Effects of combined treatments of zinc and pH on attached bacterial abundance and activity

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Abstract
The response of attached bacteria to Zn at three pH levels was studied in outdoor stream mesocosms. Streams containing water from Crooked Lake, Indiana, were dosed with either 0.1 or 0.5 mg Zn/L at ambient pH (pH 8.4), pH 7.0, or pH 5.5. Samples were taken at 2, 5, 10, and 20 days and analyzed for total biomass, chlorophyll content, total bacterial numbers, and heterotrophic activity. All measures of community abundance and activity were highest in the treatment receiving 0.1 mg/L at a pH of 8.4. Treatments at lower pH levels and the higher Zn concentration showed significantly lower biomass, chlorophyll content, and total bacterial numbers. Bacterial heterotrophic activity did not demonstrate a statistically significant response as pH decreased, although it did show a statistically significant decrease at a concentration of 0.5 mg Zn/L. The portion of the bacterial community able to grow on nutrient agar demonstrated adaptation to the increased toxicity of Zn at lower pH levels by an increase in the proportion of bacteria resistant to Zn. In addition, the structure of the culturable bacterial community was altered at low pH, as demonstrated by a reduction in the Shannon–Wiener diversity index. The results indicate that pH can modulate Zn toxicity to the attached community. Key words: attached bacteria, zinc, heavy metal resistance, acid rain, species diversity.

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