Role of Sulfate-Reducing Bacteria in Mercury Methylation in Soil of the Water-Level-Fluctuating Zone of the Three Gorges Reservoir Area
CHEN Rui;CHEN Hua;WANG Ding-yong;XIANG Yu-ping;SHEN Hong;College of Resources and Environment,Southwest University;Chongqing Engineering Research Center for Agricultural Non-point Source Pollution Control in the Three Gorges Reservoir Area;Chongqing Key Laboratory of Agricultural Resources and Environment;
Soil simulation experiments with or without addition of external mercury and a non-indigenous bacterium were conducted under laboratory conditions,in order to investigate whether sulfate-reducing bacteria( SRB) are the dominant mercury methylators in the water level fluctuation zone( WLFZ) of the Three Gorges Reservoir,which is a typical periodical wet and dry alternating environment,locating in the Shibaozai Xinzhen Village,Zhong County,Chongqing,China( E108° 12' 3″ and N30° 24' 36″). The original soil from the WLFZ was firstly sterilized and then inoculated with or without Desulfovibrio africanus( D. africanus DSM-2603,commercially purchased from the Culture Preservation Center of Microorganisms in Germany,DSMZ) as the control( Test A). The sterilized soils were respectively mixed with 0,1,or 5 mg·kg~(-1)Hg~(2+)soil under submerged and non-submerged conditions to simulate changes in the concentrations of total mercury,methylmercury and the growth of D. africanus,and the effects of environmental factor on such changes. The results showed that under the submerged situation the release of total mercury( THg) from soils( Test B)increased with the increase of external Hg~(2+)addition,and the higher the external Hg~(2+),the quicker the Hg~(2+)release was.Meanwhile,D. africanus displayed an ecological adaptability to mercury and its colony numbers were significantly correlated with the MeHg concentrations in the soil. In soil with 5 mg·kg~(-1)of external Hg~(2+),the total number of D. africanus reached the highest( 3. 65× 104cfu·g~(-1)) while the highest MeHg concentration was 7. 60 × 104ng·kg~(-1). However,on the one hand,the averaged numbers of soil SRB in the test B was only 193 cfu·g~(-1). On the other hand,in soil with 5 mg·kg~(-1)of external Hg~(2+),the MeHg concentration was as low as 5. 54 × 103ng·kg~(-1),while no significant relationships were observed between SRB and MeHg. As a result,our results indicated that SRB was not a predominant mercury methylator,while other bacterial groups,either aerobic or anaerobic bacteria,might play an important role in mercury methylation in the WLFZ of the Three Gorges Reservoir,China.