RESPONSE OF MICROCYSTIS AERUGINOSA TO ARSENATE UNDER DIFFERENT PHOSPHATE REGIMES

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 cyanobacterial 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 BC-11 medium for 14 days in order to completely consume phosphate stored in the cyanobacterial cells. Then, these phosphate-starved cells were inoculated in modified BC-11 media adjusted as following two cases; PO43- was added at 1 mu M for the phosphate-limited medium, and PO43- was absent as the phosphate-deprived medium. Arsenate as Na2HAsO4 was added to the culture media at concentrations from 10(-8) to 10(-4)M. It was worthwhile mentioning that the phosphate concentration used in this study (1 mu mol/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(-2.79)mol/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 microcystin production in the blooms in Dianchi Lake.