Interactions between A-toxicity and siderophore production of fungi

Al and Fe are the third and fourth most abundant elements respectively in the earth crust. However, their availability is extremely low under normal environmental conditions, which is, from the point of organisms, a benefit what the former and a handicap what the latter is concerned. Ionic radii of the trivalent cations AlIII and FeIII are quite close, and thus these two metals may substitute for one another. Although several investigations found connections between Al and Fe metabolisms, only very few studies (always conducted with bacteria) pursued this matter further and included siderophores into their considerations. Several investigations - mainly dealing with structural aspects - found stable complexes of siderophores on the one hand and metals like Al, Ga, Cu and Mo on the other hand. Although there is clear evidence for the connection between Al-toxicity and siderophores, not a single systematic investigation including several siderophore types exist. Both free living and mycorrhizal fungi play a central role in all soil systems. Nevertheless, in the anyway few investigations, fungi have been completely neglected in the above mentioned connection although a lot of them are known to be potent siderophore producing organisms. This leads to a number of questions such as 1. Is quality and/or quantity of fungal siderophore production influenced by Al-availability? 2. Are toxic effects of Al towards fungi influenced by the availability and production of siderophores? 3. Is Al taken up into microbial cells via the uptake systems designed for ferri-siderophores? For investigating the above mentioned questions we will use the organisms Penicillium sp. (for production of ferrichrome), Neurospora crassa (for coprogene), Aspergillus nidulans. (fusarine) and Rhizopus microsporus (rhizoferrine) which guarantees that all important siderophore types are involved in the investigation. An array of methods for the investigation of fungal growth and physiology, of siderophore production and of bio-metallurgical questions has been established in our laboratory and will be applied to investigate the above questions.

Iron is for all organisms (except for lactobacilli) essential whereas Al is usually classed in the small group of elements with solely negative effects towards all forms of life. Several effects of Al towards micro-organisms (antimicrobial liniments), plants (forest decline), animals (sudden fish death in acidic lakes) and man (several neurodegenerative aging processes) are well known since decades. However, although a couple of toxicity mechanisms were established it is still not clear how (if at all) the trivalent Al-ion passes the plasma membrane and arrives in the cell. Although it is well known that Al3+ and Fe3+ may substitute for one another for their similar ion-diameters and although this is also true for siderophores, which are iron transporting substances of micro- organisms and plants, not a single investigation is available dealing with fungal siderophores and Al-toxicity. We chose fungi for the present investigation because i) there is a striking lack of investigations regarding this group of organisms ii) fungi play a central role in various eco-systems (e.g. when forming mycorrhiza) and iii) fungi are eukaryotes and have therefore more similarities with higher plants and animals than prokaryotic bacteria have. Three fungi (Aspergillus nidulans, Neurospora crassa and Hymenoscyphus ericae) and several mutants of A. nidulans were used so that all important groups of fungal siderophores were engaged in the study. In the coarse of the now finished project we were able to show, that - siderophores are actually mobilizing Al, even in soil, the redox potential Eh is distinctly decreased in suspensions of microbial cultures within several days of incubation. As at low Eh the ratio of Fe2+/Fe 3+ increases the necessity for a high affinity Fe3+-uptake system should decreases, various, often species-dependent interactions between Al-availability on the one hand and Fe-metabolism on the other hand are significant, Al inhibits the production of extracellular siderophores - an effect which is significant even at low Al- concentrations which do not inhibit microbial growth per se, these extracellular siderophores amplify negative effects of Al (probably through an enhanced transport towards or into the microbial cell), negative effects of Al can be ameliorated by inhibiting the synthesis of extracellular siderophores, intracellular siderophores (e.g. ferricrocin) decrease toxic effects of Al, Al-effects are caused by Al per se and not by the accompanying lowered pH (Al 3+ acts as a cationic acid), storage and thus intracellular Al-contents depend to a great extend on intracellular siderophores.