| Mercury is a ubiquitous toxic heavy metal and is a well-known global contaminant that can affect the health of people and ecosystems. In fact, most of Hg accumulated in biota is its organic form, methylmercury(Me Hg), which is a potent lipophilic, high neurotoxic and the most toxic form of Hg found in the environment. Inorganic mercury can be methylated to Me Hg by biotic and abiotic factors, of which microbe play a vital role in the content of Me Hg in the environment. Previous studies on mercury methylation have focused on anaerobic environment, and mainly on methanogenic archaea(MA), sulfate-reducing bacteria(SRB) and iron-reducing bacteria(Fe RB) as principal Hg methylators, among which SRB was known as the most important factor in reservoirs.The Three Gorges Reservoir(TGR) is an extremely large reservoir in the world with its fluctuating water level between 145 m above the sea level(a.s.l.) in summer and 175 m a.s.l. in winter. Taking the varying water levels into account, it has a flooded area more than 350 km2, representing the seasonally water- level fluctuation zone(WLFZ). TGR is a typical "Mercury sensitive ecosystem". The inundated soil and plants can release mercury to the water column by a series of active or passive processes. Meanwhile, soil organic matter and nutrients from the decomposition of plants can supply a plentiful of nutrition for the microorganisms, which made the anaerobic microbes, for instance SRB thriving and prosperous and thus promoted the formation of Me Hg. Previous researches were mainly focused on the investigation of the mercury content of soil, water and fish, the characteristics of methylation, and the migration and transformation of mercury. There is a lack of data about the specific microbial species, the process and influence factors related to mercury methylation in the TGR. In this study, laboratory simulation experiments were performed:(1) inoculation experiments: The experimental system was designed to investigate the influence of SRB to mercury release and methylation under different concentration of initial mercury. Before experiment, all the soils from the water level fluctuation of the TGR were sterilized and added with 0, 1 and 5 mg?kg-1 of Hg2+(Hg C l2), respectively. These treatments were then inoculated with the SRB strain Desulfovibrio africanus(D. africanus, DSM-2603) at 104 cfu?g-1.(2) Original soil experiments: The experimental system was designed to investigate the characteristics of mercury release, methylation, and the influence factors under inundated conditions of the TGR. In this section, all the soils were unsterilized and added with 0, 1 and 5 mg?kg-1 of Hg2+(Hg C l2), respectively. The results indicated that:(1) In the inoculation experiments, the THg and Me Hg contents of CK treatments were at a very low level both in soil and water. Soil THg was declined of the treatments added with Hg2+, and the higher initial Hg contents, the more Hg released. The water THg was increased after an initial decline in all treatments. At the 30 d, the THg content of the high Hg treatment(A5) was 6.02 and 3.26 times higher than that of CK(A0) and low Hg treatment(A1), respectively. Soil Me Hg in three treatments showed an increasing trend, which were 12.85, 92.74 and 122.47 times higher than the initial level at the 20~25 d, respectively. The mercury methylation activation was enhanced after added w ith Hg2+, and the higher Hg added, the stronger methylation. Similarly, water Me Hg also performed a rising trend, and the Me Hg level was obviously higher in the high Hg treatment than others.(2) In the inoculation experiments, the amount of D. africanus in soils of CK was decreased after an initial increase; overall, it was under a relative low level. However, D. africanus of A0, A1 and A5 showed an opposite trend to CK. The D. africanus amount was the highest among three treatments(3.70′104 cfu·g-1), although it was inhibited by the high Hg concentration at the early stage. Generally, the higher initial Hg concentration, the higher D.africanus amount. Soil Me Hg content showed a significantly relationship with D. africanus amount. Furthermore, External Hg can promote the growth of D. africanus.(3) In the original soil experiments, soil THg showed the same trend with the inoculation experiments. While under the same initial Hg conditions, the THg descent rate of the original soil experiments was slower than the inoculation experiments. Water THg showed a decreasing trend, and the higher initial Hg added, the greater the descend range. Soil Me Hg was increased and then decreased, and the increased range of B5 treatment(0.55?104 ng·kg-1)was 2.20 and 1.34 times higher than that of B0 and B1. The results indicated that high Hg concentration was conducive to the formation of Me Hg. In addition, water Me Hg showed the same trend with soil Me Hg, and the maximum level was found in the B5 treatment, which was as high as 39.94 ng·L-1.(4) Overall, the amount of total SRB of the original soil experiments were under a relatively low level, which showed a periodical characteristic of ―decrease-increase‖. The amount of total SRB of B5 was 2.69 and 2.62 times higher than that of B0 and B1. This result indicated that high Hg environment was conducive to ―induce‖ the increase of total SRB amount. However, there was no significant relationship between Me Hg content and SRB amount(P>0.05), and the correlation coefficient(r2) were 0.01、0.36、0.02, respectively. Therefore, SRB were speculated not the major mercury methylators in the WLFZ of the TGR.(5) In the inoculation experiments, soil p H of each treatment was increased followed by a decrease trend, while conductivity was reverse; SOM appeared a decreasing tendency throughout the time. In addition, soil Me Hg showed no significantly correlation with both THg and conductivity(P>0.05), while performed a remarkably negative relationship with p H and SOM(P<0.05). However, in the original soil experiments, soil p H and SOM performed a downward trend, while conductivity was increased after an initial decline. Furthermore, soil Me Hg content showed no correlation between THg, p H, SOM and conductivity respectively. |
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