Title

The Mechanisms and Pathways of the Uptake of Inorganic Mercury and Methylmercury Species in Escherichia coli: Possible Implications for Mercury Cycling in the Marine Environment

Date of Completion

January 2011

Keywords

Chemical Oceanography|Biology, Microbiology|Engineering, Chemical

Degree

Ph.D.

Abstract

The objectives of this research were to construct a biosensor that can quantify CH3HgII uptake, and use this bioreporter to document the pathways of inorganic mercury (HgII) and CH 3HgII uptake in the microbe Escherichia coli. To this end we modified the mer-lux plasmid pRB28 by adding the gene merB, such that when CH3Hg II is in the cell it is broken down to Hg2+ which then triggers the release bioluminescence in proportion to the amount of Hg 2+ present. Using this tool we investigated the impact of chloride ion, humic acids, and amino acids, on the uptake of CH3Hg II. The role of glutathione transport proteins in the uptake of Hg II and CH3HgII was also investigated by comparing uptake rates in a strain that has a functioning ABC transport system to a strain where this transport system has been disabled by deleting two essential genes. 2 sections of Long Island Sound (LIS) were sampled for total mercury and thiols, and a mechanistic model was developed to predict the speciation of HgII and CH3HgII as well as the bioavailable proportions of HgII and CH3Hg II.^ It was observed that the bioreporter displayed a minimum detection limit of 2.5 nM CH3HgII, and chloride reacts with CH 3H+ to a form neutral complex that passively diffuses into the cell. Humic acids reduced the bioavailability of CH3Hg II by forming bulky complexes that contained charged groups, and their binding constants (log K) with CH3Hg+ ranged from 10.8 to 12. The formation of the Hg(cysteine)2 and CH3Hg--cysteine complexes resulted in increased uptake of HgII and CH 3HgII. This was attributed to the activity of transport proteins. There were differences in uptake rates of the HgII and CH3HgII in the strain that had a functioning transporter when compared to the strain whose transport system had been disabled in complex medium. The formation of Hg(glutathione)2 and CH 3Hg-glutathione complexes in minimum medium resulted in decreases in uptake. Nanomolar quantities of thiols were measured in LIS, and glutathione was found to control the speciation of both HgII and CH 3HgII. Active processes were also found to be important in the uptake of HgII and CH3HgII in LIS.^