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Abstract

Mercury (Hg) is one of the contaminants of most concern due to its high toxicity and
accumulative behavior in biota. Elevated Hg concentrations can exceed maximal allowed
levels in top predator fish, in which Hg is mostly present in its most toxic form
monomethylmercury (MMHg). However, the factors controlling its prevalence in fish are
largely unknown.
Significant differences in Hg biogeochemistry exist between the coastal and open sea.
In coastal waters methylated mercury (MeHg) is primarily produced in coastal sediments,
while in open ocean MeHg is formed in a water column. The extent to which MeHg
produced in coastal sediments and open waters is accumulated in fish is not well
understood.
An overview of the previous research on Hg abundance, distribution and speciation in
environmental compartments in the Adriatic Sea revealed that Hg concentrations in
seawater are decreased during past decades due to the closure of industries that released
Hg to the coastal environments. A decreasing gradient in Hg concentrations from the
northern to the southern Adriatic indicates that the presence of legacy mercury still
contributes to Hg in seawater. Concentrations of Hg in plankton and mussels follow the
spatial and temporal trends observed in water, however, concentrations in fish do not follow
these trends indicating the complexity of MeHg formation and accumulation in fish.
The main objective of this work was to better understand the interaction between Hg
species and the abundance and composition of microbial community in the central Adriatic
Sea, more precisely in the transect from Hg contaminated coastal environment in Kaštela
Bay to the Island of Vis. Monthly sampling was performed over a two-year period in
collaboration with the Institute of Oceanography and Fisheries in Split and the research
vessel Bios Dva. Mercury analysis included total mercury (THg), dissolved gaseous mercury
(DGM), and MeHg. Microbial structure analysis included high and low nucleic acid
bacteria, nanoflagellates, picoeukaryoes, Prochlorococcus, and Synechococcus. Physicochemical
properties of seawater were also measured.
One of the obstacles to study MeHg dynamics in open waters are its very low
concentrations, usually present in the femtomolar range, which represent an analytical
challenge. Therefore, the first objective of the work was to develop and validate a method
based on MeHg-hydride generation. The selected method was fit for purpose due to low
reagent blanks and quantitative recoveries from seawater. Method’s limit of detection was
3.94–15.9 fmol L–1 and the expanded uncertainty of the method 11.1–21.3%.
The results revealed that Hg methylation in seawater is possibly a metabolismdependent
transformation promoted by the heterotrophic activity of low nucleic acid
bacteria under the absence of probable phosphorus limitation (NO–PP-limitation). Under
these conditions, MeHg was also related with the concentration of nutrients and the
abundance of Prochlorococcus, whose decay promotes heterotrophic activity. DGM was
related with the abundance of heterotrophic bacteria, autotrophic picoplankton and
chlorophyll a concentration, indicating biotic DGM production that is possibly not
metabolically dependent. MeHg biomagnification from microseston to mesozooplankton was observed through the significant increase of corresponding bioaccumulation factors.
Results revealed that Hg uptake by plankton might be enhanced under NO–PP-limitation
conditions.