Zinc-induced stress on cells of Halamphora luciae (Bacillariophyceae)

The aim of the present paper was to study the effect of zinc on the benthic diatom Halamphora luciae. Bioassays using the metal concentration corresponding to the ICzn50 were terminated in two selected endpoints: day 4 (acute phase) and day 9 (chronic phase). A 1.4- and 2.2-fold reduction in Zn2+ with respect to the initial cation concentration was estimated by atomic absorption at days 4 and 9, respectively and was attributed both to precipitation of insoluble carbonates and oxides at alkaline pH and to biosorption. Carrying capacity of Zn-exposed cultures diminished even when growth rate remained unchanged, suggesting some degree of acclimation of the strain to metal stress. Zn stimulated cell bleaching together with the shift into the lipogenic phase, denoted by total lipid quantification and bigger lipid bodies volumes. Lipid peroxidation and phenolic compounds dosage revealed no major metal-induced oxidative stress. On the other hand, metal exposure had an antagonistic effect on the activation of alkaline phosphatase activity after phosphorus starvation. Superoxide dismutase (SOD) activity, cell motility and mucilage secretion were enhanced only during acute exposure. Soluble extracellular polymeric substances (S-EPS) from Zn-exposed cultures denoted a higher heterogeneity in monosaccharide composition. Frustules from metal-exposed cells presented a reduction in areolae diameter together with a lower area:volume ratio of the cells due to the addition of cingular bands.