Award: OCE-1634024

Mercury is a very toxic element common in all waters. It has both natural and man-made sources in these waters. However, mercury is found in many forms in natural waters, and not all forms are equally toxic to living things, including human consumers of seafood. The most toxic form of mercury is methylmercury, and this form of mercury is produced by diverse types of bacteria. Most of the mercury introduced into natural waters by human activities is in an inorganic form (not merthylmercury). It has been shown that methylmercury is the form of mercury that displays biomagnification in food chains, where the concentration is higher in the tissues of predators than in tissues of the prey; inorganic forms of mercury do not display biomagnification. Hence the form of mercury in marine fish is overwhelmingly methylmercury. Methylmercury is also the most toxic form of mercury to humans, and seafood consumption is the predominant source for people. Because of its danger to public health, it is critical to better understand the cycling and fate of methylmercury in seawater. It has previiously been shown that some forms.of bacteria are capable of converting inorganic mercury in seawater to mercury gas, and that gas can volatilize out of the surfacce ocean to the air above it. However, no previous study has demonstrated whether methylmercury can be converted to another form, especially a gas, by natural assemblages of marine microorganisms. This project aimed at understanding the influence of common bacterial communities in surface waters of Long Island Sound and in surface Atlantic seawater on the conversion of methylmercury to mercury gas, and to measure the rate and extent of this conversion and the rate of loss of the produced mercury gas to the air above. To facilitate rapid and accurate measurements of these rates, we used a radioactive isotope to trace the fate of the mercury and employed this radioisitope of mercury to synthesize radioactive methylmercury in the laboratory. We also designed a novel monitoring tool to trap all the released mercury gas using gold beads.The approach used here for studying the release of mercury to the air from seawater is presumably also applicable to other volatile organic metallic compounds, assuming that gamma-emitting isotopes of the other metals or mealloid elements are available. It was found that (a) natural bacteria assmblages are capable of rapidly converting methylmercury to mercury gas at natural, or near-natural, concentrations in seawater and that the mercury gas was released rapidly to overlying air at typical prevailing environmental conditions,(b) that the rate of gas evasion from water to air is directly proportional to the bacterial biomass and metabolic activity but independent of light and of mercury concentrations, and that c) reduction of inorganic mercury to mercury gas is controlled by the bacterial enzyme mercuric reductase. Our study demonstrated that by mediating this process, bacteria can reduce the amount of methylmercury that is present in seawater at a particular location and thus reduce its build-up in marine food chains, including those animals harvested for human consujmption. Last Modified: 06/18/2020 Submitted by: Nicholas S Fisher