02.08.: Bacteria as ecosystem engineers

Bacteria as ecosystem engineers - important role of microorganisms for natural phosphorus removal in the oceans

Phosphorus is an essential nutrient for all marine organisms. High concentrations of phosphorus disturb the balance of marine ecosystems and have been identified as the main culprit for coastal eutrophication.

To remove phosphorus permanently from oxygen-free seawater has proven difficult. Of critical importance is to understand the process of apatite formation, a calcium phosphate mineral that is the only stable inorganic form of phosphorus in oxygen-free sediment and water.

In a recent publication in the journal Nature Geoscience, Tobias Goldhammer, Volker Brüchert, and colleagues Tim Ferdelman and Matthias Zabel report on a new process in the apatite riddle – bacterial removal of phosphorus and apatite formation catalyzed by bacteria. In their study, they used sediment from the Benguela upwelling system off Namibia. Why this sediment? Sediments off Namibia form in permanently anoxic waters in one of the most productive ecosystems on Earth – a naturally eutrophied marine ecosystem. These sediments also host modern deposits of phosphorites - large-scale apatite deposits at the seafloor. So why does apatite form here? Earlier a Bremen colleague, Prof. Heide Schulz, discovered giant sulfur oxidizing bacteria, known as Thiomargarita namibiensis, in these sediments. She further proposed that these bacteria also stored phosphate and might be intimately tied to apatite formation there.

Using a radioactive isotope of phosphorus, phosphorus-33, as a tracer in a series of experiments, Goldhammer and his colleagues were able to show that apatite formation is indeed channelled through hydrogen sulphide-oxidizing bacteria. This process occurs in two steps - first storage of phosphorus in bacterial cells as polyphosphate and secondly, sequestration as apatite and organic phosphorus.

This mechanism was tested in the presence and absence of oxygen, and in sterilized sediment. Apatite formation was only possible when living bacteria were present.

How efficient is this process? First results suggest that the rate of phosphorus sequestration by the bacteria is comparable to the rates by which phosphorus is deposited on the seafloor bound to plankton.

”This finding is particularly encouraging because it suggests that the anoxic conditions did not enhance phosphorus dissolution. It could also indicate that we have identified an important natural negative feedback process that prevents run-away eutrophication in coastal marine ecosystems”, says Volker Brüchert, Associate Professor for Biogeochemistry in the Department of Geological Sciences at Stockholm University. “Moreover, it is a striking example how microorganisms act as ecosystem engineers, keeping nutrient cycling in the oceans in balance”, adds Tobias Goldhammer, Researcher at MARUM Center for Marine Environmental Sciences, Bremen.

Could this process also exist in the Baltic Sea? This is very well possible because close relatives to the bacteria identified off Namibia, so-called Beggiatoa bacteria, are also known to be common in Baltic Sea sediment. "Our next studies are going to show whether this process also operates in the anoxic waters of the Baltic", says Volker Brüchert.


The report of which we refer to is published online in Nature Geoscience 18th July:
Goldhammer, T, Brüchert, V, Ferdelman, T.G., and Zabel, M (2010) Microbial sequestration of phosphorus in anoxic upwelling sediments. Nature Geoscience, DOI: 10.1038/NGEO913
 

Further information / Requests for interviews / Photos:

Jana Stone
MARUM Public Relations
Tel.: 0421 218 65541
E-mail:
www.marum.de

Dr. Tobias Goldhammer
Marine Geochemistry
MARUM Bremen
Tel.: 0421 218 65116
E-mail:

Dr. Volker Brüchert
Dept. of Geological Sciences
University of Stockholm
Tel.: 0046 8164755
E-mail:

In the experiment the phosphate mineral apatite is formed in the presence of the sulphur bacteria Thiomargarita. In the mineral the radioactive marked phosphate molecules can be found, which were incorporated by the bacteria before. (Source: modified after "Ingall, E. (2010) Phosphorus burial. Nature Geoscience 3, 521-522")