Inhibited surface-thiols in Geobacter sulfurreducens : Mercury uptake and methylation rates

Abstract: Due to daily human (anthropogenic) industrial activity, a lot of elemental mercury (Hg0) has been released to the atmosphere. The mercury can then spread globally, since mercury has the ability to remain atmospheric for up to a year. When mercury deposits, it can end up in anaerobic environments such as sediments and soil. In these oxygen-free climates, microbes such as Geobacter sulfurreducens exist. Such microbes can transform the mercury into another form, methylmercury, which is the most toxic and bio-transferrable type of mercury. This results in mercury accumulation throughout the food-chain, as smaller creatures are eaten by larger predators, concentrating the mercury in each consumption step. This is indeed concerning for humans, as we eat both mercury-accumulating plants and animals. The interesting phenomenon with G. sulfurreducens, is not only its ability to survive in an environment with mercury, but that it can take up the mercury inside itself to form methylmercury, and then excrete it to the external environment. However, the uptake mechanisms of mercury are poorly understood, whether the uptake is active or not (energy-dependent or independent), how many different uptake pathways exist, and if there are any working mercury transport proteins. Incidentally, thiol functional groups on the surface of the outer-membrane of the bacteria act as “anchoring-points” that mercury can bind to. Blocking of these anchoring-points in previous experiments has not shown any significant change in mercury methylation rates, in G. sulfurreducens. However, in this study, by utilizing a high concentration of a fluorophore (qbbr) to block the surface-thiols, we present data that indicate affected mercury distribution and methylation rates, when treating the cells with qbbr. Moreover, we examine the utilization of washing-methods to separate each sample into different fractions of mercury, investigating the cellular distribution of mercury. By separating samples in total, extracellular, intracellular, surface-adsorbed and methyl-mercury fractions, it appeared that the adsorbed and methylmercury fractions were affected and decreased by the qbbr blocking. Mercury analysis was accomplished by using cold-vapor ICP-MS, thermal desorption GC-ICP-MS and isotope dilution, as an internal standardization method.Furthermore, our microscopic analysis exhibited the notion that the Pi/Syto9 stained cells were stable, even with a relatively high concentration of qbbr, for the duration of the blocking experiments (4-5h after incubation).Understanding the uptake mechanisms of mercury, as well as the mercury methylation process, can lead to future innovations. The results in this study indicate that both uptake and methylation changed with blocked surface-thiols, which opens up for further research. Perhaps, in the future, it could aid the development of new bacterial-membrane filters, and methods to clean mercury polluted water, soil, and sediments.