Response of attached bacteria to zinc in artificial streams

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Abstract
To study the response of natural communities of attached bacteria to Zn, artificial streams were dosed with Zn at levels of 0, 0.01,0.1,0.5, 1.0, and 10.0 mg/L in duplicate treatment groups. Changes in total biomass, chlorophyll a content, total bacterial numbers, and heterotrophic activity were measured over a 5-week exposure period. The adaptation of the culturable bacterial community to Zn was assessed by comparing viable counts on unsupplemented medium with counts on media supplemented with several concentrations of Zn. The structure of that portion of the bacterial community capable of growth on nutrient agar was assessed by characterizing randomly selected colonies with respect to 20 nutritional capabilities and grouping the strains by numerical taxonomy. Streams receiving 0.5, 1.0, and 10.0 mg Zn/L were significantly inhibited with respect to all variables studied when compared with streams receiving 0, 0.01, or 0.1 mg Zn/L. Bacteria developed tolerance to the exposure concentration of Zn but were resistant to higher Zn concentrations. Increasing concentrations of Zn produced alterations in the structure of the culturable bacterial community, resulting in differing assemblages of bacterial groups and a decrease in diversity as measured by the Shannon–Wiener index and rarefaction. These results suggest that streams receiving 0.5 mg Zn/L or more may be adversely impacted. Key words: attached bacteria, zinc, heavy-metal resistance, species diversity.