Role Of Extracellular Polymeric Substances From Chlorella Vulgaris In The Uptake Of Ammonium Under The Stress Of CTAC

Ammonium wasterwater discharged into water body has caused eutrophication.For this reason,the emission standards of municipal wastewater in China（carrying out level A standards）has been improved.Thus,it is necessary to set a higher demand for ammonium treatment technology.Algae have attracted more attentions due to their high removal efficiency of ammonium and resource reutilization of algal cells.As a high molecular polymer secreted by algal cell,extracellular polymeric substances（EPS）stably adheres to the cell surface,and plays a vital role in the nutrients uptake by algae.However,some potential toxic chemicals,such as surfactants,which are often detected in municipal wastewater,will affect the ammonium removal by algae.But to date,the effects of EPS on ammonium uptake by algae under the stress of surfactants are very limited.This paper selected hexadecyl trimethyl ammonium chloride（CTAC）,a common surfactant in wastewater,as the target and investigated the effects of CTAC on ammonium uptake by Chlorella vulgaris,a dominant unicellular green alga.Based on the above study,we further explored the role of EPS on the ammonium uptake in C.vulgaris under the stress of CTAC by controlling the light/dark cycle.Moreover,the algal cell properties（hydrophobicity,cell membrane properties,glutamine synthetase（GS）and chlorophyll autofluorescence）have been investigated to clarify the role of EPS on ammonium uptake by algae under the stress of surfactants.1)CTAC significantly inhibited the NH₄⁺-N uptake by C.vulgaris.The uptake efficiency of NH₄⁺-N,the content of intracellular NH₄⁺-N and the proteins in cells decreased under the stress of CTAC.Under the condition of 0.5 mg·L^-1 CTAC,the uptake efficiency of NH₄⁺-N decreased to 57.95%,which accounts for 64.80% of the control group（without CTAC）.The content of NH₄⁺ in cells decreased to 44.44×10-7 μmol·cell^-1,which accounts for 52.26% of the control group.Similarly,the content of proteins in cells decreased to 5.52×10-7 μg·cell^-1,only 19.41% of the control group.2)EPS weakened the toxicity of CTAC on ammonium uptake by C.vulgaris.Either in the lighting or dark conditions,the uptake dose of ammonium in EPS-Covered（EPS-C）cells was higher than that in EPS-Free（EPS-F）cells after exposed in CTAC for 48 h.With 0.5 mg·L^-1 of CTAC,the uptake dose of ammonium in EPS-F cells and EPS-C cells were 138.56 and 116.83 μmol·L^-1,respectively,which decreased by 135.84 and 114.11 μmol·L^-1 compared with the controlled group（EPS-C cells without CTAC）.The presence of EPS slowed down NH₄⁺ flow under the stress of CTAC,and played a significant role in NH₄⁺ transportation by C.vulgaris.After the algae illuminated for 6h,the NH₄⁺ flow in EPS-C cells(-52.22 pmol·cm^-2·s^-1)was higher than that in EPS-F cells(-40.36 pmol·cm^-2·s^-1)under the stress of 0.5 mg·L^-1 CTAC.The proportion of ammonium uptake in EPS-C cells was 2.90%,which was higher than that in EPS-F cells（2.04%）.Interestingly,algal cells treated with CTAC released NH₄⁺ in solution under the dark condition.The NH₄⁺ flow(25.72 pmol·cm^-2·s^-1)was lower than that(33.99 pmol·cm^-2·s^-1),which increased to 25.72 in EPS-C cells and 33.99 pmol·cm^-2·s^-1 in EPS-F cells compared with the controlled group（EPS-C cells without CTAC）.3)EPS weakened the inhibition of CTAC on cell hydrophobicity and enriched nutrients toward the cell surface.After treated with 0.5 mg·L^-1 CTAC,the cell hydrophobicity in EPS-C cells（44.77%）was lower than that in EPS-F cells（49.35%）after the algae illuminated for 6 h.The result was similar with that in the dark for 6 h.CTAC damaged the cell membrane of C.vulgaris,but EPS weakened the membrane damage and maintained ammonium transport through cell membrane.Both in the light or dark,the membrane permeability in EPS-F cells increased more than that in EPS-C cells after treated with CTAC,and the lipid fluidity decreased in EPS-F cells.In addition,CTAC increased the activity of glutamine synthetase（GS）,but EPS alleviated the inhibition of CTAC on GS activity.Under the stress of 0.5 mg·L^-1 CTAC,the GS activity in EPS-C cells(155.05 U·mg^-1)was higher than that in EPS-C cells(153.01 U·mg^-1)under the light for 6 h.The result was similar with the dark condition（6 h）.Furthermore,EPS reduced the inhibition of CTAC on chlorophyll autofluorescence,which was benefit for the uptake of ammonium.