Intracellular Versus Extracellular Iron Accumulation in Freshwater Periphytic Mats Across a Mine Water Treatment Lagoon

Despite the importance of periphyton-metal interactions in bioremediation schemes and in phosphorus (P) cycling, the processes controlling metal accumulation in periphytic mats are still poorly understood. Iron (Fe) accumulation in periphytic mats was examined across a Fe settlement lagoon receiving mine drainage in Scotland, UK, between March and June 2008. Quantification and mapping of intracellular and extracellular Fe concentrations in periphyton samples using scanning electron microscopy-energy dispersive spectroscopy suggested that Fe accumulation was dominated by the association of Fe-rich precipitates with the extracellular polymeric substances matrix, rather than biotic uptake. Intracellular Fe concentrations were significantly higher in periphyton samples exposed to the highest dissolved Fe concentrations. Neither intracellular nor extracellular Fe concentrations were significantly affected by light availability or cell density. While diatoms dominated the periphyton communities there was no significant association of diatom functional groups with Fe accumulation, indicating that community composition may not affect the function of periphytic mats with respect to Fe removal. Scale-up calculations based on the mean measured Fe accumulation rate by periphyton substrates of 0.021 g m(-2) day(-1) showed that exposure of large surface areas of periphyton substrate in the settlement lagoon would only increase the Fe removal efficiency of the lagoon by c.1%.