The In Jury Effects Of Arsenate Exposure To Chloroplast In Two Species Of Green Algae With The Influence Of Phosphorus Deficiency

Arsenate is the main arsenic species in contaminated water.Its toxic effects are still in doubts under the influence of its congener-phosphate.This study was aimed at investigating the photosynthetic damage effects on green algae by arsenate and the influence from phosphorus deficiency on toxic effects of arsenate.In order to achieve these goals,two typical species of green algae were selected,which were Chlorella pyrenoidosa and Scenedesmus quadricauda.They were cultured under constant room temperature and were used to carry out exposure experiments of dose effect.The differential sensitivity of the two green algae to arsenate and the chlorophyll contents were measured using Ultraviolet-visible spectrophotometry.The injury of subcellular chloroplast structure was observed by transmission electron microscopy(TEM).Furthermore,the inhibiting effect of arsenate on the photosynthetic efficiency of photosystem Ⅱ(PS Ⅱ)was determined by phytoplankton pulse amplitude modulation fluorometer.The influence of phosphorus deficiency was also analyzed on the photosynthetic damage of arsenate to green algae.The results are listed as follows:(1)Growth inhibition of green algae by arsenate was positively correlated with the exposure dose.After exposure to 0.1～50 mmol·L-1 arsenate for 24 h,the growth rates of the two green algae,C.pyrenoidosa and S.quadricauda,were significantly reduced,and the inhibition rate increased with the ascending arsenate concentration.The 50%effective concentration(EC50)values of growth inhibition to C.pyrenoidosa and S.quadricauda were 5.14(3.20～8.00)mmol·L-1 and 1.06(0.81～1.36)mmol·L-1,respectively.It indicated that S.quadricauda was more sensitive to arsenate,which may be chosen as the bioindicator of arsenate contamination.(2)Arsenate exposure reduced the cellular chlorophyll contents and caused damage to subcellular chloroplast structure in green algae.48 h exposure to 5.0 mmol·L-1 arsenate significantly decreased the chlorophyll a(chla)content and chla/chlb ratio in C.pyrenoidosa,but chlorophyll b(chlb).However,chla and chlb were both decreased in S.quadricauda,along with the increasing of chla/chlb ratio.Arsenate-induced reduction of cellular chlorophyll content and change of chla/chlb ratio revealed its destructive effect on chlorophyll-protein complex.Further TEM images showed that chloroplast was the target of arsenate.After 24 h exposure to 5.0 mmol·L-1 arsenate,it could be inferred from messy or vacant thylakoid membrane,increasing number of starch grains and diminishing pyrenoid that chloroplast structure was severely damaged.(3)Arsenate reduced PS Ⅱ efficiency in green algae.The maximum relative electron transport rate(ETRmax),initial slope of rapid light curve(a)and half saturation light intensity(Ik)were significantly reduced after 24 h exposure to 1.5-5.0 mmol·L-1 arsenate,and the reduction was positively related with the concentration of arsenate.After 24 h exposure to 5.0 mmol·L-1 arsenate,ETRmax,α,and Ik were decreased by 44.0±8.7%,17.5±5.7%,and 39.0±13.2%respectively,in C.pyrenoidosa,and by 90.4± 13.6%,36.6±4.9%,85.0± 12.5%in S.quadricauda.It demonstrated that the effect of arsenate on the maximum relative electron transport rate,light utilization efficiency,and strong light tolerance of S.quadricauda were more significant than those of C.pyrenoidosa.(4)The inhibition on photosynthetic efficiency by arsenate was not impacted by short-term phosphorus deficiency.No marked influence on PS Ⅱ photosynthetic efficiency and no increase in the growth inhibition effect by arsenate were detected after short-term(24 h)phosphorus deficiency.At the same time,there was no enhancement on the photosynthetic inhibition effect with response to ETRmax,a and Ik.TEM images revealed that arsenate exposure under phosphorus deficiency caused striking diminishment of pyrenoid and starch granules,while damage to the thylakoid was similar to that in phosphorus rich group.