Seasonal changes of mercury reduction and methylation in Gulf of Trieste (north Adriatic Sea)

Abstract:

We have successfully improved and applied the ¹⁹⁷Hg radiotracer method during the sampling campaign from March until November 2011, collecting and incubating sediments and waters with low ¹⁹⁷Hg²⁺ additions without significantly increasing natural levels. The evolution of Me¹⁹⁷Hg and DGM¹⁹⁷ was followed. In addition, we have performed Hg speciation of the water column and sediment, determined diversity of microbial community and investigated microbial resistance to Hg through presence of merA and merB genes. Our results showed repeatedly that methylation does not occur in the water column of the GoT, and confirmed that sediments are the principal methylation site, as well as the source of MeHg to the water column. Its formation seems to be closely linked to nutrient cycling at the sediment-water interface, where degradation of organic matter with accompanying oxygen consumption significantly stimulates MeHg production (range 0.85 pM – 3.39 pM). The water column showed a pronounced capability for ¹⁹⁷Hg²⁺ reduction (up to 25% d^-1), confirming that the GoT is a source of Hg to the atmosphere. Whether reduction was directly linked to genetic resistance; was a consequence of non-specific redox reactions or of other microbial mechanisms could not be demonstrated. Neither merA nor merB genes were detected, but the microbial community structure was changing in the water column seasonally, as did the reduction rates in the experiments. Most importantly, it was shown that ¹⁹⁷Hg methodology is sensitive enough to follow Hg biogeochemical transformations at environmental levels. The advantage is that the minimal additions of ¹⁹⁷Hg do not disturb the natural processes occurring in the environment and that very small changes can be detected. Hg stress in the Gulf can directly manifest itself in biota and consequently result in a threat to environmental and public health and therefore needs to be seen in the light of changing global climate and marine environment.