Seasonal variations in microbial populations and environmental conditions in an extreme acid mine drainage environment

Abstract

Microbial populations, their distributions, and their aquatic environments were studied over a year (1997) at an acid mine drainage (AMD) site at Iron Mountain, Calif. Populations were quantified by fluorescence in situ hybridizations with group-specific probes. Probes were used for the domains Eucarya, Bacteria, and Archaea and the two species most widely studied and implicated for their role in AMD production, Thiobacillus ferrooxidans and Leptospirillum ferrooxidans. Results show that microbial populations, in relative proportions and absolute numbers, vary spatially and seasonally and correlate with geochemical and physical conditions (pH, temperature, conductivity, and rainfall). Bacterial populations were in the highest proportion (>95%) in January. Conversely, archaeal populations were in the highest proportion in July and September ( approximately 50%) and were virtually absent in the winter. Bacterial and archaeal populations correlated with conductivity and rainfall. High concentrations of dissolved solids, as reflected by high conductivity values (up to 125 mS/cm), occurred in the summer and correlated with high archaeal populations and proportionally lower bacterial populations. Eukaryotes were not detected in January, when total microbial cell numbers were lowest (<10(5) cells/ml), but eukaryotes increased at low-pH sites ( approximately 0.5) during the remainder of the year. This correlated with decreasing water temperatures (50 to 30 degrees C; January to November) and increasing numbers of prokaryotes (10(8) to 10(9) cells/ml). T. ferrooxidans was in highest abundance (>30%) at moderate pHs and temperatures ( approximately 2.5 and 20 degrees C) in sites that were peripheral to primary acid-generating sites and lowest (0 to 5%) at low-pH sites (pH approximately 0.5) that were in contact with the ore body. L. ferrooxidans was more widely distributed with respect to geochemical conditions (pH = 0 to 3; 20 to 50 degrees C) but was more abundant at higher temperatures and lower pHs ( approximately 40 degrees C; pH approximately 0.5) than T. ferrooxidans.

FIG. 1

Location map of Iron Mountain.

FIG. 2

Schematic map of the Richmond five-way and entrance tunnel. Sampling sites are marked with asterisks.

FIG. 3

Monthly rainfall totals during 1997 at Iron Mountain. Data was provided by Stauffer Management Co. and reflects readings taken at the treatment facility.

FIG. 4

Environmental conditions measured at B-drift and the tunnel during 1997.

FIG. 5

Microbial populations at B-drift for January, July, September, and November 1997. The numbers used to plot the relative proportions of cells represent the sum of cells on surfaces and in suspension associated with the sediments. Cell numbers are normalized to the sum of the domain counts and thus reflect viable cell proportions. Error bars reflect standard errors for counting procedures (see text). Only the lower error bars are shown for stacked data.

FIG. 6

Image of probing of archaeal cells in B-drift sediments. For both the upper and lower sets of images, the cells on the left are stained with DAPI and viewed under UV fluorescence and the cells on the right are hybridized with the CY 3 archaeal probe. The upper two images show cells in suspension, while the lower two show cells adhering to pyrite sediment surfaces (separated from solution and rinsed with ethanol prior to probing). Arrows point to clusters of cells, stained (left) or hybridized (right), that are out of the plane of focus due to the irregular shape of natural pyrite sediments.

FIG. 7

Example of eukaryotic filaments at the Richmond five-way. (Left) DAPI-stained cells (viewed under UV); (right) filaments hybridized with the CY 3 eukaryote probe.

FIG. 8

Cells per milliliter for each domain in B-drift sediments over the course of 1997. Error bars reflect standard errors for counting procedures (see text).

FIG. 9

Comparison of microbial populations at the tunnel and B-drift at the two end-member extremes under environmental conditions represented in Fig. 4. Patterns are the same as were used for Fig. 5. Error bars reflect standard errors for counting procedures (see text). Only the lower error bars are shown for stacked data.