Morphological, biochemical, and physiological responses of a mixed periphytic diatom community to single and concurrent exposure to elevated Fe and Hg levels.

Abstract

Industrial effluents discharged into aquatic bodies often contain multiple metal species simultaneously. However, research on toxicity assessment tends to focus on the effects of individual metals. The present study evaluated the impact of Fe and Hg, considering single and combined (Fe + Hg) exposure, on the morphological, biochemical, and physiological parameters of a diatom community. Several non-taxonomic parameters, including percent motile frustules (PMF), percent alteration in protoplasm, lipid bodies (LBs), pigment composition, and deformities, showed noticeable changes at different levels of metal exposure. Compared with the control, responses of non-taxonomic parameters were significantly greater under combined exposure to Fe and Hg than under either treatment alone. Significantly higher deformed frustules were examined under higher Fe (0.6 ppm) and Fe + Hg concentrations [High 1 (0.3 ppm Fe + 1.5 ppb Hg) and High 2 (0.6 ppm Fe + 2.0 ppb Hg)]. The severity of metal toxicity was also examined in terms of the dominance of type 4 (mixed) deformities at higher concentrations of Fe (0.6 ppm), Hg (2.0 ppb) and Fe + Hg [High 1 (0.3 ppm Fe + 1.5 ppb Hg) and High 2 (0.6 ppm Fe + 2.0 ppb Hg)]. Although lipid bodies showed some potential as indicators of overall metal stress, they were comparatively less effective than other non-taxonomical parameters in elucidating interactions between Fe and Hg under co-exposure conditions. Based on the observed responses, the overall order of metal toxicity was Fe < Hg < Fe + Hg, highlighting the enhanced toxicity of metal mixtures. Nevertheless, the examined non-taxonomic parameters are promising because they provide relatively accessible information on both acute and chronic effects, require less reliance on detailed taxon-specific identification, and can be applied across diverse biogeographical settings with limited influence from genus-specific variability.