Study of the Microflora of Pore Solutions in Rocks of the Kolyvan-Tomsk Fold Zone
Abstract Generated abstract
This paper examines microorganisms inhabiting pore solutions in rocks of the Kolyvan-Tomsk fold zone and their possible role in forming groundwater chemistry. The authors isolated autotrophic bacterial cultures, primarily related to thionic bacteria, from pore waters of weathering-crust clays using selective media supplemented with sterile pore solution or rock extracts, then tested their ability to leach metals from sulfide minerals. Several cultures differed morphologically from described forms and showed selective effects on mineral oxidation: one strongly accelerated zinc leaching from sphalerite, another weakly enhanced zinc leaching but not copper release from chalcopyrite, and a halophilic culture accelerated copper leaching from chalcopyrite. The results suggest that pore-solution microflora can contribute to sulfide oxidation and to the high mineralization of pore waters, making it a potentially important factor in groundwater composition.
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STUDY OF THE MICROFLORA OF PORE SOLUTIONS IN ROCKS OF THE KOLYVAN–TOMSK FOLD ZONE
P. A. UDODOV, G. A. CHUGUNOVA, V. A. SHAMOLIN
The processes by which the chemical composition of groundwater is formed have until recently remained poorly studied. This was due to the fact that the approach to their study was highly simplified. The role of such an important factor as pore solutions was not taken into account.
The study in Siberia of pore solutions in rocks was undertaken only in 1968 by the Problem Geological Laboratory of the Tomsk Polytechnic Institute. In the first year, primary attention was given to determining their chemical macro- and microcomposition. Bound waters of some rock samples have a low pH value (1.5–2.0) and a high content of metals, in particular total Fe, up to 48.7 g/l.
In 1969 we established the presence of microflora in pore solutions. In terms of their physicochemical characteristics, these solutions constitute a specific environment for microorganisms, which differ significantly from the microflora of freely exchanging waters. Thus, in the pore waters we studied, isolated from dark-gray clays of the weathering crust of the Lower Carboniferous (point 1, a) and from bleached clays, likewise of the weathering crust (point 2, a), inoculation on meat-peptone agar revealed no heterotrophs, which are characteristic of freely exchanging waters.
Among autotrophs, we studied mainly the group of thionic bacteria. By inoculating pore solutions onto selective media, along with the known species of thionic bacteria Th. thioparus, Th. thiooxidans, and Th. ferrooxidans, a number of microorganisms were isolated that have morphological differences from those described in the literature. These cultures were isolated on nutrient media for thionic bacteria with the addition of sterile pore solutions or an extract from the natural substrate.
With the aid of a series of experiments, we attempted to determine the capacity of these bacterial cultures to leach metals from ores.
The experiments were carried out according to the following scheme. Into sterile flat-bottomed flasks, 5 g of crushed sulfide ore were introduced, 40 ml of the appropriate nutrient medium and 10 ml of the bacterial emulsion under study were added. In the control flasks, the same weighed portion of ore was covered with 50 ml of the appropriate nutrient medium (without introducing bacteria). Aeration and mixing were carried out with the aid of shakers.
Cultures isolated on modified Beijerinck media (for Th. thioparus) and Lieske media (for Th. olenitrificans) did not possess leaching ability.
On Leten medium (for Th. ferrooxidans) with the addition of sterile pore solution, by alternate subculturing on solid and liquid media from the pore solution of dark-gray clays of the weathering crust (point 1, a), a bacterial culture was isolated that differed greatly from those described in the literature: bacterial colonies on solid medium were irregular in shape, 0.5–1 cm in size, wrinkled, yellow, and released abundant yellow pigment into the medium. Microscopy revealed large, 1.5–2 µm bodies surrounded by a dense membrane and filled with smaller spherical formations. They resemble a fungal sporangium filled with spores. However, no other features characteristic of microscopic fungi were found.
To determine the ability of this culture to leach sulfide minerals, sphalerite was subjected to its action. The results of the experiment are given in Table 1.
Table 1
Oxidation of sphalerite by a bacterial culture isolated from pore solution (point 1a) on Leten medium
(metals passed into solution, mg/l)
| Time (days) | Experiment | Control | Experiment | Control | Experiment | Control |
|---|---|---|---|---|---|---|
| 1 | 8.2 | 7.8 | 9.4 | 8.9 | 0.01 | 0.01 |
| 5 | 8.7 | 8.3 | 8.9 | 8.3 | 2 | 1.2 |
| 7 | 9.7 | 8.8 | 10 | 8.7 | 50 | 1.2 |
At the beginning of the experiment the pH of the medium was set at 2.5; after a week it rose to 3.0–3.5. The results of this experiment show that, after seven days of action, the bacteria accelerated the leaching of zinc from sphalerite by a factor of 41.6.
From the pore solution of bleached clays of the weathering crust (point 2, a), on Waksman medium (for Th. thiooxidans) with the addition of an extract from the parent rock, by repeated subculturing on liquid and agarized media, a culture was isolated that also leached zinc from sphalerite.
The colonies of this culture on dense medium are small, slightly pinkish, of regular round shape, slightly convex, and transparent. The bacterial cells have the appearance of short rods (1 µm in length) with rounded ends. This culture was used to act upon sphalerite and chalcopyrite. The results of the experiment are given in Table 2.
The initial pH was set as neutral. In the experimental flasks, by the end of the period it had become 4.2; in the control it did not change. On the 21st day after the beginning of the experiment, the culture under study accelerated the leaching of zinc from sphalerite by a factor of 2.6 compared with the control, but proved inactive toward chalcopyrite, since no leaching of copper was observed. The results of the experiment indicate the selective action of the culture.
Table 2
Oxidation of sphalerite and chalcopyrite by a culture of bacteria isolated from pore solution (point 2a) on Waksman medium
| Time (days) | Sphalerite (Zn passed into solution, mg/L) (experiment) | Sphalerite (Zn passed into solution, mg/L) (control) | Chalcopyrite (Cu passed into solution, mg/L) (experiment) | Chalcopyrite (Cu passed into solution, mg/L) (control) |
|---|---|---|---|---|
| 1 | 1.6 | 1.3 | — | — |
| 7 | 0.38 | 0.3 | — | — |
| 14 | 0.20 | 0.28 | — | — |
| 21 | 2.4 | 0.9 | — | — |
A selective action on ores was also found in the bacterial culture isolated from the pore solution of bleached clays of the weathering crust (point 2a) on Nathanson medium for halophilic bacteria. The colonies of this culture on solid medium are very small, colorless, round, and drop-like. Microscopy revealed small (0.5–0.7 μm in length), nonmotile rods. This culture, unlike the culture isolated on Waksman medium, did not leach zinc from sphalerite, but accelerated the leaching of copper from chalcopyrite. Thus, after its two-month action, the amount of copper in the experimental sample increased to 12 mg/L, whereas in the control sample it was 1.4 mg/L; that is, copper leaching was accelerated by more than eight times in comparison with the control.
The results of the experiments we conducted showed the undoubted influence of certain microorganisms isolated from pore solutions on the oxidation of the sulfide minerals tested. It is natural to suppose that, under natural conditions as well, the microflora under study actively affects the oxidation of rocks, causing the high mineralization of pore solutions in comparison with freely flowing waters, and thus constituting an important factor in the formation of the chemical composition of groundwater.
Further study of the properties of the cultures mentioned here is currently being carried out.