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

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 hybri dizations with group-specific probes. Probes were used for the domains Euca rya, Bacteria, and Archaea and the two species most widely studied and impl icated for their role in AMD production, Thiobacillus ferrooxidans and Lept ospirillum ferrooxidans, Results show that microbial populations, in relati ve proportions and absolute numbers, vary spatially and seasonally and corr elate with geochemical and physical conditions (pH, temperature, conductivi ty, and rainfall), Bacterial populations were in the highest proportion (>9 5%) in January. Conversely, archaeal populations were in the highest propor tion in July and September (similar to 50%) and were virtually absent in th e winter. Bacterial and archaeal populations correlated with conductivity a nd rainfall. High concentrations of dissolved solids, as reflected by high conductivity values (up to 125 mS/cm), occurred in the summer and correlate d with high archaeal populations and proportionally lower bacterial populat ions. Eukaryotes were not detected in January, when total microbial cell nu mbers were lowest (<10(5) cells/ml), but eukaryotes increased at low-pH sit es (similar to 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, ferrooxida ns was in highest abundance (>30%) at moderate pHs and temperatures (simila r to 2.5 and 20 degrees C) in sites that were peripheral to primary acid-ge nerating sites and lowest (0 to 5%) at low-pH sites (pH similar to 0.5) tha t were in contact with the ore body. L. ferrooxidans was more widely distri buted with respect to geochemical conditions (pH = 0 to 3; 20 to 50 degrees C) but was more abundant at higher temperatures and lower pHs (similar to 40 degrees C; pH similar to 0.5) than T, ferrooxidans.