铜绿微囊藻在不同供磷水平下对砷胁迫的响应

RESPONSE OF MICROCYSTIS AERUGINOSA TO ARSENATE UNDER DIFFERENT PHOSPHATE REGIMES

采用磷饥饿培养后的微囊藻细胞进行不同供磷水平的五价砷(As(V))暴露实验,考察单一胞外磷变化的情况下As(V)对滇池分离出的铜绿微囊藻FACHB905生长及产毒的影响。结果表明胞外磷浓度变化不会影响铜绿微囊藻对As(V)的耐受阈值(-10-7mol/L)。少磷条件下的半数抑制浓度(IC50)为10-2.79mol/L,比无磷条件下的IC50(10-5.81mol/L)高3个数量级,并且少磷条件下As(V)与细胞活性位点的结合常数要远低于无磷条件,因此胞外磷在As(V)对微囊藻的毒性效应上具有关键的作用。As(V)对微囊藻单细胞的叶绿素含量没有显著影响,但是对毒素产量具有剂量效应。在少磷条件下,As(V)浓度大于10-7mol/L可促进微囊藻FACHB905的胞内产毒量;而在无磷条件下,所有As(V)处理组的胞内产毒量均上升78%左右。由上可知,微囊藻在产毒方面与As(V)具有协同效应,这对于全面了解滇池水华暴发期间毒素的变化规律具有一定的参考价值。

Arsenic is ubiquitous in the environment and potentially toxic to humans. Arsenate, thermodynamically the dominant specie of arsenic in marine and estuarine surface waters, was shown to be taken up by the phosphate transport systems of phytoplankton and plants due to its similar structure to the phosphate. Considerable evidence has been found that Microcystis luxuriously uptake phosphate to form polyphosphate bodies in phosphate-rich environments. The eyanobaeterial sensitivity to arsenate has often been linked to the structural similarities of arsenate and phosphate, and intracellular polyphosphate was shown to be related to the sensitivity to arsenate. Therefore, the present research was intended to explore the effects of arsenate on the growth and microcystin production of Microcystis aeruginosa FACHB905, which isolated from Dianchi Lake when only the extracellular phosphate concentration was changeable. The cells of M. aeruginosa FACHB905 were cultivated in the phosphate-free BG-11 medium for 14 days in order to completely consume phosphate stored in the cyanobaeterial cells. Then, these phosphate-starved cells were inoculated in modified BG-11 media adjusted as following two cases ： PO4^3- was added at 1 μM for the phosphate-limited medium, and PO4^3- was absent as the phosphate- deprived medium. Arsenate as Na2HAsO4 was added to the culture media at concentrations from 10^-8 to 10^-4M. It was worthwhile mentioning that the phosphate concentration used in this study （1 μmol/L） was similar to that under natural conditions. We measured the density of cultures, chlorophyll content and microcystin content of this cyanobacterium responding to arsenate under both the phosphate regimes. This study showed that the extracellular phosphate concentration had no reference to the threshold doses （ - 10^-7 mol/L） of M. aeruginosa FACHB905 to arsenate. However, the IC50 value under phosphate limitation was 10^-2.79mol/L and three magnitudes greater than that under phosphate deprivation. The apparent association constant of arsenate to the cyanobacterium under phosphate limitation was much lower than that under phosphate deprivation. Thus, it presumed that extracellular phosphate had the key role on protecting cyanobacterial cells from arsenate. Otherwise, arsenate did not affect the chlorophyll content per cell, but had dosage effect on the cellular microcystin content. Arsenate, higher than 10^-7 mol/L, could promote the cellular microcystin content per cell under phosphate limitation, while the microcystin content of all arsenate treatments was stimulated about 78% of that of the control under phosphate deprivation. The synergistic effect of arsenate and microcystin production of M. aeruginosa FACHB905 is of definite significance for complete understanding the mierocystin production in the blooms in Dianchi Lake.