The Effect Of Heavy Metal Stress On The Removal Of Nitrogen And Phosphorus By Dominant Species Of Algae And Its Mechanism

Algae are the primary producers in aquatic ecosystems, and sewage treatment byalgal biotechnology is focused by many researchers. With a large amount of nitrogenand phosphorus in industrial, agricultural and domestic wastewater being dischargedinto the environment, eutrophication occurs frequently. Thus, aquatic ecologicalbalance and the safety of drinking water have been affected. In view ofeutrophication caused by excessive nitrogen and phosphorus emissions, removal ofnitrogen and phosphorus by algal biotechnology has become a research hotspot. Dueto the discharge of heavy metals and other toxic pollutants into the water withoutproper disposal, sewage treatment by algae is easy to be affected by heavy metalsand other toxic pollutants. Therefore, it is crucial to study heavy metal stress on theremoval of nitrogen and phosphorus by algae.In this paper, by comparing the removal of nitrogen and phosphorus inwastewater by four algal species （Spirulina platensis, Spirogyra, Chlamydomonasreinhardtii and Inclined Scenedesmus）, S. platensis, a dominant alga was firstselected. Then, the influence of initial algal density, nitrogen and phosphorusconcentration, ratio of nitrogen to phosphorus （N/P）, effect of starvation andinorganic carbon source were studied. Meanwhile, the removal conditions of S.platensis was optimized. Furthermore, the effect of heavy metal stress on nitrogenand phosphorus removal by S. platensis was investigated combined with cellularanalysis of protein content, antioxidant content in algal cell. This paper has obtainedthe following main conclusions:（1）The four tested freshwater algae （Spirulina platensis, Spirogyra,Chlamydomonas reinhardtii and Inclined Scenedesmus） all have certain abilities toremove nitrogen and phosphorus. The removal efficiency of NH₄-N and PO4-Preached82.6%and93.0%treated by S. platensis, which was the highest in the fourtested algae. Removal efficiency of NO₃-N by Spirogyra was the best, which couldreach70.3%. S. platensis was finally selected as the dominant alga due to its strongeststain resistance and highest special growth rate.（2） The removal of nitrogen and phosphorus was related to algal density, initialconcentrations of nitrogen and phosphorus, ratio of nitrogen to phosphorus and otherfactors. When initial algae density （OD560） was in the range of0.1-0.3, the initial concentration of nitrogen and phosphorus were25mg/L and2.5mg/L respectively,the optimal conditions of N/P from5:1to10:1, starvation treatment of2-3d andwithin inorganic carbon source, the removal efficiency of nitrogen and phosphorus byS. platensis could reach90.2%and100%, respectively. The nutrient absorptioncharacteristic of S. platensis showed that the absorption of NO₃-N、NH₄-N and PO4-Pby S. platensis could be explained with Michaelis-Menten equation. Km and Vmaxwere26.82mg/（g d）,10.29mg/（g d） and13.34mg/L,6.87mg/L, respectively.（3） Through studying the impact for four heavy metal ions (Pb²⁺, Zn²⁺, Cd²⁺andMn²⁺) on the growth of S. platensis, it was found that low concentrations of heavymetals （0-10mg/L） promoted cell growth, while high concentrations（10-160mg/L）presented inhibition. In the four tested metals, the toxicity of Cd²⁺on S. platensis wasmaximum.The removal of NH₄-N and PO4-P increased firstly, and then decreased withthe concentration of Cd²⁺increasing. When the concentration of Cd²⁺was10mg/L, theremoval of NH₄-N and PO4-P reached highest value at62.9%and88.1%, whichincreased5.5%and11.2%compared with control group, respectively. However,algal soluble protein content increased slightly compared to the control.The changeof （Superoxidase dismutase）SOD,（Peroxidase）POD,（Catalase）CAT activity wassimilar to that of algal soluble protein content. The activity of CAT with Cd²⁺concentration at160mg/L increased by673%compared to the control.（4） It was observed that algal cells appeared breakage and the degree of breakageincreased with the concentration Cd²⁺by Scanning Electron Microscopy （SEM）.Algal cells were wizened and tabula, furthermore, the surface of algal cells had deepwrinkles. Part of Algal cells was unable to maintain morphology under highconcentrations of Cd²⁺.