Effect of selenium on microbial communities in laboratory microcosms and outdoor streams

Ecological effects of selenium (as sodium selenite) on naturally derived microbial communities were evaluated in laboratory microcosms and in outdoor experimental streams at the U.S. Environmental Protection Agency (U.S. EPA) Monticello Ecological Research Station (MERS). Microcosms were continuously dosed for 21 days at selenium concentrations ranging from 0 to 160μg Se/L. Outdoor streams were continuously dosed at 0, 10, and 30 μg Se/L, the highest concentration approximating the current U.S. EPA water quality criterion. In laboratory microcosms, protozoan species richness was reduced by 20%, and chlorophyll and hexosamine levels were reduced by 40% at μ80 μg Se/L. Total biomass and carbohydrate levels decreased with increasing Se, but these effects were not significant. Selenium had no effect on microcosm production to respiration ratios. In outdoor streams, microbial community biomass collected on artificial substrata was 2–3 times greater than in the laboratory. In general, adverse effects were not observed, confirming laboratory estimates of no adverse effects at <80 μg Se/L. However, low doses (10μg Se/L) consistently stimulated microbial biomass (protein, chlorophyll, hexosamine) and elevated production to biomass (P/B), consistent with reports of Se stimulation of algal growth. The highest Se dose (30 μg/L) caused decreased primary production and decreased P/B compared to controls. Experiments showed that ecological responses of laboratory microcosms and outdoor experimental ecosystems are similar, and are at least as sensitive as standard toxicological responses.