were analyzed.The Box-Behnken design(BBD)was used to optimize the protein extraction conditions.The optimal extraction conditions were:pH 11.0;ultrasonic power 600 W;extraction time 24 min;extraction temperature 20?;solid-liquid ratio 1:15,after experiment,it was verified that the optimal extraction conditions were effective.Finally,the extraction rate of protein was increased from 53.55±0.98%to 63.24±0.83%.By measurement,the isoelectric point of the protein was pI=4.3.At pH 12.0,the solubility is reach to 89.74%,and the primary structure is relatively stable.The molecular weight is mainly concentrated at 75 kDa.The thermostable temperature of the high-density protein is 109.17?,similar to the spirulina,Chinese quince and peanut protein.It is higher than the thermal stability of mung bean and red bean protein;the emulsification and emulsion stability of protein are 61.11%and 84.55%,respectively,similar to soybean protein,and the emulsion stability is high,which can be used as a binder for different foods.Polysaccharide was extracted by microwave assisted constant temperature extraction method.The optimal extraction process of polysaccharide was determined by orthogonal test:extraction temperature 70?,microwave power 700 W,solid-liquid ratio 1:35,microwave extraction time 20 min,constant temperature extraction for 2 h,The crude polysaccharides were separated and purified by DEAE-52.In total four eluents of water(DAP1),0.25 mol L-1 NaCl(DAP2),0.5 mol L-1 NaCl(DAP3)and 1.0 mol L-1 NaCl(DAP4)were fractionated.Four polysaccharides were analyzed,and the four polysaccharides were composed of rhamnose,fucose,arabinose and mannose.Gel Permeation Chromatography(GPC)analysis showed that the four polysaccharides had a uniform molecular weight distribution,and the molecular weights of DAP1,DAP2,DAP3 and DAP4 were 3.4 kDa,33.4 kDa,53.4 kDa and 59.1 kDa,respectively.Scanning electron microscopy showed that the surface structure of DAP1 was smooth and dense,but not flat,DAP2,DAP3 and DAP4 were amorphous solids and flaky.In vitro antioxidant results showed that DAP2 and DAP3 had strong antioxidant activity.By feeding Desmodesmus armatus B38 as the first feed of the Babylonia areolata and the feed of the larvae carp,the effect of Desmodesmus armatus B38 on the growth and survival of Babylonia areolata and carp were studied.The results showed that on the basis of the artificial formulated diet,the addition of Desmodesmus armatus B38 as the first feed was not conducive to the growth and survival of Babylonia areolata,But Desmodesmus armatus B38 can promote the growth of larvae carp,The specific growth rate(SGR),weight gain rate(WGR)and condition factor(CF)were the highest in the compound feed group(0.48%,43.75%and 7.65%,respectively),and the CF of the Desmodesmus armatus B38 group was 7.28%higher than that of the artificial formulated diet group,which was 7.28%,and the difference was significant. |
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