Characteristics of attachment and growth of Thiobacillus caldus on sulphide minerals: a chemotactic response to sulphur minerals?

To further our understanding of the ecological role of sulphur-oxidizing mi croorganisms in the generation of acid mine drainage (AMD), growth and atta chment of the chemoautotrophic sulphur-oxidizing bacterium, Thiobacillus ca ldus, on the sulphide minerals pyrite, marcasite and arsenopyrite was studi ed. Growth curves were estimated based on total cells detected in the syste m (in suspension and attached to mineral surfaces). In general, higher cell numbers were detected on surfaces than in suspension. Fluorescent in situ hybridizations to cells on surfaces at mid-log growth confirmed that cells on surfaces were metabolically active. Total cell (both surface and solutio n phase) generation times on pyrite and marcasite (both FeS2) were calculat ed to be approximate to 7 and 6 h respectively. When grown on pyrite (not m arcasite), the number of T. caldus cells in the solution phase decreased, w hile the total number of cells (both surface and solution) increased. Addit ionally, marcasite supported about three times more total cells (approximat e to 3 x 10(9)) than pyrite (approximate to 8 x 10(8)). This may be attribu ted to the dissolution rate of marcasite, which is twice that of pyrite. Ep ifluorescent and scanning electron microscopy (SEM) were used to analyse th e cell orientation on surfaces. Results of Fourier transform analysis of fl uorescent images confirmed that attachment to all three sulphides occurred in an oriented manner. Results from high-resolution SEM imaging showed that cell orientation coincides with dissolution pit edges and secondary sulphu r minerals that develop during dissolution. Preferential colonization of su rfaces relative to solution and oriented cell attachment on these sulphide surfaces suggest that T. caldus may chemotactically select the optimal site for chemoautotrophic growth on sulphur (i.e. the mineral surface).