"Milk" on the Baltic Sea

Oxygen-free zone in the depths of the Baltic Sea explored by scientific maiden voyage of Maria S. Merian

Maria S. Merian at sea © Arne Dübecke, IOW
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On her first scientific voyage, Maria S. Merian, the latest German research vessel, collected highly interesting data and images from the oxygen-free zones at the bottom of the deep basins of the Baltic Sea. The aim of the cruise was to explore the biogeochemical processes that take place from the surface of the water down to the uppermost layers of the seabed. The researchers were particularly interested in the two deepest points of the Baltic Sea, the Landsort low (to about 450 meters depth) and the Gotland low (to about 270 meters depth), since there is no oxygen at the seabed. They have taken spectacular shots of a milky layer, where the oxygen disappears. So far there are theories, but no final confirmation of the composition of this layer.

In the oxygen-free zones of the oceans, special microorganisms produce the hydrogen sulfide that is poisonous for life. The scientists' program aimed to explore how the microorganisms living above, on and below the seabed shape this habitat and respond to external influences. To do this, the scientists are researching which biological, physical and chemical processes control this system and how they influence each other. Samples taken at different depths help them to find out where the oxygen is being consumed by the microorganisms.

Targeting water samples

"We use the so-called pump CTD, which was built by the Institute for Baltic Sea Research in Warnemünde (IOW). She is constantly pumping water from the different depths directly into the laboratories on board. This allows us to see exactly where the various processes are taking place and how the biological, physical and chemical values ​​change, "explains geochemist Professor Michael Böttcher from the Leibniz Institute for Baltic Sea Research. "Particularly interesting are particles, small aggregates of microorganisms and other substances that slowly sink through the water. The interaction between solid material, microorganisms and surrounding water causes a series of processes that influence the composition of the sediments. "

Milky layer of elemental sulfur?

Samplers © Arne Dübecke, IOW

"When immersing in the redox clinker, where the oxygen disappears and the hydrogen sulfide begins to accumulate, the camera suddenly plunged into a milky layer several meters thick, " says Michael Böttcher of the event. "We are still puzzling over the cause of this unusual phenomenon. A hot candidate, however, are the tiniest twists of elemental sulfur, but the evidence is still pending.

The mineralogist Dr. Thomas Leipe from the IOW took a closer look at these particles from the water column and discovered something astonishing: manganese and iron, for example, react with oxygen and phosphate. For example, tens of microns of spider-like particles of manganese dioxide and iron phosphate are distributed in the water column. The first measurements with the scanning electron microscope show that they only occur at certain depths in the area of ​​the redox boundary layer. The role of these solid particles in biogeochemical processes will continue to occupy the scientists in the future. They want to be better able to explain how nutrients, metals, oxygen and microbial processes are distributed in the water and how to incorporate this knowledge into models. "Even the fact that new bacterial groups appear here already shows that we can still learn a lot from our data, " says a delighted marine biologist Professor Klaus J rgens from the IOW. display

Removal of organic material variable

Samples from the seafloor, where it penetrates the oxygen barrier, have also brought new insights: the white fluff of Beggiatoa bacteria that exists there also contains elemental sulfur. The oxidation product of hydrogen sulfide is found here but within the bacteria. In high resolution, including microsensors, the scientists studied the water that was in the pores of the deposits and modeled the results. How fast the organic material in Baltic Sea basins is degraded varies considerably, depending on how the sediments are put together and where they are in relation to the oxygen boundary layer.

Maria S. Merian Arne D becke, IOW

As many samples could have been examined during the exit in the modern laboratories of Maria S. Merian, most of the work now starts in the laboratories of the home institutes. We have collected hundreds of samples that can give us answers: answers to the question of how and why oxygen depletion occurs in the depths of the Baltic Sea and what microorganisms are involved in it. The depth distribution of a key organism, a bacterium which oxidizes hydrogen sulphide with the aid of nitrate, has already been analyzed by means of gene probes. Klaus J rgens, the leader of the voyage MSM01 / 1war.

Not only the scientists from the Leibniz Institute for Baltic Sea Research in Warnemünde, the University of Rostock, the Center for Oceanography in Southampton, UK, the Russian Shirchov Institute, and others are involved in the search for these answers the Bremen Max Planck Institute, who were on board, but also other employees of the respective institutes.

(Kirsten Achenbach, DFG Research Center Ocean Frontier, 16.06.2006 - AHE)