Mercury in the Black Sea: New Insights From Measurements and Numerical Modeling.

Redox conditions and organic matter control marine methylmercury (MeHg) production. The Black Sea is the world's largest and deepest anoxic basin and is thus ideal to study Hg species along the extended redox gradient. Here we present new dissolved Hg and MeHg data from the 2013 GEOTRACES MEDBlack cruise (GN04_leg2) that we integrated into a numerical 1-D model, to track the fate and dynamics of Hg and MeHg. Contrary to a previous study, our new data show highest MeHg concentrations in the permanently anoxic waters. Observed MeHg/Hg percentage (range 9-57%) in the anoxic waters is comparable to other subsurface maxima in oxic open-ocean waters. With the modeling we tested for various Hg methylation and demethylation scenarios along the redox gradient. The results show that Hg methylation must occur in the anoxic waters. The model was then used to simulate the time evolution (1850-2050) of Hg species in the Black Sea. Our findings quantify (1) inputs and outputs of HgT (~31 and ~28 kmol yr-1) and MeHgT (~5 and ~4 kmol yr-1) to the basin, (2) the extent of net demethylation occurring in oxic (~1 kmol yr-1) and suboxic water (~6 kmol yr-1), (3) and the net Hg methylation in the anoxic waters of the Black Sea (~11 kmol yr-1). The model was also used to estimate the amount of anthropogenic Hg (85-93%) in the Black Sea.

Effect of trace metal availability on coccolithophorid calcification.

The deposition of atmospheric dust into the ocean has varied considerably over geological time. Because some of the trace metals contained in dust are essential plant nutrients which can limit phytoplankton growth in parts of the ocean, it has been suggested that variations in dust supply to the surface ocean might influence primary production. Whereas the role of trace metal availability in photosynthetic carbon fixation has received considerable attention, its effect on biogenic calcification is virtually unknown. The production of both particulate organic carbon and calcium carbonate (CaCO3) drives the ocean's biological carbon pump. The ratio of particulate organic carbon to CaCO3 export, the so-called rain ratio, is one of the factors determining CO2 sequestration in the deep ocean. Here we investigate the influence of the essential trace metals iron and zinc on the prominent CaCO3-producing microalga Emiliania huxleyi. We show that whereas at low iron concentrations growth and calcification are equally reduced, low zinc concentrations result in a de-coupling of the two processes. Despite the reduced growth rate of zinc-limited cells, CaCO3 production rates per cell remain unaffected, thus leading to highly calcified cells. These results suggest that changes in dust deposition can affect biogenic calcification in oceanic regions characterized by trace metal limitation, with possible consequences for CO2 partitioning between the atmosphere and the ocean.

Interactions of algal ligands, metal complexation and availability, and cell responses of the diatom Ditylum brightwellii with a gradual increase in copper.

A continuous culture experiment was conducted to study interactions between copper-binding ligands released by light-limited Ditylum brightwellii, and toxic effects of Cu on this diatom. Over 6 months, the Cu concentration in the medium has been increased in seven steps (3-173 nM). At each Cu addition, Cu speciation, characteristics of Cu sorption to cellular binding sites, and cell characteristics were determined. Physiological effects of Cu were studied, using indicators for metal detoxification (thiols) and lipid peroxidation (malondialdehyde). Minor amounts of Cu (<1.4%) were chelated by a minimum amount of EDTA (57 nM), required to maintain a stable long-term continuous culture. The responses of D. brightwellii to Cu were monitored. (1) From 3 to 47 nM added Cu, decreasing pools of glutathione, increasing malondialdehyde contents, an increased release of lipophilic ligands, and cell lysis indicated the enhancement of lipid peroxidation. (2) From 47 to 94 nM Cu, a 16-fold increase in high-affinity (strong) hydrophilic ligands was measured (conditional stability constants K' approximately 10(12)) that complexed most Cu (maximum 97%); sexual reproduction was stimulated and cell volumes increased. (3) From 126 nM Cu, glutathione pools increased again, whereas cell division rates decreased slightly. (4) At 142 nM Cu, the number of lysed cells reached a maximum, as did the production of lipophilic compounds that complexed approximately 2% Cu. As the binding sites of the strong ligands became Cu-saturated above 142 nM Cu, larger amounts of Cu were bound to low-affinity (weak) dissolved ligands (3-30%) and cellular binding sites (0.2-2.5%). Probably due to saturation of organic complexes at 142 nM Cu, the MINEQL-calculated Cu2+ concentrations increased markedly; pCu values decreased from >11 to approximately 10; division rates were further inhibited; gamma-glutamylcysteine (phytochelatin precursor) was produced. (5) At 157 nM Cu, phytochelatin synthesis started, and Cu-sorption capacities (cell walls and internal binding sites) increased. (6) At 173 nM Cu, the phytochelatin pool sizes and the number of cellular Cu-binding sites increased further. These results suggest that ligands released by a dense bloom of D. brightwellii, either by active excretion or lysis, would have lower affinities for Cu (K' approximately 10(9)-10(12)) and moderate the availability of Cu less effectively than ligands in natural environments (10(13)-10(14)). In this diatom, the concurring release of ligands, enhanced malondialdehyde production, increasing numbers of presexual cells and cell enlargement may serve as early-warning signals for Cu toxicity, rather than metal-specific phytochelatins that appeared at a stage when cell division was already clearly inhibited.