Slađana Strmečki

Copper complexing properties of exudates and metabolites of macroalgae from the Aegean Sea.

Macroalgae are a significant source of extracellular organic material in the coastal areas of the Mediterranean, including organic ligands which serve as modulators of metal complexing capacity. This paper examines the release of copper-complexing ligands by 24 macroalgal species, including chlorophyta, rhodophyta, ochrophyta as well as Posidonia oceanica, common throughout the Mediterranean. Metabolites isolated from the algae Dictyota dichotoma and Pterocladiella capillacea were examined for the first time regarding their Cu-complexing properties. The concentration of ligands (LT) and the copper-binding strength (logKapp) of exudates and metabolites were determined by electrochemical methods. All algal species released ligands giving LT concentrations ranging from 109 to 744 nM in unfiltered samples. An increase in the concentration of ligands up to 15 times in comparison to the blank was observed after 48 h of culturing macroalgae in artificial seawater. The binding strength (log Kapp) varied among species from 7.0 to 8.6, except for the metabolites for which it was lower (log Kapp=6). Most of the ligands released were in the dissolved phase, with the contribution of particulate and/or colloidal organic matter (up to 46% of total ligands) appearing to be important only in specific macroalgal species.

Constant current chronopotentiometric stripping characterisation of organic matter in seawater from the northern Adriatic, Croatia

Environmental context We determined seasonal changes in the organic matter content of the northern Adriatic with newly applied electrochemical techniques able to measure catalytically active organics. The inflow of the Po River and its nutrient load are responsible for the observed changes in the type and concentrations of organic matter in the area. Abstract Catalytically active polysaccharides (Cat PSs) and nitrogen-containing polymeric organic material (N-POM) were determined in seawater from the northern Adriatic station ST101. Catalytically active organics were measured by applying electrochemical methods of adsorptive transfer chronopotentiometric stripping with medium exchange and chronopotentiometric stripping in unmodified seawater. Their concentrations were expressed in milligrams per cubic decimetre–3--> of equivalents of the model calibrating substances, polysaccharide xanthan and protein human serum albumin. The optimal electroanalytical conditions for determination of Cat PSs in seawater were evaluated and defined. Seasonal changes of Cat PSs and N-POM were observed during the period 2011–2013. The highest values were determined in the spring–summer period and the lowest in winter. Cat PSs and N-POM were present in both the dissolved and particulate organic carbon fractions. Cat PSs and N-POM showed a statistically significant positive correlation with the concentrations of surface-active substances. A weak but statistically significant correlation was found between Cat PSs and dissolved organic carbon concentrations. Copper complexing capacities in the period 2011–2013 were in the range of 41–130nmoldm–3.

Assessing trace metal contamination and organic matter in the brackish lakes as the major source of potable water

On small and medium karstic coastal islands in the Adriatic Sea, brackish lakes are often the only source of freshwater. Therefore, it is important to adequately evaluate the biogeochemical processes occurring in these complex water systems, as well as to determine the origin of contaminants present. In this study, the distribution and origin of trace metals (Tl, Hg, Cd, Pb, Cu, Zn, Ni, Co) and organic matter in the water column, sediment, and surrounding soil of the brackish lakes on Mljet Island, South Adriatic Sea, Croatia, were evaluated. Thallium and mercury concentrations in the lake water were up to two orders of magnitude higher compared to ranges found in the adjacent coastal sea water. Elevated thallium concentrations were of anthropogenic origin resulting from previous use of rodenticide, while elevated mercury content was naturally enhanced. Levels for the other metals were characteristic of uncontaminated water systems. Speciation modelling showed that dissolved trace metals such as Cu, Pb, and Zn were mostly associated with organic matter, while Tl, Co, and Ni were present predominantly as free ions and inorganic complexes. The presence of organic matter (OM) clearly influenced the speciation and distribution of some trace metals. OM was characterised by the determination of the complexing capacity for Cu ions (CuCC), surface active substances, and catalytically active compounds. Reduced sulphur species (glutathione and other thiols) representing significant Cu-binding ligands were determined and discussed as well.

Adsorption/desorption of biomacromolecules involved in catalytic hydrogen evolution

Previously, it has been shown that proteins and some polysaccharides (PSs) catalyse hydrogen evolution, producing electrochemical signals on mercury electrodes. The catalytic hydrogen evolution reaction (CHER) of the above-mentioned biomacromolecules was studied by voltammetric and chronopotentiometric stripping (CPS) methods. To obtain more information about electrode processes involving CHER, here we used protein such as BSA, and chitosan as a PS; in addition, we investigated dextran as a control PS not involved in CHER. We studied biomacromolecules by phase-sensitive alternating current (AC) voltammetry. Using phase-in AC voltammetry, for CHER-involved biomacromolecules we observed a CHER peak at highly negative potentials, similar to that observed with other voltammetric and CPS methods. On the other hand, by means of the adsorption/desorption processes studied in phase-out AC voltammetry, we uncovered a sharp and narrow decrease of capacitive current in the potential range of the CHER peak, denominated as the tensammetric minimum. This minimum was closely related to the CHER peak, as demonstrated by similar dependences on specific conditions affecting the CHER peak such as buffer capacity and pH. A tensammetric minimum was not observed for dextran. Our results suggest specific organization of biopolymer layers at negative potentials observed only in biomacromolecules involved in CHER.