Metallic environmental mops

Inductively coupled plasma spectroscopy and mass spectrometry revealed that natural metal-chelating agents, so-called siderophores, which are produced by bacteria and plants, make much better absorbers of cadmium, and potentially other toxic metals, than synthetic EDTA, and so could improve the efficiency of phytoremediation systems.

C.O. Dimkpa and E. Kothe of the department of Microbial Phytopathology, at the Friedrich Schiller University, in Jena, Germany, working with D. Merten and G. Büchel in the department of Applied Geology, and A. Svatos of the Mass Spectrometry Research Group, Max Planck Institute for Chemical Ecology, Jena, Germany, point out just how problematic is cadmium in environmental bioremediation.

"As a toxic metal, cadmium (Cd) affects microbial and plant metabolic processes, thereby potentially reducing the efficiency of microbe or plant-mediated remediation of Cd-polluted soil," the team explains. They have investigated the role of siderophores produced by the soil microbe Streptomyces tendae F4 in the uptake of cadmium by bacteria and plants.

The study reveals important information that could help improve bioremediation efforts that utilise plants, such as the sunflower (Helianthus annuus) to soak up cadmium from contaminated land. The study shows that the natural siderophores are more effective than their synthetic equivalents, chelating compounds, such as EDTA, ethylene diammine tetra-acetic acid.

In order to test their premise, the team cultured the bacterium under conditions that would induce it to produce high levels of siderophores. Bacteria commonly use siderophores to encapsulate toxic metals in their environs and so protect themselves, but also as a means to fish for nutrients in the medium in which they grow, whether natural or in a Petri dish in the microbiology lab.

The team studied the kinetics of siderophore production and identified the various siderophores produced using electrospray ionization mass spectrometry. ESI-MS also allowed them to identify the metal-bound forms of any siderophores.

The researchers then turned to inductively coupled plasma spectroscopy to measure iron (Fe) and Cd content in the bacterium. The same technique would also allow them to determine cadmium levels in sunflower plants grown in soil contaminated with the toxic metal.

They found that the presence of siderophores significantly reduced cadmium uptake by the bacterium, but simultaneously supplied it with nutrient iron. Their tests also showed that bacterial culture filtrates containing three hydroxamate siderophores secreted by S. tendae F4 also promoted sunflower plant growth significantly and enhanced uptake of both iron and cadmium by the plant, compared with control plants.

The team adds that application of the siderophores to sunflowers also improved cadmium uptake slightly relative to EDTA used as an alternative, but iron uptake was similar in both cases. As such, the team explains that their study shows that microbial processes can indirectly influence the availability and amount of toxic metals taken up from the rhizosphere of plants.

Moreover, they suggest that chelator-enhanced phytoremediation might be improved by the use of cultured microbial siderophores rather than the application of EDTA. Microbial siderophore production being less costly and relatively sustainable compared to synthetic EDTA production, would be "ideal for this purpose," especially as unlike EDTA it is degradable and does not cause transportation by leaching of the chelated metal ions to distant soil, they add.
The next step is to further investigate the effect of increased siderophore-
expressing microbial strains, and to test the siderophores on different metals and with different metal-accumulating plant species.