The thiosulfate reductase gene (phsABC) from Salmonella enterica serovar Typhimurium was expressed in Escherichia coli to overproduce hydrogen sulfide from thiosulfate for heavy metal removal (or precipitation). A 5.1-kb DNA fragment containing phsABC was inserted into the pMB1-based, high-copy, isopropyl-␤-D-thiogalactopyranoside-inducible expression vector pTrc99A and the RK2-based, medium-copy, m-toluate-inducible expression vector pJB866, resulting in plasmids pSB74 and pSB77. A 3.7-kb DNA fragment, excluding putative promoter and regulatory regions, was inserted into the same vectors, making plasmids pSB103 and pSB107. E. coli DH5␣ strains harboring the phsABC constructs showed higher thiosulfate reductase activity and produced significantly more sulfide than the control strains under both aerobic and anaerobic conditions. Among the four phsABC constructs, E. coli DH5␣ (pSB74) produced thiosulfate reductase at the highest level and removed the most cadmium from solution under anaerobic conditions: 98% of all concentrations up to 150 M and 91% of 200 M. In contrast, a negative control did not produce any measurable sulfide and removed very little cadmium from solution. Energy-dispersive X-ray spectroscopy revealed that the metal removed from solution precipitated as a complex of cadmium and sulfur, most likely cadmium sulfide.Heavy metals are commonly found at many hazardous waste sites in industrialized countries. Many soluble metals can form insoluble complexes with hydroxides, carbonates, phosphates, and sulfides (21). One of the best-known natural metal precipitation mechanisms is due to sulfide production from sulfate by sulfate-reducing bacteria (SRB) found in anoxic sediments containing high concentrations of lead and mercury (9). A recent bioremediation technology utilizes hydrogen sulfide generated by SRB in anaerobic bioreactors to precipitate soluble metal species in aqueous waste streams as insoluble metal sulfides (25). The primary focus of this study was to develop a genetically engineered bacterium capable of producing sulfide under aerobic, microaerobic, or anaerobic conditions for heavy metal precipitation.Among several bacterial hydrogen sulfide-generating systems, we chose the thiosulfate reductase gene (phsABC; phs represents production of hydrogen sulfide) from Salmonella enterica serovar Typhimurium to overproduce hydrogen sulfide. Thiosulfate reduction is a common but incompletely understood feature among bacteria (17). Thiosulfate reductase catalyzes the dissimilatory reduction of inorganic thiosulfate to hydrogen sulfide and sulfite (6). The enzyme has been purified from Desulfovibrio vulgaris (1), D. gigas (13), and a thermophilic iron-oxidizing bacterium, strain TI-1 (22).Mutant and biochemical tests suggested that thiosulfate reductase activity from S. enterica serovar Typhimurium has an absolute requirement for the F 0 F 1 -ATP synthase (20). Sequence analyses of the chromosomal phsABC region from S. enterica serovar Typhimurium revealed a functional operon with three open readin...

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The phytoplankton community structure in a riverine zone of Pal'tang Reservoir was monitored weekly from April to November 1997. During the heavy rainfall period, phosphate and silicate concentrations sharply increased while dissolved oxygen, pH and chl a decreased. Nitrate concentration was relatively stable over the period. Chl a concentration showed an inverse relationship with phosphate and silicate concentrations. A high DIN/DIP ratio ranging from 25 to 1,8 13 led chlorophytes to be predominant over the whole period. During autumn, silicate concentration considerably decreased and played an important role as a limiting factor for diatoms. The physical disturbance of the water column due to rainfall seemed to be the major factor determining phytoplankton biomass in the water. Additionally, phytoplankton groups displayed the seasonal succession: domination of bacillariophytes, cryptophytes and chlorophytes during spring, chlorophytes in summer, and cyanobacteria in autumn. The composition and succession of phytoplankton species were typical of these of very eutrophic lakes.

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