The activities of mineral-oxidising microorganisms play a crucial role in global
mineral cycles, and are of huge importance to the mining industry. Their
activities in situ often result in the production of acid mine drainage (AMO),
with dire environmental consequences. While the microbial populations of
AMO itself are relatively well studied, very little is known about the microbial
populations of solid-phase mine wastes and products. Increased
understanding of these populations will allow greater .understanding of
pollution genesis in mine wastes, such as the longevity of the problem. It may
also be possible to identify novel organisms that may be important for the
biotechnological processing of mineral ores, while increasing the
understanding of microbial interactions therein. Therefore, the microbial
populations of material from seven mine sites located in the U.K., mainland
Europe and the U.S.A. were investigated using culture-based and
biomolecular methods. Samples were obtained from heaps at five metal
mines and one coal mine. The heaps varied greatly in terms of their
mineralogies and ages since deposition. There was considerable variation in
the microbial populations between all the materials analysed. The simplest
population was observed in water droplets forming on the surface of pyritic
rock in an abandoned sulfur mine. This was dominated by Acidithiobacillus
ferrooxidans, while Leptospirillum spp. were the only other microorganisms
detected. The microbial populations of spoil and tailings from an abandoned
Portuguese mine were restricted to a group of iron-oxidising 11 Firmicutes11 and the sulfur-oxidiser, At. thiooxidans. I ran-oxidising 11 Firmicutes11 also dominated a currently inactive pilot-scale heap bioleaching operation at a copper mine in Utah, suggesting that this group of organisms are important in mineral heaps in semi-arid zones. Enrichment cultures from both the Portuguese tailings and the U.S. bioleach heap were capable of extensive sulfide mineral oxidation.
The majority of organisms detected at the remaining four sites could not be
cultured with the media used, and therefore, in many cases, their metabolic
activities could not be ascertained. However, a novel iron-oxidising bacterium
was isolated from the oldest site investigated, and was characterised in detail.
This was found to be a member of the Rubrobacteridae subclass of the
Actinobacteria, which has not previously been found to include any iron- or
sulfur-oxidising species. Mineral-oxidising Bacteria were detected in samples
from all seven mine sites, although Acidithiobacillus was the only genus to be
detected at every site. Microbial biodiversity varied greatly between the sites,
and appeared to be primarily dependent on pH, although deposit age was
also a factor. Biodiversity was enhanced where mine wastes had been
landscaped and planted with trees as part of a remediation process. However,
this did not appear to restrict the amount of mineral oxidation, and therefore
potential genesis of acid mine drainage waters; geochemical data implied that
sulfide mineral oxidation was on-going, even in mine wastes that had been
deposited at least 120 years ago. The data indicated that mineral oxidation
and hence the environmental hazard posed by mine wastes can be a very
long-term phenomenon. Further work should be undertaken to overcome
problems associated with sample processing, to sample a greater number and
diversity of sites and to further characterise novel and potentially useful
organisms isolated in this study.