In this article we will discuss about the microbes that are involved in leaching.
Algae and oligotrophous nonspore forming bacteria belonging to the genera Pseudomonas, Corynebacterium and Arthrobacter predominate in the rocks. Many of them are capable of fixing atmospheric nitrogen. Rocks disintegrate mainly along cracks and at the surface.
Accumulation of organic substances, leaching of elements and formation of clayey minerals occur through weathering of rock-crust which are accompanied by large deposits of bauxites, kaolinite clays and nickel, are more complex land microenvironments.
Their formation is related to the degradation of original rocks under certain oxidative/reductive conditions. The weathering crusts are characterised by general distribution of autotrophic bacteria which oxidise reduced compounds of nitrogen, sulphur and iron.
Nonspore forming bacteria prevail among heterotrophs, particularly belonging to the genera Arthrobacter, Corynebacterium and Mycobacterium in a pure culture which assimilate phenols, ethanol and other alcohols besides growing on nitrogen-poor rocks.
The microflora of sedimentary rocks in the zone of hypergenesis is more diverse and the geochemical activity of various bacterial groups depends upon the reduction-oxidation conditions of the medium.
It is interesting to note that microbes are involved not only in the leaching and migration of elements but also in the formation of minerals under present conditions (Table 33.2). The second source of organic matter for heterotrophs in rocks is metabolites of photo- and chemolithotrophs.
Algae and thiobacilli are known to liberate 5-50 and 20-50% of fixed carbon, respectively, as organic compounds into the medium. A possibility of growth and biomass accumulation of heterotrophs in cultures of thiobacilli and nitrifying bacteria has been proved beneficial. Hence, it is obvious that rocks contain organic substances at concentrations which considerably exceed those required for the beginning of growth of microbial cenoses.
Metal ores are present in various forms in nature. Rocks are the rich source of various metals. Ore deposits are characterized by a considerable concentration of elements, indicating that biogeochemical process of their transformation. Of course, ore deposits are an excellent ecological niche for the activity of a specific autotrophic microflora.
Thiobacilli utilising the energy of oxidation of iron, sulphur and its reduced compounds predominate in the deposits of sulphur and sulphide ores. Leptospirillum ferrooxidans (Fe2+ → Fe3+), Thiobacillus organoparus and T. acidophilus (S → SO4) are found in ore deposits under the favourable physicochemical conditions; elements are leached from ores and transferred with ore waters. Leaching often proceeds on such a large scale that it can be a source for obtaining considerable amount of metals.
The microbial leaching process starts with the acid reaction in which ores are percolated down, rich in the mineral, which is collected and allowed to precipitate, purified and recovered.