Attachment of acidophilic bacteria to solid substrata
Bioleaching is the dissolution of metal sulfides such as pyrite (FeS2) by bacterial and archaeal oxidation-driven processes. Leaching microorganisms attach to mineral surfaces, thus enhancing metal sulfide dissolution. Bioleaching has two contrary aspects: The negative aspect of acid mine/rock drainage (AMD/ARD), which results in water pollution, and the positive aspect, the environmentally friendly application in industry. Consequently, there are two targets: the prevention of AMD/ARD and the optimization of bioleaching as industrial technology. In industrial applications such as in reactors, bioleaching is controlled and conditions are optimized for leaching microorganisms to achieve high oxidation rates. One important factor for the optimization is a high temperature. Moderately thermophilic leaching organisms are beneficial for such applications. The attachment and biofilm formation and the physiology of pure and mixed cultures of moderately thermophilic bacteria were investigated in this thesis. Leaching activities of the metal sulfide and interactions in between the consortium were of interest. Attachment and leaching experiments under moderately thermophilic conditions indicated that Leptospirillum ferriphilum is the first colonizer of a pyrite surface. Furthermore, it was shown that mixed cultures are more effective than pure cultures with attachment and leaching. Precolonization tests exhibited that Acidithiobacillus caldus needs precolonization of active L. ferriphilum cells to establish itself in a biofilm on metal sulfide surfaces. Quorum sensing or chemotaxis related effects were determined because dead L. ferriphilum cells and their EPS residues did not increase attachment of At. caldus cells to the metal sulfide. However, active L. ferriphilum cells had an increasing effect on attachment of At. caldus cells to pyrite. Further experiments indicated that attachment and biofilm formation of L. ferriphilum and At. caldus are influenced by addition of certain N-acetyl-homoserine-lactones (AHLs). Especially, pyrite leaching with cells of L. ferriphilum was strongly inhibited by addition of 3-hydroxy-C14-AHL. Consequently, pyrite leaching by L. ferriphilum cells becomes manipulable. Biofilm formation of At. caldus on pyrite coupons was increased by the use of the C8-AHL family. L. ferriphilum and At. caldus do not produce AHLs on their own but they were able to respond to an external addition of AHLs. The addition of (5Z)-4-bromo-5-(bromomethylene)2(5H)-furanone inhibited growth, biofilm formation and leaching of L. ferriphilum, At. caldus and Acidimicrobium ferrooxidans. The inhibition is a reversible process. Extracts from supernatants of pyrite-grown L. ferriphilum cells were tested in growth experiments with other species. L. ferriphilum supernatants had a strongly inhibiting effect on iron-oxidation in several microorganisms such Leptospirillum spp., At. ferrooxidans, Acidimicrobium ferrooxidans, Acidithiobacillus ferrivorans and Sulfobacillus thermosulfidooxidans. Sulfur oxidation was not affected by the addition of the “L. ferriphilum- extract”. The inhibitory effect on iron-oxidation was also caused by an addition of extracts from three other Leptospirillum strains. A novel QS- related autoinducer molecule was detected in cultures of L. ferriphilum. The utilization of a novel Janthinobacterium based biosensor test indicated its presence. It is highly likely that this autoinducer is a derivative of a α-hydroxyketones. Bioinformatic studies indicated that genomes of Leptospirillium spp. include a homologous gene for the enzyme, 8-amino-7-oxononanoate synthase, of the jqsA gene of the Janthinobacterium. Whether the inhibiting effect on iron oxidation can be attributed to the unknown autoinducer(s), still needs to be demonstrated. Nevertheless, the data indicated that a cross-communication via AHLs and by the unknown autoinducer(s) occurs in moderately thermophilic bioleaching bacteria.