Mercury transformations in a coastal water column (Gulf of Trieste, northern Adriatic Sea)

Abstract

The mechanisms of mercury (Hg) microbial transformations in coastal marine water column are poorly understood. In an effort to study seasonal Hg transformation mechanisms and underlying processes in the coastal marine ecosystem of the Gulf of Trieste (northern Adriatic Sea), we have successfully applied the 197Hg radiotracer technique in series of incubation experiments. This area is characterized by continuous Hg input from the Soča/Isonzo River as a consequence of nearly 500 years of activity at the Idrija Mine (W Slovenia). The method proved to be valuable, since it allowed us to follow Hg transformations at concentrations similar to those found in the study site. During our study, Hg methylation in the marine water column could not be detected, suggesting that sediments are the principal methylation site and the source of MeHg to the water column. Conversely, the water column showed a pronounced Hg reduction potential, mostly of photochemical origin, suggesting that the Gulf of Trieste is a source of Hg0 to the atmosphere. However, the high Hg reduction potential observed in autumn was most likely related to a phytoplankton (diatom) autumn bloom, indicated by high Chl a concentrations measured during that period. We hypothesize that the microbial reduction was either linked to the expression of mer genes or, a consequence of non-specific redox reactions. Variations in bacterioplankton community fingerprints (as determined by Denaturing Gradient Gel Electrophoresis) suggested that community structure had little influence on microbial Hg reduction potential. Nevertheless, our results indicate that in the absence of photochemistry, interactions between microbes and organic matter may enhance biogeochemical Hg2 + reduction potential in some coastal marine environments.