engleski

Seasonal changes in mercury speciation and the composition of the microbial community in the seawater in the central Adriatic Sea

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. Physico-chemical 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 metabolism-dependent 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.

Mercury speciation ; microbial community ; Adriatic Sea

nije evidentirano