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 filamentous 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 dissolved 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 that had coursed over the algal mat had the following concentrations in milligrams 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 from 32.9 to 9.3 µg·L-1. Thus, the concentrations of potentially deleterious elements were typically reduced by between 5- and 10-fold. Scanning electron microscopy (SEM) indicated that individual filaments of the algae were encrusted with mineral precipitate. Microprobe analyses indicated that the coatings were predominantly composed of Fe with other metals, in the presence of S and P, the latter possibly associated with the algal biomass. While culturing methods indicated the presence of 104-106 Thiobacillus ferrooxidans·mL-1 in the water, epifluorescence microscopy observations using DAPI and SYTO 9 nucleic acid stains did not reveal bacteria in association with the algal filaments. Hydrated samples were also observed using confocal scanning laser microscopy (CSLM) with FITC-conjugated lectin staining, autofluorescence, and reflection imaging. These observations indicated that the algal filaments had an extensive exopolysaccharide surrounding the filaments 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 important role in mineral production.Key words: heavy metals, exopolymers, acid drainage, confocal microscopy.