Selenium Bioconcentration in Freshwater Periphyton
Selenium is an essential micronutrient for most forms of life, but it can elicit developmental toxicity in aquatic and semi-aquatic vertebrates, such as fish and waterfowl, through dietary exposure to excess organic Se compounds. When inorganic Se (as selenate or selenite) is introduced into an aquatic ecosystem as a contaminant, it is bioconcentrated by microorganisms and primary producers (algae, periphyton), biotransformed into organic Se compounds and passed on to higher trophic levels through the food chain. The enrichment of Se in algae is difficult to predict due to interspecific differences in Se bioconcentration, which have been demonstrated to vary by several orders of magnitude in planktonic algae. In addition, Se bioconcentration data are largely lacking for freshwater, periphytic species of algae, and for multi-species periphyton biofilms, adding to the challenge of modeling Se transfer in periphyton-based food webs. Therefore, this research project was designed to address specific knowledge gaps related to the enrichment of selenium in different periphyton communities, as defined by differences in photoautrophic assemblage composition. To satisfy this objective, laboratory-grown and naturally-grown periphyton biofilms were exposed to environmentally relevant concentrations of selenite [Se(IV)] or selenate [Se(VI)] (nominal concentrations of 5 and 25 μg Se L-1) under similar, controlled laboratory conditions. Laboratory-grown periphyton biofilm experiments assessed Se oxyanion bioconcentration in single-species, freshwater periphytic biofilms representative of three major algal phyla: Chlorophyta (Stichococcus bacillaris), Cyanophyta (Anabaena flos-aquae) and Bacillariophyta (Asterionella formosa). Results of these experiments revealed that there was different enrichment of selenate versus selenite for the three species of algae tested (e.g., selenite enrichment was significantly higher than selenate enrichment for A. formosa). There were also significant differences in Se enrichment when comparing similar treatments among the three species of algae tested (e.g., enrichment of selenate was 3.6-fold higher in S. bacillaris compared to A. flos-aquae for the 25 μg Se L-1 treatment). Nevertheless, interspecific Se enrichment did not vary by orders of magnitude for freshwater periphyton, but rather by less than one order of magnitude. Naturally-grown periphyton experiments assessed Se oxyanion accumulation in freshwater periphyton communities sampled from five different water bodies. Results revealed that unique periphyton assemblages were derived from the five different field sites, as confirmed by light microscopy and targeted DNA sequencing of the plastid 23S rRNA gene in algae. Selenium accumulation demonstrated a maximum of 23.6-fold difference for selenite enrichment and 2.1-fold difference for selenate enrichment across the periphyton/biofilm assemblages tested. The assemblage from one field site demonstrated both high accumulation of selenite and iron, and was subjected to additional experimentation to elucidate the mechanism(s) of accumulation. Selenite accumulation was assessed in both unaltered and heat-killed periphyton, and in periphyton from the same site grown without light to exclude phototrophic organisms. All periphyton treatments showed similar levels of Se accumulation, indicating that much of the apparent uptake of selenite was due to non-biological processes (i.e., surface adsorption). The results of this study highlight the need for further exploration of the ecological consequences of extracellular adsorption of selenite to periphyton and will also help to reduce uncertainty in the prediction of Se dynamics and food-chain transfer in freshwater environments.
selenium, periphyton, bioconcentration
Master of Science (M.Sc.)