| A large quantity of pollutants containing nitrogen and phosphorus have been discharged into aquatic environment, leading to serious eutrophicationof rivers, lakes and other water bodies, and the threat of drinking water security and aquatic ecological balance. Algae can efficiently take up nitrogen and phosphorus, and then assimilate them into its own complex organic material. Therefore, algae-based biofilm technology has been widely studied in the removal of nitrogen and phosphorus in recently. However, organic comtaminants, e.g., quaternary ammonium cationic surfactants (QACs), always coexist with nitrogen and phosphorus in wastewater. When the concentrations of QACs are lower than certain threshold, they may induce the hormesis on algae. Though the hormesis of some toxicnats on algal growth has been reported, the effects of low concentrations of QACs and on algal growth and removal of nitrogen and phosphorus and the cooresponding mechanisms are still very limited.In this study, Chlorella vulgaris (C. vulgaris) was employed to investigate how low concentration of QACs (i.e., Cg-TMAB, C10-TMAB, DTAB, TTAB and CTAB as representative) induced hormesis on algal growth, removal efficiency of ammomia nitrogen (NH4+) and total phosphorus (TP) and algal cell activities. Furthermore, choosing CTAB as a representive, we investigated the biochemical components, activities of three key enzymes related to NH4+ and TP absorption, fluorescence parameters and adenosine energy charge (AEC) and explored the corresponding hormesis mechanism. The following conclusions were obtained:(1) In the range of the low concentration C8-TMAB, C10-TMAB, DTAB, TTAB and CTAB, which were 0-0.6 mg/L,0-8 ?g/L,0-1 ?g/L,0-0.5 ?g/L and 0-10 ng/L, respectively, the five QACs all promoted the algal biomass and increased the removal efficiencies of NH4+ and TP of. Meanwhile, the concentrations of the five QACs inducing hormesis decreased gradually with the increase of their carbon chain length. While in the range of high concentration of the above five QACs (>0.8 mg/L,10 ?g/L, 1.0 ?g/L,0.5 ?g/L and 15 ng/L, respectively), they inhibited the algal biomass and the removal efficiency of NH4+ and TP. When C8-TMAB, C10-TMAB, DTAB, TTAB and CTAB was at the concentration of 0.6 mg/L,4 ?g/L,0.2 ?g/L,0.05?g/L and 5 ng/L separately, the maximum algal biomass and the removal efficiency of nitrogen and phosphorus were obtained. NH4+ removal efficiencies were increased by 24.87%, 29.77%,32.89%,16.97% and 14.66%, TP removal efficiencies were increased by 41.41%,47.06%,48.29%,40.05% and 13.31%, compared to the control (without QACs), respectively.(2) Low concentration of QACs, C8-TMAB (0-0.6 mg/L), C10-TMAB (0-8?g/L), DTAB (0-1 ?g/L), TTAB (0-0.5 ?g/L) and CTAB (0-10 ng/L), enhanced the photosynthesis of C. vulgaris by increasing the content of chlorophyll and decreasing the chlorophyll autofluorescence intensity. Furthermore, low concentration of QACs improved the esterase activity through increasing the fluorescence intensity. However, the algal photosynthesis and esterase activity were limited when the concentrations of the above five QACs were higher than 0.8 mg/L,10 ?g/L,1.0 ?g/L,0.5?g/L and 15 ng/L, respectively. In addition, the maximum value of chlorophyll a (19.29,16.83, 15.87,15.91 and 17.63 mg/L), chlorophyll b (8.51,12.92,13.93,13.55 and 11.73 mg/L), total chlorophyll (27.81,29.68,29.73,29.39 and 29.77 mg/L) and the strongest fluorescence intensities of esterase (5344.51,4991.92,10587.31,6764.53 and 7770.22 a.u.) occurred at 0.6 mg/L (C8-TMAB),4 ?g/L (C10-TMAB),0.2 ?g/L (DTAB),0.05 ?g/L (TTAB) and 5 ng/L (CTAB), respectively, meanwhile the intensity of chlorophyll autofluorescence were decreased by 41.41%,47.06%,48.29%,40.05% and 13.31%, respectively, compared to the control.(3) When the concentration of CTAB was less than 10 ng/L, the cellular biochemical components proteins, polysaccharides, and total lipids were increased by 58.17%,38.78% and 11.87%, respectively, comparing to the control. Additionally, some key enzymes related to nutrient metabolism (i.e., glutamine synthetase, acid phosphatase and H+-ATPase), fluorescence parameters (Fv/Fm, ??I, Fv/Fo and rETR) and AEC of algal cells were also enhanced. The maximum values of enzymes activity mentioned above were 406.94 U/g,310.56 U/g and 354.00 U/g, respectively, and the values of fluorescence parameters were 0.6,0.4,1.5 and 74.32 umol·m-2·s-1, with 5 ng/L of CTAB. The underlying mechanism was mainly due to an enhanced algal photosynthetic activity indicated by an increase in the values of Chlorophyll fluorescence parameters and adenylate energy charge (AEC) (from 0.68 to 0.72). While CTAB was higher than 15 ng/L, it inhibited the synthesis of biochemical components, reduced the activity of key enzymes related to NH4+ and TP uptake and decreased the values of chlorophyll fluorescence parameters and AEC of C. vulgaris.(4) Based on the above results, we hypothesis low concentrations of CTAB induced-hormesis on the algal growth and nutrient removal, may be attribute to the raise of photosynthetic activity and the synthesis of adenosine triphosphate (ATP). Hereby, it increases algal key enzyme activity for the absorption of nitrogen and phosphorus, and then leads to adjust the various physiological functions of C. vulgaris and promotes the synthesis of biochemical components. Hence, we consider the leading mechanism of low concentrations of CTAB induced-hormesis is overcompensation. |
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