The contribution of cyanobacteria to the Baltic Sea mercury emission (Hg-Cyano)

The Baltic Sea shows a maximum of elemental mercury (Hg⁰) release in mid-summer. It is investigated, if the transformation of ionic mercury to volatile Hg⁰ is actively done by cyanobacteria or if it is the result of photochemical reactions. The study comprises in situ measurements, culture experiments and microbiological analyses.

Toxic mercury affects human health and ecosystems, thus it is important to understand and quantify Hg transport and transformation processes in the environment. In a preceding project that investigated the Baltic Sea as a source for atmospheric mercury, clear seasonality of the elemental mercury (Hg⁰) emission was found (Kuss and Schneider, 2007). In winter there was no significant emission, some areas even became a weak sink for atmospheric Hg⁰. In spring and autumn elevated emission was determined, and the maximum was measured in mid-summer at intense solar radiation and widespread occurrence of cyanobacteria. Various Lab studied showed that the potential biotic and abiotic transformation pathways depend on light. In the new project it is studied, if light directly causes the transformation as a photochemical process mediated by dissolved organic carbon, or if light mainly controls biological processes that in turn cause the transformation. This is investigated by three approaches:

Some algae and bacteria seem to posses the genetic pre-condition to transform toxic ionic mercury to volatile and less toxic Hg⁰. A hypothesis would be an improved survival strategy during the evolution for organisms that show this so called mercury resistance. Increased volcanic activity during early stages of life could have caused enhanced natural mercury exposure, revealing an advantage for organisms that were able to transform and excrete toxic mercury.
-> The genetic pre-condition of mercury resistance of cyanobacteria is investigated by microbiological analyses. Moreover it will be figured out which cyanobacteria show an active MerA gene, indicating that the transformation was really done.
During two research cruises in summer 2011/2012 surface water will be sampled for ionic mercury and dissolved organic carbon analyses at IOW. Elemental mercury will be measured quasi-continuously on-board. Parallel, the composition of the plankton community is studied with special emphasis on cyanobacteria. However, for detailed species determination and counting samples are taken and preserved that are analyzed at the IOW.
For comparison deeper water layers are also analysed with regard to plankton composition, light supply, and Hg⁰ concentration. Reduced light intensity in combination with enhanced nutrient supply could favour biotic transformation by cyanobacteria in the layer between about 10 and 20 m depth.
-> Distribution pattern of Cyanobacteria, dissolved organic carbon, and light supply are statistically analysed with the concentration distribution of Hg^0.
Another central part of the investigations are incubation experiments of ambient seawater with its natural plankton community. Some samples are treated as close as possible to the natural environment, for others certain control factors are excluded. Therefore quartz glass bottles are half filled with seawater, submerged in open vessels and exposed to natural light conditions. Parallel experiments are aimed as dark control or are based on sterilized water and otherwise subjected to the same conditions. Three times a day the amount of released Hg⁰ is measured in the headspace of the bottle.
->This is used to quantify the contributions of biogenic and of the photochemical mercury transformation.

Dr. Joachim Kuss (Projekt manager) with contributions of other groups:
Dr. Matthias Labrenz , Dr. Norbert Wasmund, Dr. Christa Pohl, Dr. Klaus Nagel, Dr. Monika Nausch, Dr. Herbert Siegel und Siegfried Krüger.

References:
Kuss, J. and Schneider, B., 2007. Variability of the gaseous elemental mercury sea-air flux of the Baltic Sea. Environmental Science and Technology, 41(23): 8018–8023.