A potential role for bacterial endophytes in phytoremediation of heavy metal-contaminated soils (Thlaspi caerulescens, Brassica napus, Burkholderia cepacia, Hergaspirillum seropedicae)

This thesis aims to study the potential contribution of heavy metal resistant endophytic bacteria on heavy metal uptake and translocation processes by their host plant. The extensive study of bacterial heavy metal resistance mechanisms showed that metal bioprecipitation processes near the bacterial cell wall are often found to be a consequence of efflux based systems that reduce the bioavailable metal concentration present. Heavy metal resistant endophytic bacteria with a similar behaviour would possibly result in a lowered heavy metal bioavailability that consequently would lead to a lowered heavy metal toxicity or an improved heavy metal accumulation within the host plant. Based on this concept, new strategies to improve phytoremediation would be developed.; In a first step the presence of heavy metal resistant bacteria inhabiting the Zn hyperaccumulator Thlaspi caerulescens subsp. calaminaria growing on a soil from an abandoned Zn and Pb mining and smelter site as well as the non-hyperaccumulator Brassica napus originating from a clean garden soil was demonstrated. The occurrence of such bacterial heavy metal resistance characteristics obviously is not restricted to the heavy metal polluted environments. An efficient re-colonization of the host plant by these plant associated bacteria is an important prerequisite to allow an evaluation of the potential effect on the metal household of the plant. Re-inoculation experiments revealed that this condition was not fulfilled. For this reason, the metal sensitive endophytic bacteria Burkholderia cepacia L.S.2.4 and Herbaspirillum seropedicae LMG2284 were subsequently selected as they proved to be efficient colonizers of their respective host plant Lupinus luteus and Lolium perenne .; By means of transposition, the Ni, Co, and Cd resistance mechanism ncc-nre of Ralstonia metallidurans 31 A was introduced which resulted in stable B. cepacia L.S.2.4::ncc-nre and H. seropedicae LMG2284::ncc-nre transconjugants. An increased Ni resistance was observed accompanied by the ability to reduce bioavailable Ni levels in the growth medium up to 35% and 15% respectively. Re-colonization experiments with B. cepacia L.S.2.4::ncc-nre revealed that under the circumstances used, significant increased Ni levels (30%) actually were measured in the lupinus roots. In contrast, inoculation with the wild type strain L.S.2.4 didn't influence the concentration of Ni measured in the roots compared to the non-inoculated control plants. On the other hand, the presence of H. seropedicae LMG2284::ncc-nre, equipped with limited Ni removal capacities, didn't seem to affect the Ni uptake capacity of Lolium perenne. Efficient colonization as well as efficient Ni removal properties probably are essentially required in order to detect a clear effect.; To obtain a better understanding of the interactions between endophytic bacteria and their host plant, additional information concerning the mechanisms of plant colonization on one hand and the plants' metal household from uptake to translocation and storage on the other hand certainly would be necessary in order to interpret these results in a broader context. Additionally, in situ experiments remain to be performed before practical applications of these plant-bacterium interactions can be evaluated.