The aim of this thesis was to further the knowledge on microbial weathering, by looking at the effect of model organisms (Acidithiobacillus ferrooxidans and Geobacter metallireducens) to natural microbial communities (from basaltic glass). It was found that the medium water-rock ratio (50:1) provided the optimum conditions for A. ferrooxidans growth, whilst the low water-rock ratio (1:1) had a lower release of iron because of pH. The pH affected the release of iron and REEs, with less released the higher the pH. In addition, it was found that, though localised areas of hematite were found on the treated rocks, there were also oxidised layers that did not correspond to specific mineralogy. The lack of specific mineral signatures on the rock surface, but the apparent oxidation of the surface, suggested that the surface had been passivated with Fe3+ binding to the mineral surface. In contrast, G. metallireducens did not affect the production of Fe2+ from basalt glass when compared to controls. However, when low water-rock ratios and hematite were tested, a difference was observed between abiotic and biotic flasks. It was suggested the low water-rock ratio possibly allowed G. metallireducens to obtain the iron more easily by affecting the pH of the solution which in turn affected the stability of the bound iron. In terms of studying microbial communities on rocks, it was found that community structure in Icelandic basaltic glass changed over time, becoming more diverse, with a switch from r- to K-selected microorganisms over the course of the year, similarly to results obtained in the field. DGGE results showed each flask had a distinctive population - with no correlation between ratios, and replicates different in composition to each other. It is suggested that, though community does change over time (as shown by the clone libraries), the ratios do not have an effect and each flask is developing with its own ‘microbial island’. However, the results of the chemistry of the flask solutions indicated that the biological experiments showed differences in pH and elemental release between ratios. Elemental release rates were faster in the biological experiments. The natural communities affected mineral dissolution, possibly through the release of organic acids, which would also account for the drop in pHs observed in the biological experiments. It was noted that there were differences in dissolution rates between the results reported in this thesis and previous literature. It is suggested that these are caused by the rock surface area as in previous studies the rocks have typically been crushed into powder and fine particles. This crushed powder would have provided fresh rock surface for the microorganisms and also greater surface area for reactions to take place, accounting for generally higher weathering rates in previous literature per unit weight of material.