Snow in the ocean
The absorption of CO2 by the sea is the focus of intense research
By Helle Ploug
Helle Ploug (Fotos: privat).
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It is well known that the CO2 concentration in the earth's atmosphere is rising. The ocean covers about 70 per cent of the earth's surface and its potential to absorb CO2 from the atmosphere is a topic of intense research. One of the mechanisms by which atmospheric CO2 is drawn down by the ocean and transported to the deep sea is referred to as the "biological carbon pump": In the upper ocean CO2 is transformed into organic matter in the light by growing phytoplankton (microscopic plants of the ocean). Single phytoplankton cells sink very slowly. Phytoplankton debris, living phytoplankton and bacteria, fecal matter, and miscellaneous detritus, however, stick together and form fast-sinking macroscopic aggregates upon wind-driven collision in the upper ocean.
Such aggregates often occur at densities of about 10,000 per m3 of sea water. They are referred to as "marine snow" because these flakes sinking through the water column very much resemble falling snow. Marine snow sinks through the water column at speeds of up to several hundred meters per day, and part of the CO2 fixed in these aggregates thereby sinks into the ocean's interior where it may become isolated from the atmosphere for millennia. In the surface waters of the ocean, however, marine snow is quickly colonized by bacteria. Some bacteria are motile and can chemically sense marine snow. They swim towards the aggregates and colonize and exploit them as a nutrient and carbon source in the ocean. Bacteria solubilize, assimilate and respire the organic matter and a large fraction of the CO2 originally fixed by the phytoplankton is thus respired back and released as CO2 by bacteria. How much and how fast bacteria degrade and respire marine snow relative to the "time frame" set by sedimentation of these particles into the ocean's interior is one of the central themes of my research.
In the research group at the Max Planck Institute in Bremen microsensors with tip sizes as small as the size of a single bacterium are being developed.
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During my Humboldt Fellowship I am working in the Microsensor Group at the Max Planck Institute for Marine Microbiology in Bremen. Here, electrochemical microsensors with tip sizes as small as the size of a single bacterium are being developed. The chemical microenvironment of marine snow is often distinctly different from that of the surrounding sea water. Using microsensors, it is possible to directly measure bacterial metabolic processes and growth conditions within marine snow. These measurements are done in a flow system in which the marine snow aggregate is suspended by an upward-directed flow balancing its sinking velocity (see photo). This hydrodynamic condition is similar to that of a marine snow aggregate sinking through the water column. Using different staining and molecular techniques, the bacteria are visualized for enumeration and characterization afterwards.
At the Max Planck Institute for Marine Microbiology, I enjoy the possibility to learn and use new technologies as well as the international atmosphere and the pleasure to work with people with different educational and cultural backgrounds. From this institute I am collaborating with colleagues from the University of Oldenburg, the Max Planck Institute for Biogeochemistry in Jena, the Institut für Gewässer und Binnenfischerei in Neuglobsow, and the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven as well as with colleagues from Denmark and the USA.
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