Chelators and Siderophores

The bioavailability of metals can be altered by forming metal complexes with chelating molecules (or minerals, as discussed in the next section). The excretion of chelating substances has been described for bacteria and fungi, with the most active components being siderophores that are involved in iron acquisition (Crowley et al. 1984). Under oxic conditions, iron is only minimally bioavailable, and the very low solubility of Fe3+ hinders the uptake of iron required for electron transport chain components and in active centers of redox-active enzymes. This is especially true of aerobic, soil-dwelling organisms, including plant roots. While plants mainly acquire iron through the acidification of the rhizosphere and the excretion of organic acids such as oxalate, citrate or malate, bacteria apply another strategy. They excrete low molecular weight, high iron binding affinity substances - siderophores, which are delivered and in many cases taken up after iron loading. Many different siderophores have been described for both Gram-negative and Gram-positive bacteria. Hydroxamate siderophore production was verified in Streptomyces acidiscabies E13, a nickel-resistant isolate from the field test site (Dimkpa et al. 2008). The three different hydroxamate-type siderophores can be produced irrespective of the presence of nickel in the cultures, showing that their respective production mechanisms are not influenced by metals other than iron. The effects of the siderophores on the bioavailabilities of both iron and nickel were shown in plant experiments, where the addition of filtrates of hydroxamate-producing cultures of S. acidiscabies E13

had a plant growth promoting effect on cowpea seedlings (Dimkpa et al. 2008). The three hydroxamates found in the culture filtrates are the desferroxamines DFOE (with a circular structure) and DFOB (with an open structure), as well as coelichelin, which has an open structure with even less rigidity in the backbone. In accordance with these structural predispositions, the amount of siderophore bound to iron versus that bound to nickel varied among the three molecular types of siderophore, and with time. Similar amounts of iron and nickel were found to bind to the siderophores in the overwhelming presence of nickel, the production of all three siderophores was possible at the same time, and the production of all three siderophores changed over time (Dimkpa et al. 2008). While DFOE predominantly chelated nickel, DFOB showed a preference for iron, especially at the peak in production, while large portions of coelichelin remained unchelated, even though nickel was present at 2 mmol L-1 in the experiment while iron was limiting. Thus, the excretion of siderophores leads to nickel complexation in the surroundings of the bacterial hyphae, and while nickel is chelated, iron is still solubilized, aiding iron reduction and uptake into the cell. The siderophore thus plays a dual role in releasing nickel stress and allowing sufficient iron to be taken up for biosynthesis and metabolism.

Another observation made about the heavy metal resistant strains isolated was that many of them produced a dark pigment. Soluble brownish pigments are known to be formed by many streptomycetes, and it has been suggested that these melanin-like pigments are able to sequester metals from the vicinity of the growing cell. Indeed, cultures grown in media with increasing amounts of added nickel were darker, indicating the increased production of the melanin-like pigment (Fig. 10.2). In order to address these questions, mutant strains were derived from a UV-directed mutagenesis. White mutants were obtained, but did not show an unequivocally lower nickel resistance. It can thus be concluded that nickel sequestration is not driven mainly by melanin excretion, although the substance may still have a slight protective effect. Such an effect could also be due to the release of ROS stress, as melanin is also known to scavenge oxygen radicals through the

Fig. 10.2 Increased production of a melanin-like brown pigment under nickel stress by S. acidiscabies E13

radical-assisted formation of higher molecular weight melanins. The role of the siderophores clearly exceeded the potential function of the melanins in protecting from nickel stress.

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