Role of Sulfate-Reducing Bacteria in Mercury Methylation in Soil of the Water-Level-Fluctuating Zone of the Three Gorges Reservoir Area

Abstract

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 Hg2+ 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 Hg2+ addition, and the higher the external Hg2+, the quicker the Hg2+ release was. Meanwhile, D. africanusdisplayed 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 Hg2+, the total number of D. africanus reached the highest (3.65×104 cfu·g-1) while the highest MeHg concentration was 7.60×104 ng·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-1of external Hg2+, the MeHg concentration was as low as 5.54×103 ng·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.