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A team led by microbiologist Heide Schulz-Vogt was able to show that conspicuous phosphorus anomalies in the Black Sea can be attributed to the fascinating abilities of certain large bacteria. Until now, the phenomenon could not been explained by the scientific community. In an article published in The ISME Journal, the authors now show that so-called magnetotactic bacteria, which are capable of accumulating polyphosphate and can migrate in a directed manner within the water column thanks to their magnetic properties, are the main causes of phosphate displacements. In this way, they help to control the phosphate content in surface water.
The Black Sea is an ideal place to study the effects of oxygen deficiency on marine matter cycles, as here, the boundary between the two worlds "oxic” and "non-oxic” (anoxic) is very stable. Moreover, in contrast to many other sea areas with oxygen deficiency, a water body several decimetres thick underlies the oxic layer, which no longer contains oxygen but is still free of H2S. Processes that occur after the depletion of oxygen can be studied in this suboxic zone in high resolution.
It was discovered as early as the mid-1980s that this suboxic zone appears to have a major influence on the distribution of phosphorus in the water: With the onset of the oxygen deficiency, i.e. at the upper limit of the suboxic zone, the concentrations of dissolved phosphate reach their minimum, while at the lower end, where H2S spreads, a pronounced maximum occurs. Between the two extremes, a significant accumulation of particulate phosphorus can always be detected.
For no other nutrient a comparable distribution can be determined, which is why the phenomenon was referred to in the scientific literature as a "phosphorus anomaly". A first explanation, according to which the dissolved phosphates are included in the precipitation of iron oxides in the uppermost region of the suboxic zone and released again when these compounds dissolve in contact with H2S, could explain the phenomenon only insufficiently. Further processes had to exist to trigger such depletion and accumulation.
Heide Schulz-Vogt, head of the Biological Oceanography department at the IOW, and her colleagues from Warnemünde now report in the international journal The ISME Journal on a process that can comprehensively explain the phosphorus anomaly. They postulate that large magnetotactic bacteria, such as Magnetococcus, store phosphates in the form of polyphosphates at the beginning of the oxygen deficiency zone and release them again in contact with H2S, i.e. at the lower boundary of this zone. Heide Schulz-Vogt has long been interested in the fascinating world of large bacteria: "When we refer to large bacteria, we are talking about 5 µm - only one twentieth the diameter of a hair, but at least 5 times the size of normal bacteria". Such a difference in size enables the large bacteria to absorb 125 times more polyphosphates than their small relatives. "We were able to detect Magnetococcus in the respective zone and were also able to show by gene expression in the area of the phosphate maximum that polyphosphates are degraded by different bacterial groups," said Heide Schulz-Vogt explaining the methodological approaches of the detection. However, the decisive indication came via the scanning electron microscope, the SEM: "At a 10,000-fold magnification of phosphor particles from the suboxic zone, we were able to recognize chain structures as they are typical for magnetotactic bacteria. Only then did we know what to look for."
Magnetotactic bacteria are highly mobile. Their embedded magnetosomes help them to orient themselves along the earth's magnetic field. This enables them to swing back and forth between "top" and "bottom" and build up an effective phosphate shuttle.
This bacterial transport of phosphates into deeper water layers keeps these nutrients away from the productive zone and prevents them from triggering cyanobacterial blossoms, for example. The role this process plays in the Baltic Sea, where cyanobacteria blooms occur every summer in the form of huge carpets that dramatically increase oxygen consumption in deep water after their death, needs further investigation. However, there is also evidence of such a bacterial phosphorus shuttle in the Baltic Sea.
IOW is a member of the Leibniz Association with currently 95 research institutes and scientific infrastructure facilities. The focus of the Leibniz Institutes ranges from natural, engineering and environmental sciences to economic, social and space sciences as well as to the humanities. The institutes are jointly financed at the state and national levels. The Leibniz Institutes employ a total of 19.100 people, of whom 9.900 are scientists. The total budget of the institutes is 1.9 billion Euros. (www.leibniz-association.eu)
Prof. Dr. Heide N. Schulz-Vogt | phone: +49 381 – 5197 200 | heide.schulz-vogt@io-warnemuende.de, Department Biological Oceanography, Leibniz Institute for Baltic Sea Research Warnemünde
Schulz-Vogt, H. N., Pollehne, F., Jürgens, K., Arz, H. Beier, S., Bahlo, R., Dellwig, O., Henkel, J. V., Herlemann, D. P. R., Krüger, S., Leipe, T., Schott, T (2019): Effect of large magnetotactic bacteria with polyphosphate inclusions on the phosphate profile of the suboxic zone in the Black Sea. The ISME Journal online first, https://doi.org/10.1038/s41396-018-0315-6
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