三峡库区消落带土壤中硫酸盐还原菌对汞甲基化作用的影响

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

为探究三峡水库这一特殊的周期性干湿交替环境中,硫酸盐还原菌(sulfate-reducing bacteria,SRB)是否在汞的生物甲基化过程中起主导作用,本文以库区消落带原土为研究对象,以灭菌土+接种硫酸盐还原菌Desulfovibrio africanus(D.africanus,DSM-2603)为对照(试验A),在每公斤土壤分别添加0、1、5 mg汞浓度条件下,模拟研究原土(试验B)中总汞(THg)和甲基汞(MeHg)含量的动态变化、总硫酸盐还原菌(SRB)数量变化,以及影响土壤中甲基汞含量的环境因子分析.结果表明,淹水状况有利于THg从土壤中释放,且Hg含量越高,释放越快;菌株D.africanus对汞具生态适应性,其菌数与MeHg含量显著正相关:在5 mg·kg^(-1)Hg条件下,D.africanus菌数可达3.65×10~4cfu·g^(-1),MeHg含量也高达7.60×10~4ng·kg^(-1).值得注意的是,试验B处理中,一方面消落区原土中SRB菌数较少,平均仅193 cfu·g^(-1);另一方面,土壤MeHg含量较低:在5mg·kg^(-1)Hg条件下,MeHg含量仅为5.54×10~3ng·kg^(-1),且总SRB菌数与MeHg含量无显著相关性.由此可以推测,在三峡水库消落区这种非严格厌氧土壤环境中,SRB并非优势菌群,其中还存在着其它对生物汞甲基化起主导作用的好氧或兼性厌氧微生物种群.

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-1Hg2＋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. 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-1of external Hg2＋,the total number of D. africanus reached the highest（ 3. 65× 10^4cfu·g-1） while the highest MeHg concentration was 7. 60 × 10^4ng·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 × 10^3ng·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.