Natural attenuation of aqueous metal contamination by an algal mat

Naturally occurring mechanisms of attenuation for metals in the environment are important for understanding and remediating acid rock drainage. A fila mentous green algae was found forming an extensive mat below an outflow of acidic, metal-laden groundwater, at Macintosh Creek, MacMillan Pass, Yukon Territory, Canada. Emerging waters had a pH of 3.3 and the following dissol ved metal concentrations in milligrams per litre: Al = 156, Fe = 298, Mn = 1.1, Cd = 0.13, Ni = 2.52, Cu = 0.69, and Zn = 5.0. In contrast, waters tha t had coursed over the algal mat had the following concentrations in millig rams per litre: Al = 26.4, Fe = 29.3, Mn = 0.18, Cd = 0.03, Ni = 0.4, Cu = 0.2, Zn = 0.88. In addition, the concentration of dissolved As declined fro m 32.9 to 9.3 mu g.L-1. Thus, the concentrations of potentially deleterious elements were typically reduced by between 5- and 10-fold. Scanning electr on microscopy (SEM) indicated that individual filaments of the algae were e ncrusted with mineral precipitate. Microprobe analyses indicated that the c oatings were predominantly composed of Fe with other metals, in the presenc e of S and P, the latter possibly associated with the algal biomass. While culturing methods indicated the presence of 10(4)-10(6) Thiobacillus ferrom idans.mL(-1) in the water, epifluorescence microscopy observations using DA PI and SYTO 9 nucleic acid stains did not reveal bacteria in association wi th the algal filaments. Hydrated samples were also observed using confocal scanning laser microscopy (CSLM) with FITC-conjugated lectin staining, auto fluorescence, and reflection imaging. These observations indicated that the algal filaments had an extensive exopolysaccharide surrounding the filamen ts and that mineralization occurred within the matrix. This suggested that factors such as the Eh and pH proximal to the algae may be playing an impor tant role in mineral production.