engleski

Relationship between mercury species and microbial communities in the seawater of the central Adriatic Sea

Natural microbial processes in seawater and sediments can convert inorganic mercury into its toxic form, methylmercury (MeHg). Our research tries to identify relationship between mercury species and microbial abundance in seawater. We performed series of samplings during oceanographic cruises aboard the research vessel Bios Dva from March 2014 to December 2015 in the Central Adriatic Sea. Research was constrained in transect from the Bay of Kastela to the island of Vis. We collected non-filtered water samples for determination of total mercury (THg), dissolved gaseous mercury (DGM), methylmercury (MeHg) and microbial species in Adriatic coastal and open waters. Plankton samples for the determination of THg and MeHg were collected using vertical towing nets (53 and 200 μm). THg concentrations are the lowest in the pristine environment of the island of Vis (0.14-1.10 ng/L). The highest THg values (0.92-5.58 ng/L) are found in the Bay of Kastela which had been affected by previous contamination from chlor-alkali industrial waste waters. DGM always shows slightly higher values in contaminated areas (31.8-351 pg/L) than in the pristine environment (22.1-245 pg/L). MeHg concentrations vary, but range from 1.07-34.3 pg/L for all stations, with the highest values found in the Bay of Kastela. Percentage of THg present as MeHg is low (1.2% and 3.1% in coastal and open sea, respectively) indicating deficiency of conditions for mercury methylation or high MeHg demethylation. MeHg concentration in seawater is correlated with abundance of picoeukaryotes, indicating possible microbial methylation in the water column. Number of picoeukaryotes is the highest in the Bay of Kastela (0.44-31.8 millions/L) which has been affected by industrial effluents from surrounding cities. The lowest number (0.63-19.9 millions/L) is found near the island of Vis. The highest THg concentrations in plankton are found in the Bay of Kastela (49.8-1115 ng/g and 103-249 ng/g d.w. for 53 and 200 μm fraction, respectively), while the lowest values are found at the Split Channel station (39.5-96.5 ng/g and 5.27-96.8 ng/g d.w. for 53 and 200 μm fraction, respectively). MeHg concentrations are more variable (0.29-14.2 ng/g and 2.00-24.3 ng/g d.w. for 53 and 200 μm fraction, respectively). THg concentrations in plankton show biodilution effect which is best seen in the Bay of Kastela. On the contrary, MeHg bioaccumulation along trophic levels is observed through higher MeHg concentrations and MeHg percentage in 200 μm fraction, compared to 53 μm fraction.

Mercury species ; Microbial communities ; Adriatic Sea

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