Effects Of Nitrogen And Phosphorus Levels On Gracilaria Bailinae Growth And The Polyculture Effect Of Algae-shrimp

Gracilaria bailinae is a kind of macroalgae with economic and ecological value.Economically,it can be used as an important raw material for extracting agar.It is rich in dietary fiber,minerals and vitamins,and has high protein content and low fat content.It has also been developed into flavor food and pharmaceutical products.Ecologically,Gracilaria bailinae and other macroalgae can absorb a large number of nutrients in the growth process,such as C,N and P.It is of great significance to maintain the health of water environment.At present,little research has been done on Gracilaria bailinae,the research on N and P nutrient requirements of Gracilaria bailinae and its application in Litopenaeus vannamei culture have not been reported,Therefore,under laboratory conditions,1.the specific growth rate of Gracilaria bailinae has measured under different nitrogen and phosphorus concentration gradients with NaNO₃,（NH₄）₂SO₄,CH₄N₂O as nitrogen source and KH₂PO₄as phosphorus source.2.Using NaNO₃,（NH₄）₂SO₄,CH₄N₂O as nitrogen source and KH₂PO₄as phosphorus source,the changes of soluble protein content of Gracilaria bailinae under different nitrogen and phosphorus concentration gradients were determined.In order to achieve the purpose of high-efficiency cultivation and increase production and income,we explored the N and P conditions that affect the demand of algae in the process of artificial cultivation of Gracilaria bailinae,and laid the foundation for the large-scale artificial cultivation of Gracilaria bailinae.3.In this study,Gracilaria bailinae and Litopenaeus vannamei were polycultured,and five different density gradients were set up to determine the specific growth rate,photosynthetic characteristics,NH₄⁺-N,NO₂^--N and NO₃^--N concentrations.In order to explore the optimal ratio of Gracilaria bailinae and Litopenaeus vannamei growth and the purification effect of algae on NH₄⁺-N,NO₂^--N and NO₃^--N under the condition of zero water exchange,the effects of algae density on the dual polyculture system of shrimp and algae were studied.The results are as follows:1.Using NaNO₃,（NH₄）₂SO₄,CH₄N₂O as nitrogen source and KH₂PO₄as phosphorus source,Gracilaria bailinae was cultured in laboratory for 12 days under different nitrogen（0,100,400,800,1600μmol/L）and phosphorus（10,20,40μmol/L）concentration gradients.The specific growth rate of the algae in each treatment group was determined.The results showed that different nitrogen sources had significant effects on specific growth rate（P<0.05）.With NaNO₃as the nitrogen source,the maximum specific growth rate was obtained at different phosphorus concentrations and the ratio of nitrogen to phosphorus was 40:1.When the concentration of nitrogen and phosphorus was 1600 and 40μmol/L,the specific growth rate of algae reached the maximum value（2.95±0.19%/d）;With（NH₄）₂SO₄as the nitrogen source,the maximum specific growth rate was obtained when the ratio of nitrogen to phosphorus was 10:1 at different concentrations.When the concentration of nitrogen and phosphorus was 100 and 10μmol/L,the specific growth rate of algae reached the maximum value（3.88±0.11%/d）;With CH₄N₂O as the nitrogen source,the maximum specific growth rate was obtained when the ratio of nitrogen to phosphorus was 40:1at different concentrations.When the concentration of nitrogen and phosphorus was800 and 20μmol/L,the specific growth rate of algae reached the maximum value（5.45±0.53%/d）.2.Using NaNO₃,（NH₄）₂SO₄,CH₄N₂O as nitrogen（N）source and KH₂PO₄as phosphorus（P）source,Gracilaria bailinae was cultured in laboratory for 12 days under different nitrogen（0,100,400,800,1600μmol/L）and phosphorus（10,20,40μmol/L）concentration gradients.The soluble protein content of the algae in each treatment group was determined.The results showed that different nitrogen sources had significant effects on the content of soluble protein of Gracilaria bailinae（P<0.05）.Using NaNO₃as the nitrogen source,the soluble protein content of Gracilaria bailinae was the highest when N:P ratio was 40:1 under different concentrations.When the concentration of nitrogen and phosphorus was 1600 and 40μmol/L,the soluble protein content reached the maximum value（0.12±0.01 mg/m L）.When（NH₄）₂SO₄was used as nitrogen source,the soluble protein content of Gracilaria bailinae was the highest when N:P ratio was 10:1 under different concentration.When the concentration of nitrogen and phosphorus was 100 and 10μmol/L,the soluble protein content reached the maximum value（0.15±0.01 mg/m L）.Using CH₄N₂O as nitrogen source,the soluble protein content of Gracilaria bailinae was the highest when N:P ratio was 40:1 under different concentrations.When the concentration of nitrogen and phosphorus was 800 and 20μmol/L,the soluble protein content reached the maximum value（0.17±0.00 mg/m L）.3.Under the condition of zero water change,Gracilaria bailinae and Litopenaeus vannamei were layered and mixed with 5 different algal density gradients（0,114,228,456,926 g/m²）for 20 d.Specific growth rate of Gracilaria bailinae and Litopenaeus vannamei,photosynthetic characteristics of algae,NH₄⁺-N,NO₂^--N and NO₃^--N concentrations were measured.The results show that the different algal density of Gracilaria bailinae have significant effects on it’s specific growth rate（P<0.05）,with the increase of density of algae,the specific growth rate of algae presents the downward trend after rising first,when the algal density reached to 456 g/m²and926 g/m²,the algal specific growth rate reached the maximum and the minimum,were 1.63±0.04 and 0.97±0.07%/d;The specific growth rate of Litopenaeus vannamei was significantly affected by the density of mixed algae（P<0.05）.With the increase of algae density,the specific growth rate of Litopenaeus vannamei increased firstly and then decreased,and when the algae density reached the maximum value and minimum value,which were 456 g/m²and 926 g/m²,the specific growth rate of Litopenaeus vannamei was 1.82±0.21 and 0.51±0.11%/d,respectively.With the increase of algal density,the Fv/Fm ratio of the mixed algal population increased firstly and then decreased,and reached the maximum and minimum values at 456 g/m²and 926 g/m²,which were(0.61±0.00μmol·m^-2·s^-1)and 0.54±0.00μmol·m^-2·s^-1,respectively（P<0.05）.With the increase of algal density,the Y_（II）ratio of the mixed algal population increased firstly and then decreased,and reached the maximum and minimum values at 456 g/m²and 926 g/m²,which were(0.64±0.00μmol·m^-2·s^-1)and 0.54±0.00μmol·m^-2·s^-1,respectively（P<0.05）.When the algae density was 456 g/m²,the concentrations of NH₄⁺-N,NO₂^--N and NO₃^--N were the lowest at the end of the experiment,which were significantly different from those in other treatment groups（P<0.05）.