To remove phosporus 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 Mathias Zabel report on a new process in the apatite riddle – bacterial removal of phosphorus from seawater 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 – lage-scale apatite deposits at the seafloor. So why does apatite form here? Earlier a colleague, Heide Schulz, discovered giant sulphide-oxidizing bacteria, known as Thiomargarita, in these sediments. She was the first to propose 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.
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 was 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